Demonstrationsand teaching ideas

selected by the Irish Team

Demonstrationsand teaching ideas

selected by the Irish Team

“In the matter of physics, the first lessons

should contain nothing but what is

experimental and interesting to see. A pretty

experiment is in itself often more valuable

than twenty formulae extracted from our

minds.”Albert Einstein

A. Moszkowski 1970 Conversations with Einstein (Horizon Press) p67

PHYSICS ON STAGE 3

Team and contributors

The POS3 contributors: Sean Fogarty, Paul Nugent, Alison Graham, Eilish McLoughlin,Jennifer Egan, Tim Roe, Alison Hackett, Ian Elliott, Eamonn Cunningham and Vida Given.

DisclaimerThe National Steering Committee for Physics on Stage 3 (POS3) has made every effort toensure the good quality of the information presented in this publication. Teachers shouldensure the safety of the demonstrations in their own laboratories. This document has beenproduced by volunteers and, thanks to our sponsors, is distributed free of charge. It isintended as a support resource for teachers of physics and is not published for profit.

POS3 gives permission to educational organizations to reproduce material from this bookwithout prior notification, provided that this is done for the purposes of education, is not forprofit and suitable acknowledgement to POS3 is given. POS3 would be grateful to receive acopy of any other publication using material reproduced from this booklet.

Any comments or suggestions would be welcomed by the committee and can be sent toAlison Hackett, Physics on Stage 3, c/o Department of Experimental Physics, UniversityCollege Dublin, Belfield, Dublin 4.

http://ireland.iop.org/pos3/

The POS3 team (left to right): JenniferEgan, Joanna Dullaghan, Paul Nugent,Alison Graham, Eilish McLoughlin, TimRoe, Nancy Roe and Sean Fogarty.

The conference vi

Acknowledgements viii

Experiments

pressure 1–5

forces 6–19

density 20-21

heat 22–23

waves & sound 24–27

light 28–47

electricity & magnetism 48–53

miscellaneous 54–56

PHYSICS ON STAGE 3: Contents

Physics on Stage 3,Noordwijk, the Netherlands,8–15 November 2003

“The wealth of ideas and material

on display is staggering, and in my

18 years of teaching I have not

attended anything quite like this.

There are so many new

experiments and approaches that

can be used in the class that I feel I

have learned more ‘tricks of the

trade’ in one week than I did in all

my previous years’ teaching.”Sean Fogarty, Irish delegate

What is Physics on Stage?Physics on Stage (POS) is a unique Europe-wide programmethat aims to raise public awareness of physics and relatedsciences by bringing physics teachers together for a week,allowing them to exchange ideas and assess the currentsituation in physics education. POS3 was hosted by theEuropean Space Agency (ESA) at its European SpaceResearch and Technology Centre in Noordwijk, theNetherlands. Some 400 delegates from countries all overEurope set up their national stands to display physicsexperiments of various kinds.

PHYSICS ON STAGE 3: The conference vi

The Irish stand at POS3.PHYSICS ON STAGE 3

PHYSICS ON STAGE 3: The conference vii

PHYSICS ON STAGE 3

Ireland takes a prizeThe centrepiece of the Irish stand was a demonstration of theCoriolis effect. Tim Roe, a retired physics lecturer from GMIT,brought a model that splendidly demonstrated the effectthat’s responsible for causing the rotary movements of theatmosphere associated with weather patterns. The deviceeven shows the different rotations set up in the northern andsouthern hemispheres. Tim’s model deservedly won the awardfor “most original demonstration of a scientific principle”.Other countries’ demonstrations, in keeping with the show’stheme, Physics and Life, included a full model of the workingsof the inner ear on the Greek stand and another illustratingthe principles behind the MRI scanner on the Belgium stand.

Broken hardware experimentsMany stands made use of materials that are commonlyavailable by scavenging old appliances, such as computersand microwaves. For example, the Czech stand had a range ofexperiments performed using items rescued from defunctcomputer CD and hard drives – colour mixing, magnetism,gyroscopes and the conversion of mechanical energy toelectrical energy were all demonstrated with this equipment.These were just a few of the hundreds of good ideas and nicetricks to be seen at the fair – far too many to mention here.

Performances and workshops Time was set aside for performances, presentations andworkshops. The performances covered all areas of physicswith titles such as “Simple experiments on the physics ofvision”, “Food for life” and “Alice in quantum-land”. They useda variety of methods from large-scale demonstrations andtheatrical performances to audience participation, all toconvey information about the principles of physics. Theworkshops, which everyone was encouraged to attend,covered topics such as “New resources for science teachers”,“Training”, “Equipment”, “Performance in teaching” and“Research links” – all areas that could result in improved pan-European science education.

At the time of going to print the next European Conference forPhysics Teachers is planned to be called Science on Stage.This will be held on 21–25 November 2005 at CERN with thetheme Science for Humanity. Details can be found atwww.physicsonstage.net.

Tim Roe and the Coriolis effect.

Vaclv Piskac recycling hardware.

A physics performance at POS3.

This project was made possible by the generous sponsorship of Discover Science andEngineering, Physics on Stage and the Institute of Physics in Ireland. The Department ofEducation and Science kindly provided substitution cover for the four full-time teachers whoattended the conference.

The gratitude of the thousands of teachers and educators who receive this free booklet ofdemonstrations and teaching ideas must principally go to the very hard-working authors: PaulNugent, Sean Fogarty, Alison Graham, Jennifer Egan, Eilish McLoughlin, Vida Given and TimRoe. They all work full time, yet, despite this, they produced this excellent selection ofdemonstrations selected from the Physics on Stage 3 (POS3) conference. This meant testingand developing the demonstrations and creating images for them. This publication would notexist without their very professional commitment. The team has also been most generousgiving their time to present many of these demonstrations at physics and science teachers’conferences and in-service meetings in both the Republic of Ireland and Northern Ireland.

The seven mentioned above were delegates at POS3, but the Irish delegation also includedJoanna Dullaghan and Nancy Roe, whom we thank for their time and dedication. Ourcongratulations to Tim Roe for being the creator and presenter of the excellent Coriolisdemonstration, which won one of the main prizes at POS3 (p56).

Gerard McMahon deserves special thanks for his very hard work as the secretary of POS3 inthe early stages, including arranging the mailing of a request for demonstration submissionsfrom physics teachers in every school, setting up the first website and making many of thefirst communications with POS3.

Thanks are due to Eilish McLoughlin for taking on the role of treasurer and to EamonnCunningham, former POS treasurer, for remaining on the committee and providing guidanceand help with the financial management of POS3. Our thanks also go to the other committeemembers, who helped with planning for POS3 and with editing: Ian Elliott, Michael Grehan,John Hennessy and Diarmuid O Leary. Special thanks go to Daniel Robinson for his excellentartwork on the eye and to Michael Grehan for his help with some of the digital images.

The National Steering Committee for POS3 would also like to thank the following institutionsand people for providing meeting rooms and resources to help with this project: Tim Regan,physics coordinator, Second Level Support Service; Anna Walshe, Junior Science SupportService; Dublin City University Physics Department; Belvedere College Dublin; University CollegeDublin Department of Experimental Physics.

Thanks are also due to the management of the following schools: St Dominic’s High School,Santa Sabina, Sutton, Co. Dublin; Sandford Park School, Ranelagh, Co. Dublin; St Mary’s,New Ross, Wexford; and St Joseph’s Cluny, Killiney, Co. Dublin.

Alison Hackett Chair, Physics on Stage 3Paul Nugent Delegation chair, Physics on Stage 3

PHYSICS ON STAGE 3: Acknowledgements viii

PHYSICS ON STAGE 3

At constant temperature, volume increases as pressure decreases

pressure 1The incredible shrinking marshmallowSpain/Austria

BackgroundBoyle’s Law states that whenthe temperature is heldconstant, the volume of agas is inversely proportionalto its pressure.

Follow these steps1 Place a cylindricalmarshmallow inside a largeplastic syringe.2 Place your thumb over theend to seal the nozzle.3 Push the plunger in andthen out and note whathappens now.

So what happened?The marshmallow shrinkswhen you push the plunger inbut then regains its originalsize when you pull theplunger out again.

The structure of amarshmallow is such thatthere are many bubbles of airtrapped by sugar molecules.There is a fixed number of airmolecules inside themarshmallow–syringe system.

As the plunger is pushedin, the pressure inside thesyringe increases and thebubbles of air decrease insize, therefore the volume ofthe marshmallow decreases.Exactly the opposite thenoccurs when the plunger ispulled outwards.

Eventually the marshmallowresponds less well becausebubbles of gas inside it havebeen punctured becausetheir internal pressure ismuch greater than theexternal pressure.

What next?Boyle’s Law can also bedemonstrated by placing amarshmallow or a partiallyinflated balloon inside a bell-jar and then evacuating thejar – the marshmallow/balloon swells as thepressure is reduced.

Surprisingly, this alsoworks well with ice cream.

PHYSICS ON STAGE 3

You will need... a large plastic syringe fresh marshmallowsNote: you can alsoperform this demonstrationwith a wine-bottle pump(used to expel air from awine bottle to prolong thelife of the unfinished wine)

2 pressureCan you feel the force?The Czech RepublicYou can really experience and feel atmospheric pressure

You will need... a large paperclip a CD a piece of string a large sheet of

newspaper

BackgroundWe live at the bottom of anocean of atmosphere.Atmospheric pressure isequivalent to the weight of10 tonnes over a squaremetre.

Follow these steps1 Attach the CD to the stringusing the paperclip.2 Feed the string through asmall hole in the centre ofthe sheet of newspaper.3 Place the newspaper flaton the ground withoutallowing any wrinkles.4 Exert a firm upwards tugon the string.

So what happened?It is surprisingly difficult to liftthe newspaper because ofthe atmospheric pressureacting on it.

PHYSICS ON STAGE 3

pressure 3

PHYSICS ON STAGE 3

The balloon ‘sucked’ into a bottleBelgiumWitness the effects of atmospheric pressure

You will need... a bottle a balloon a piece of string boiling water

BackgroundThis experiment is a variationon the egg-in-a-milk-bottleexperiment, which is oftenused to demonstrate theeffect of atmosphericpressure. In this version awater-filled balloon is used.

The advantage of theballoon is that its size can beadjusted by changing theamount of water in theballoon, allowing it to becustomized to fit whateverbottle you have.

Follow these steps1 Attach a piece of string tothe neck of the balloon.2 Pour some boiling waterinto the bottom of the bottle.3 Place the balloon over theneck of the bottle so that itmakes an airtight seal.4 Allow the water to cool.

So what happened?As the temperature falls thewarm vapour condenses andthe internal pressure drops,so the balloon is sucked (or,more correctly, pushed byexternal pressure) slowly intothe bottle. If you stand thebottle in cold water you canspeed up the process.

The experiment proceedsslowly compared with thedemonstration that uses anegg and burning methylatedspirits in a milk bottle.Students therefore have moretime to observe what is goingon and work out what mightbe happening.

The string attached to theballoon makes it easy toremove the balloon from thebottle at the end of theexperiment.

What next?This can lead to a discussionabout atmospheric(condensing) engines, whichwere the first type of steamengine that worked on thisprinciple and were first usedin coalmines.

4 pressure

PHYSICS ON STAGE 3

Pressure with a differenceBelgiumPressure is dependent on depth

You will need... a sealed container

(e.g. a sweet tin) a piece of wooden

dowel a container of water

BackgroundThe pressure exerted on afluid depends on the depthof the fluid.

Follow these steps1 Glue a piece of dowel tothe centre of the tin (seepicture).2 Make two holes in the lidof the tin, about 1–2 mm indiameter, one each side ofthe dowel, about halfwaybetween the rim of the tinand the dowel. The holes anddowel should all lie along astraight line.3 Ask the students what willhappen if you lower the tininto the water while keepingthe tin level.4 Do it and see if the waterenters the tin via the holes.5 Repeat the exercise, thistime tilting the tin slightly asyou lower it so that one holeis higher in the water thanthe other. Will the water enterthe holes this time?

So what happened?Once the tin has beenlowered into the water, thesurface tension of the waterat the holes and the pressureof the trapped air prevent thewater from entering the tin.

If, however, you tilt the tinso that one of the holes ishigher in the water than theother, the water enters the

lower hole and bubbles of aircome from the upper hole.

Why? The holes are atdifferent depths, so thepressure at the upper of thetwo holes is less than that atthe lower hole. This causesthe water to enter at thelower hole at the same timeas air is forced out of theupper hole.

What next?See what happens if youmake the holes bigger.

pressure 5Air race between identical pieces of paperThe Czech RepublicAir resistance has less effect on dense objects

PHYSICS ON STAGE 3

You will need... a coin two small pieces of

paper (same weightand dimensions)

BackgroundA skydiver relies on airresistance to allow him/herto fall through the air forseveral seconds beforedeploying a parachute.

Follow these steps1 Place a small piece ofpaper on a large coin.2 Drop the coin and paperas well as an identical pieceof paper at the same time.3 Watch which piece ofpaper reaches the floor first.

So what happened?The paper on the coinreaches the floor firstbecause it does notexperience the slowing-downeffect of air resistance asmuch as the piece of paperwithout the coin.

What next?This leads on neatly to adiscussion of slipstream,coefficients of dragaerodynamics, etc.

6 forces

PHYSICS ON STAGE 3

Water rocketAustriaBreaking surface tension

You will need... a bowl of water some card a pair of scissors some washing-up liquid

BackgroundSome small insects can walkon the surface of water owingto surface tension.

Follow these steps1 Using the template shown,cut out a paper rocket.2 Place the rocket onto thesurface of the water.3 Put a drop of washing-upliquid in the central circle ofthe rocket (dip a pencil intothe detergent then dip it intothe water inside the circle)and watch what happens.

So what happened?When the detergent is addedit locally reduces the surfacetension of the water. Owingto the shape of the card, therocket shoots forward.

What next?Try repeating this exerciseseveral times in the samewater. You will find that itonly works two or three timesbefore there is too muchdetergent present. Addingmore has no effect – now thesurface tension is low all overthe surface.

water rocket

detergent

For every action there is an equal and opposite reaction

forces 7If you push the wall, will it move?Ireland

You will need... two metre sticks Blu-Tack a straight pin a small piece of mirror

or CD (1 ×1 cm) a laboratory bin a laser pointer a retort stand a flat, smooth surface

(e.g. hardback book) a masonry wall

BackgroundThis uses a simple classroomoptical lever to demonstrateNewton’s Third Law and themicroscopic flexures ofmasonry walls created byhuman-scale contact forces.

Follow these steps1 Upturn the bin, place aflat, smooth surface on top ofit and position it about 0.5 mfrom a masonry wall.2 Attach the small mirror tothe pin with Blu-Tack.3 Place a metre stick on itsedge on top of the flatsurface, one end attached tothe wall with Blu-Tack.4 Rest the other end of themetre stick freely on top ofthe bin.5 Place the pin and mirrorbetween the metre stick andthe flat surface so that theend of the stick rests freelyon top of the pin and mirror.6 Position the laser pointerin the retort stand, directingthe beam onto the mirror.7 Position a second metrestick, to act as a scale, in theline of the reflected beam.8 Observe the reflected spoton the scale.9 Get a student to pushagainst the wall and watchthe reflected spot move upand down on the scale.

So what happened?According to Newton’s ThirdLaw, for every force (oraction) between two bodiesthere is always an equal butopposite force (or reaction).So, as you push against thewall, the wall yields.

As the wall flexes, the firstmetre stick moves back andforth, rotating the pin andmirror and thus deflecting thelaser spot at varying anglesback to the scale.

What next?Ideally, use a wall that youcan both push and pull.

You can read more aboutthis demonstration in

Daniel L MacIsaac andMichael Nordstrand 2001Demonstrating and measuringthe flexure of a masonry wallPhys. Teach. 39 212.

PHYSICS ON STAGE 3

8 forcesHas Newton’s Law gone wrong?SlovakiaAcceleration due to gravity acts at the centre of gravity of an object

You will need... a metre stick several coins

BackgroundObjects come in all shapesand sizes. When we applyNewton’s law we assume thatall of the mass is in thecentre.

Follow these steps1 Rest one end of a metrestick on the edge of a bench.2 Support the free end of thestick with your hand so that itremains level with the top ofthe bench.3 Pile two or three coins onthe free end of the stick.4 Let go of the stick and allowit to fall a short distancebefore catching it again.5 Watch (and listen) todiscover what happens.

So what happened?You should hear the sound ofthe coins landing on the stickafter you catch it. The coinsmust be falling at a slowerrate to that of the metrestick. However, how can thisbe true if the accelerationdue to gravity is the same forall objects, as Newton’s Lawof gravity tells us? Was hewrong?

Newton was not wrong. Inthis case the centre of gravityof the stick is at its midpoint

and it is here that theacceleration due to gravity isthe standard 9.8 ms–2. Theouter end of the stick, as itfalls, is actually rotatingabout the suspended end onthe bench. This end is 50 cmfarther away from the centreof gravity, so its totalacceleration (angular andlinear) will be greater and itmoves faster than the coins,which are accelerating at9.8 ms–2.

What next?Discuss how pole vaulterscan curve their bodies overthe bar so that their centre ofgravity is below the bar.

PHYSICS ON STAGE 3

forces 9

PHYSICS ON STAGE 3

Opening flowersAustriaCapillary action

You will need... a bowl of water some paper (coloured,

if possible) a pair of scissors

BackgroundPlants rely on capillaryactions to draw water upthrough a network of thintubes.

Follow these steps1 Using the template shown,cut out some paper flowers.2 Fold in the points of eachflower across its centre.3 Place the flowers onto thesurface of the water andwatch what happens.

So what happened?The base of the flowersabsorbs the water and thewater moves out through thepetals in a capillary action.As the petals become soakedwith water they open outowing to the force of thewater travelling through thepaper fibres.

What next?Discuss whether thisexperiment would work withplastic flowers.

flower template flower, folded in

10 forcesCan you balance six nails on one?HungaryPuzzle stimulates discussion on balance and centres of gravity

You will need... seven nails

(6 inch/15 cm) a small block of wood

Follow these steps1 Hammer one of the nailsinto the block of wood.2 Pose the followingproblem: how do youbalance the other six nails ontop of the one in the woodenblock?

So what happened?Did any of the students figureit out? This is how it’s done:1 Lay the six nails flat on thetable.2 Arrange the nails as shownin the picture at top right.“Three heads are better thantwo” may be a useful way toremember the arrangementat each end.3 Gripping the arrangementat the centre, carefullybalance it on the upright nail.

PHYSICS ON STAGE 3

forces 11How can a balloon lift a glass?Slovakia Pressure and friction conspire to produce an entertaining effect

PHYSICS ON STAGE 3

You will need... a drinking glass a balloon

Follow these steps1 Place the balloon insidethe glass.2 Slowly inflate the balloon.3 Holding just the neck ofthe balloon, lift the glass.4 Ask the students to explainwhat they see.

So what happened?A seal is formed between theglass and the balloon. Theresulting frictional force, dueto the increased pressure, isgreater than the weight(downward force) of theglass, so the glass doesn’tslip off the balloon.

What next?Discuss whether this wouldwork with a paper bag, or ifthere was a layer of oilbetween the glass and theballoon.

12 forcesAnyone for tennis?BelgiumAcceleration due to gravity is independent of the mass of an object

PHYSICS ON STAGE 3

You will need... three identical tennis

balls some water a needle and syringe

(for filling one of theballs with water)

Safety note Remove breakables

from the vicinity and donot stand over the ballsas you bounce them.

BackgroundStudents intuitively expectheavy objects to fall fasterthan light ones.

Follow these steps1 Out of view of thestudents, inject one tennisball with as much water as itwill hold.2 Ask a student to drop twonormal tennis balls from thesame height and notewhether or not they hit theground at the same time.3 Then ask them to hold twotennis balls, one of whichyou have filled with water, tofeel the difference in weight.4 Get them to drop the twodifferent tennis balls from anequal height and note whenthey hit the ground.5 Ask the students to try toexplain what they see.

So what happened?Students will expect theheavier of the two differentballs to hit the ground first. Infact they should reach theground at the same time.

This exercise is used todemonstrate how gravity isindependent of the mass ofan object. As the balls arethe same size, students canignore differences in airresistance, etc.

What next?There is a link to the videoclip of the hammer and thefeather being dropped on themoon, which demonstratesthe same principle. You cansee the video clip(broadband is required) athttp://www.hq.nasa.gov/office/pao/History/alsj/a15/a15v_1672206.mpg.

“Bouncing balls” follows onfrom this experiment.

forces 13Bouncing ballsBelgiumConservation of momentum – the effect of collisions

PHYSICS ON STAGE 3

You will need... a tennis ball a basket ball a lofty room (e.g. a

sports hall)

Safety note Remove breakables

from the vicinity and donot stand over the ballsas you bounce them.

BackgroundMomentum is the product ofmass and velocity.

Follow these steps1 Drop the tennis ball andthe basketball independentlyfrom the same height andsee how high they bothbounce.2 Hold the tennis ball on topof the basketball, then dropthem both at the same time.3 Watch how high the twoballs bounce now.

So what happened?If the tennis ball bounces offthe top of the basket ballthen it bounces high into theair – approximately ninetimes as high as previously.

This is a demonstration ofconservation of momentum.Some of the momentum fromthe basket ball is transferredto the tennis ball, therebycausing the tennis ball tobounce higher.

There are also otherreasons why the ball bounceshigher, such as the differentsizes, elasticity and airresistance of the two balls.

What next?A number of people havelooked at the maths of thisexercise. You can find outmore at http://physics.ucsd.edu/students/courses/fall2001/physics2a/tennis-basket-balls.pdf and athttp://www.physics.otago.ac.nz/teaching/PHSI110/jakub/Momentum.html.

14 forcesWhich syringe is easiest to push?AustriaForce is proportional to area if the pressure is constant

You will need... two syringes of different

sizes (e.g. 5 and 20 ml) some plastic tubing some water

Follow these steps1 Connect the apparatus(see diagram), filling thesyringes and plastic tubingwith water. The nozzle ofeach syringe should be ofequal size to connect theplastic tubing.2 Hold a syringe in eachhand and press evenly onthem.3 Note which syringe plungeris easier to push down. Youmay need to swap thesyringes from one hand tothe other to notice thedifference, because thedifference in the strength ofyour hands may counteractthe difference in forcerequired.4 Try varying the amount ofwater in the system. About athird full should work best. Ifthere is too little water it canbe difficult to detect thedifference in forces required.The greater the difference inthe size of the two syringesthe easier it will be to detectthe difference in forcerequired.

So what happened?The plunger of the smallersyringe is easier to pushdown. As the pressureremains constant the forcerequired is proportional tothe area of the syringe.

What next?This is a good introduction tohydraulics, as used in thebrakes of a car, as the basicprinciple is the same.

PHYSICS ON STAGE 3

forces 15Centripetal forceIrelandForces acting on rotating objects

PHYSICS ON STAGE 3

You will need... two circular bowls

(Danone yoghurt comesin an ideal plasticcontainer; see picture)

a table-tennis ball ormarble

BackgroundWhen going round a cornerquickly in a car you feel as ifyou are pushed to theoutside of the bend. In factthe car is exerting a force onyou, called centripetal force,which makes you go roundthe corner with the car.

Follow these steps1 Put the ball under one ofthe containers, placed facedown.2 Ask a student to move theball from this container to theother container withouttouching the ball or theempty container.

So what happened?You may have to give thestudent some clues. Theyneed to move the first bowlin a circular motion, spinningthe ball round fast enoughthat it moves up the sides ofthe bowl. This will allow themto lift the bowl quickly whilethe ball is still forced againstthe sides and place it overthe other bowl, allowing theball to drop into the secondbowl without them having totouch either the second bowlor the ball.

Centripetal force, whichkeeps the ball against thesides of the first bowl, isresponsible for preventing itfrom dropping out of thebowl until the student stopsmoving the bowl in a circularmotion.

What next?When a hammer throwerreleases the hammer, inwhich direction does it go?

This experiment can alsolead to a discussion aboutgovernors used in engines,such as those on steamengines (e.g. the threshingengines seen at field days).

bowl A bowl B

16 forcesSoap bubbles and films: 2-D and 3-D shapesSpainSurface tension determines the shape of soap films

You will need... some glycerine

(~650 ml; can bepurchased from apharmacy)

some water (~2 l) some washing-up liquid

(~1.1 l)

Follow these steps1 Stir the glycerine, waterand washing-up liquid(proportions of about 1:4:2)for about 10 min.Note: Too many bubblesforming at the top make itharder to get good shapes.2 The solution can be storedin a sealed container (e.g. anempty water bottle). In time,some of the water mayevaporate, so add more, asrequired.Note: There are otherrecipes for soap solutionsthat can be found on theInternet. If your tapwater ishard you may need to usebottled water to create thebubbles.

Soap solution 2-D shapes

Follow these stepsDemonstration 11 Make a wire circle and tiea piece of thread across it.2 Dip the wire circle andthread into the soap solution.3 Break the bubble on oneside of the thread and notewhat happens to the thread.Demonstration 21 Drill two Perspex sheets atthe four points of a squareand join the sheets with thescrews through these holes,leaving a gap of about 2 cm,so that the screws form theedges of a cuboid shape.2 Similarly, drill throughthree points of a triangle onthe other two sheets so thatthe screws form the edges ofa triangular prism shape. 3 Dip the 3-D shapes intothe soap solution and notewhere the film forms.

PHYSICS ON STAGE 3

So what happened?The bubbles always move toreduce the surface area to aminimum, so the threadmoves to minimize the size ofthe film; and, for both thesquare and the triangle,instead of being formed atthe outside of the screws, thefilm comes in to the centre,forming 120° angles.

You will need... some coathanger wire some thread four pieces of clear

Perspex (~10 ×20 cmeach)

a drill seven screws

forces 17

PHYSICS ON STAGE 3

Soap bubbles, continued...

3-D shapes

Follow these steps1 Construct a 3-D pyramidand a 3-D cube (see pictures,right) using the magnetic kitand coathanger wire.2 Dip the pyramid into thesoap solution and note wherethe films form.3 Burst some of the sides ofthe bubble and note whathappens.3 Dip the cube into the soapsolution and note where thefilms form.

So what happened?The bubbles always move toreduce the surface area to aminimum, so the films allform at the centre of thestructures.

What next?Ask students to measure thedistances on the 2-Dstructures around the outsideof the triangle/square wherethey expected the film toform. Compare this to thefilm of the bubble formed.

You will need... soap solution some coathanger wire,

solder and solderingiron; or a straw kit(used to createmolecular shapes); or amagnetic kit (e.g.Cultimo MagneticConstruction Set; seeright)

18 forcesMoments on a wheelThe Czech RepublicThe turning effect depends on the distance from the axis

You will need... a CD a marker pen some small round

magnets a retort stand

BackgroundPupils should know that:1 the moment of aforce = force × perpendiculardistance from the axis;2 the principle of momentsstates that in equilibrium thesum of the clockwisemoments equals the sum ofthe anticlockwise moments.This experiment allows pupilsnot only to verify the principleof moments but also to seethat the turning effect dependson the perpendiculardistance from the axiswithout doing calculations.

So what happened?Note that the CD will become unbalanced. Thedirection of the moment ofthe force created by themagnets can easily be seen, thus introducing theterms “clockwise” and“anticlockwise” as applied to moments.

Ask pupils how to get theCD to balance again. This issimply done by putting twomagnets on the oppositeside of the CD at an equaldistance from the axis.

The principle of momentscan then be verified by usingdifferent numbers of magnetsat various distances from theaxis.

Younger pupils should usedifferent numbers of magnetsat various distances from the

axis. Ask them to bring theCD back into balance usingonly one magnet on theopposite side to check thatthey have understood theprinciple of moments.

Ask older pupils what willhappen when the CD is inequilibrium if one set of themagnets is moved to theedge of the CD. The obviousanswer is that the CD willbecome unbalanced.

However, this is not thecase if the magnets aremoved down (see picture).The perpendicular distance isthe same, though the actualdistance (the radius of thecircle) has increased.

This clearly shows that themoment is calculated usingthe perpendicular distanceand not the actual distance.

What next?This can be compared to abicycle wheel and shows whyit is best for the footrest on abicycle to be able to swivelso that the maximumdistance (i.e. the diameter) isalways obtained and thus themaximum turning effect isobtained using the minimumof force.

This can then be followedwith mathematicalcalculations on moments.

Follow these steps:1 Draw a grid of equallyspaced squares on the silverside of the CD (1 cm square).2 Support the marker pen inthe clamp of the retort stand. 3 Balance the CD using themarker pen as the axis. 4 Attach two magnets, eachside, at a fixed number ofsquares away from the axisalong the diameter of the CD.PHYSICS ON STAGE 3

forces 19Impulse and momentumUKA change in momentum is greater when there is a rebound

PHYSICS ON STAGE 3

You will need... two balls of similar size

and mass (one shouldbe elastic and reboundwell and the otherinelastic with little or norebound)

a retort stand a block of wood

BackgroundMomentum = mass × velocityand is a vector quantity.Impulse = change ofmomentum.

This experiment is used toshow that, since momentumis a vector quantity, thechange of momentum isgreater when there is arebound (i.e. a change indirection).

So what happened?The wood should be knockedover much more easily byone side of the ball than bythe other. This is because theelastic ball rebounds onimpact with the wood, thuscausing a bigger change inmomentum due to thedirection change, while theother ball does not reboundand therefore has a smallerchange in momentum.

What next?This can be shown to havemany applications in sportslike tennis and squash.

Follow these steps:1 Cut the two balls in halfand, using one half of each,glue two halves together tomake a composite ball,embedding one end of alength of string in the middle.2 Suspend the compositeball from the clamp of theretort stand.3 Set up the block of wood ashort distance away from theball so that the top of thewood is level with the ball.4 Using one side of the ball,raise it through a fixed heightfrom the top of the wood andthen let it drop to see if it hasenough force to knock overthe wood.5 Repeat this process usingthe other side of the ball.

20 densityThe mystery of the vanishing liquidIrelandDifferent liquids are made up of different-sized molecules

PHYSICS ON STAGE 3

What next?The same principle can beillustrated by using materialswhere the students can seethe individual units thatmake them up (e.g. grains ofrice or dried peas take theplace of the molecules inwater and alcohol).

Discuss how quicksandand avalanches areexamples where particles canbehave like fluids.

So what happened?The total volume of the mixedliquids should be about 5%less than the sum of theprevious two volumes,because the smallermolecules of the wateroccupy some of thevacancies between the largeralcohol molecules.Follow these steps

1 Measure equal amounts ofwater and alcohol and notethe volumes poured (thesmaller the volumes thebetter – say 100 ml of each).2 Add the contents of one ofthe cylinders to the other andmeasure the total volume ofthe mixture.

BackgroundMatter is made up ofmolecules of various sizes.When different-sizedmolecules are mixed, thesmaller ones can occupy thevacancies between the largermolecules.

You will need... two graduated cylinders

(the smaller the better) some alcohol (any type) some water

100 mlwater

100 mlalcohol ? ml+ =

density 21White ball, black ballBelgiumParticles can behave like fluids

PHYSICS ON STAGE 3

You will need... a container and lid some rice a black ball-bearing a white polystyrene ball

So what happened?The heavy black ball-bearingsinks into the rice whereasthe light polystyrene ballfloats to the surface. To thestudents the ball appears tohave mysteriously changedcolour.

Follow these steps1 Pour the rice into thecontainer.2 Hide the white ball justbelow the surface of the rice,then place the black ball ontop.3 Show the container to thestudents.4 Put the lid on thecontainer and then give thevessel one vigorous verticalshake. (Some special magicwords might help here!)

BackgroundThis exercise introducesdensity and flotation andshows how, under somecircumstances, particles canbehave like fluids.

22 heatMaking a turbine from a plastic cupBelgiumConvection currents are created in warm air

You will need... a plastic drinking cup a pencil or pin a heat source (e.g. a

radiator)

PHYSICS ON STAGE 3

BackgroundWarm air rises.

Follow these steps1 Make a number ofL-shaped incisions (all alongthe same orientation) in avery light plastic drinking cupand fold back the flaps thatare created as a result (seepicture).2 Place the cup, upturned,onto a pin, pencil tip or otherpointed support on which itcan turn freely.3 Place the apparatus over asource of heat, such as aradiator.

So what happened?The rising hot air escapessideways from the holes andthe resulting force causes thecup to rotate.

What next?Discuss how steam turbinesoperate using the sameprinciple.

heat 23The blue spiral flameAustria

Safety notes Don’t be tempted to

use a plastic pipette toput the propanone intothe tube. It’s a volatileliquid so will tend tovaporize with the heatfrom your hand andsquirt out.

Don’t use too muchpropanone or the liquidwill catch fire at theend of the tube

Place the candle on themat. Excess propanonemay catch fire as itdrips out of the tubebut will burn safely onthe mat.

So what happened?A bright-blue flame spirals upthe tube with a “whoosh”.The propanone is a volatileliquid, so it vaporizes andmixes with the air as it flowsdown the tube.

When the mixture reachesthe flame, the ignitiontemperature is exceeded andthe propanone reacts withthe oxygen in the air. This isan exothermic reaction, so itspreads back through thetube as a blue flame, whichwinds its way to the top.

The air expands out of thetube with an impressivesound, which increases inpitch as the length of theresonating column of airabove the flame gets shorter.

What next?If you want to do it again itworks best if you blowthrough the tube first tomake sure that it is full of aircontaining oxygen.

Find out more aboutactivation energy by lookinginto how the diesel engineworks.

You will need... 1–2 m of clear Perspex

tubing (5–10 cm wide) propanone (acetone) a glass pipette a night-light candle matches a heat-proof mat

Follow these steps1 Clamp the tube in a spiral(see figure).2 Place the candle on theheat-proof mat at the lowerend of the tube and light it.3 Darken the room.4 Drop about 2 ml ofpropanone from the pipetteinto the top end of the tube.5 Stand clear and watchwhat happens.

An introduction to ignition temperature/activation energy

PHYSICS ON STAGE 3

24 waves & soundStanding waves in elasticThe Czech Republic/SpainStanding waves in an elastic string

You will need... a battery-operated fan

or toothbrush some plastic tubing a matchstick some hat elastic

(~1.5 m)

Follow these steps1 Insert a piece of matchinto the tube on the fan/toothbrush so that it wobbles.2 Attach one end of thethread to the vibrating end ofthe toothbrush/fan and theother to a fixed object.3 Apply tension to the threadby pulling on it with yourfinger and thumb.4 Using varying degrees oftension, set up a standingwave on the thread todemonstrate nodes andantinodes.

So what happened?This shows how standingwaves can be set up on thestrings of musicalinstruments. Different modesof vibration can be obtained

by varying the type ofthread/string used, thedegree of tension appliedand the length of thethread/string.

node

secondharmonic

antinode

fundamentalharmonic

BackgroundThe term standing wave isapplied to a resonant modeof an extended vibratingobject. The resonance iscreated by the constructiveinterference of two wavestravelling in oppositedirections, but the visualeffect is that of an entiresystem moving in simpleharmonic motion.

PHYSICS ON STAGE 3

Standing waves in a tube of gas

The flame tubeGermany

Safety note Once the gas is on,

light the jets to preventunburned gas fromescaping and open thelab door to vent anyfumes.

You will need... a metal pipe (2 m

long, 3 cm wide) some flammable gas

(e.g. natural, butane) an end cap some hosing a flow valve a signal generator a loudspeaker (3 cm in

diameter) a rubber diaphragm

(e.g. a latex glove) wooden supports a microphone an amplifier

waves & sound 25

BackgroundAn acoustic standing wave isset up in a pipe with regionsof low and high pressure. Thepipe is filled with gas thatescapes through evenlyspaced holes. The escapinggas is ignited to show aflame pattern of varyingheights along the pipe.

Follow these steps1 Drill some holes (1–2 mmwide) on one side of the pipe.2 Place the pipe (holesupwards) onto the supports.3 Weld an end cap onto oneend of the pipe, with aconnector to attach thehosing and flow valve to thegas cylinder.4 Fix the diaphragm to theother end of the pipe andposition the loudspeakeragainst it.5 Connect the loudspeakerto the signal generator.6 Open the flow valve to fillthe pipe with gas.7 Wait 10 s or so until thegas has displaced the air.8 Light all of the gas jets.9 Turn on the signalgenerator and set it atmaximum amplitude.10 Adjust the gas flow untilyou see a reasonablevariation in flame height.11 Vary the driving frequencyand observe differentresonant frequencies.

So what happened?About 2 to 10 wave crestsare observed in the200–600 Hz range. Themaximum flame height showsa region of maximumdisplacement and minimumpressure – an antinode. Theopposite situation is a node.

The separation betweenthe nodes equals half awavelength of the standingwave and depends on thefrequency of the sound wave.The pipe is closed at eachend, so nodes should beobserved at both.

What next?Both rhythm and frequencyresponse can be seen nicelyin music. An oscillator or avoice introduced using amicrophone and amplifiercan be used as a simplesource for the loudspeaker.

You can measure thevelocity of sound in a gas byrecording the wavelength, λ,of the standing wave atdifferent resonantfrequencies, f (v = f × λ).

PHYSICS ON STAGE 3

26 waves & sound

Standing waves in a column of air

Kundt’s tubeLuxembourg

You will need... a sound/signal

generator a loudspeaker a transparent plastic

pipe (50–100 cm) some small polystyrene

beads (at least 2 mm indiameter)

a filter funnel an adjustable plunger dry cork dust or

lycopodium powder a metal rod with an end

cap a piece of leather cloth

coated with resin

signal generator

loudspeaker polystyrene beads

BackgroundThis is a dramaticdemonstration and iseffective in providing anintroduction to standingsound waves.

Follow these steps1 Scatter a thin layer ofpolystyrene beads asuniformly as possible insidethe pipe along its length.2 Attach the loudspeaker toone end of the pipe andclose the other end with anadjustable plunger (the pipeis closed so that theresonances exist at oddmultiples of one-quarterwavelength).3 Switch on the signalgenerator at maximumamplitude and then vary thefrequency.4 Observe the standingwaves in the air column.

What next?Try using dry cork dust orlycopodium powder insteadof polystyrene beads.

Vibrations can also begenerated by fitting a metalrod with an end cap into thetube and stroking the endwith a piece of leather clothcoated with resin, or strokingthe tube with a violin bow.

This set-up can be used tomeasure the velocity ofsound in air by recording thewavelength (λ) of thestanding wave at differentfrequencies (f) (v = f × λ).

So what happened?The driving frequency formsstanding wave patterns in theair column inside the tube.The beads show the positionof the nodes and antinodesby forming piles at the nodelocations.

PHYSICS ON STAGE 3

waves & sound 27

PHYSICS ON STAGE 3

A storm in a bottle of waterUKInterference in an everyday context

You will need... a 2 litre bottle of water

(in our experience,River Rock works best)

BackgroundThis demonstrates standingwaves in two dimensions.

Follow these steps1 Hold the bottle by the neckwith one hand.2 Give the bottle a firmknock about midway alongits length using the knucklesof your other hand.

So what happened?A definite note is both feltand heard, but moredramatically a beautifulinterference pattern can beseen on the surface of thewater inside the bottle.

What next?See whether it matters if thecap is removed or the waterlevel changed.

28 light

PHYSICS ON STAGE 3

Which one is the tonic water?BulgariaFluorescence

You will need... four identical bottles

with screw tops, threecontaining water andone containing tonic

an ultraviolet lamp

BackgroundUltraviolet light is invisible tothe human eye but you canstill see its effects.

So what happened?The tonic fluoresces brightblue in the ultraviolet light sois easily identified. Acomponent of tonic water(quinine) absorbs theultraviolet light, which is notvisible to the human eye,thereby exciting its electrons.This is an unstable state, sothe electrons emit energy asphotons of light in the blueend of the visible spectrum.The bottle containing thetonic therefore emits a bluelight under the ultravioletlamp.

What next?Discuss why white materialsdazzle in a disco.

Also, try viewing apassport/drivers’licence/euro note underultraviolet light.

Follow these steps1 Present the four bottles tothe class and ask them tocheck that one contains tonicand the others tap water.2 Bet them (a week’shomework!) that, withouttouching the bottles, you canidentify which contains tonic.3 Get a student to muddlethem up out of your sight.4 Shine the ultraviolet lighton the apparently identicalbottles.

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PHYSICS ON STAGE 3

Dirty mirrorThe Czech RepublicAn unusual method to demonstrate interference

You will need... a concave shaving

mirror a torch some dirt

BackgroundInteresting effects arecreated when thin films areilluminated.

Follow these steps1 Sprinkle a thin layer of finedirt onto the shaving mirror.A suspension of fine mud orchalk smeared onto themirror and allowed to dryshould also work, while a filmof milk allowed to dry on themirror is a good alternative.2 Remove the head of thetorch to expose the bulb,which should be clear so thatthe filament acts as a smallpoint-source of bright light.Look directly at the mirrorwhile holding the light sourceclose to your temple.

So what happened?You should see a pattern inthe mirror consisting of brightand dark bands due toreflections from differentheights on the surface, in aneffect known as thin-filminterference.

What next?If you use a plane mirror youwill need to hold it a bitfarther away from you to seethe effect clearly (about1.5 m should be fine).

The closer you bring thelight source to your eye, thebetter is the interferenceeffect. In fact, if the light is infront of your eye, a circularpattern can be seen providedthat the bulb is not so brightas to dazzle you.

30 light

PHYSICS ON STAGE 3

Finding the refractive index of a liquid

You will need... a deep glass container two pieces of paper

BackgroundLook down at your feet whenyou’re standing in aswimming pool to see anexample of refraction.

Follow these steps1 Draw a straight line onboth pieces of paper.2 Place one of the sheetsunder the glass container.3 Looking down into thecontainer from above, raisethe second sheet of paperuntil the two lines appear tocoincide with no parallax.4 Record the real andapparent depths.5 Calculate the refractiveindex from the formula:

So what happened?The light travels more slowlythrough the water thanthrough the air so thewavefront changes direction.

What next?To improve the accuracy ofthis experiment, use a deepcontainer with straight sides,such as a chromatographyjar. You may wish to use awaterproof card placed in thewater to eliminate any errors

due to the refractive index ofthe glass in the bottom of thecontainer. If the glass is thincompared with the depth ofthe water there will not be asignificant error in yourcalculation.

= real depth

apparent depthrefractive

index

Real depth and apparent depthThe Czech Republic

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PHYSICS ON STAGE 3

The water lensThe Czech RepublicLenses can be made of many different materials

You will need... a glass of water a piece of card with an

arrow drawn on it

BackgroundThis demonstration shows theinversion of images and alsothat lenses can me madefrom several materials otherthan glass.

Follow these steps1 Place the card behind theglass of water.2 View the arrow through thewater.

So what happened?The direction of the arrow isreversed when viewedthrough the water. This is asimple way to introduce theway in which lenses work.

What next?Further work can be done byproducing ray diagrams forthe water lens.

32 light

PHYSICS ON STAGE 3

Disappearing crystalsIreland

BackgroundThe speed of light dependson the type of material thatit’s travelling through.

So what happened?The gel is visible when heldin the net bag in air.However, when immersed inwater the bag looks empty.The refractive index of the gelis the same as that of thewater. When immersed inwater, light is not refracted atthe junction between the geland the water, so thejunction can’t be seen.

What next?Explore the refractive index ofvarious materials.

You will need... a container of water a small net bag to hold

crystals (e.g. the type inwhich vegetables andfruit are often sold)

a piece of string tosecure the net

Chempak Supergelcrystals or equivalent(sold in garden centresto conserve water inhanging baskets, etc)

a spoon

Follow these steps1 Soak a few of the crystalsin lots of water. Water thathas been boiled and cooledworks best as the gel formedcontains fewer air bubbles,which are easily visiblebecause the refractive indexof air is so different from thatof water.

2 Use a spoon to transfersome of the gel to the netbag.3 Tie the bag at the top withstring and lower it into thecontainer full of water until itis completely immersed.4 Note what happens to theappearance of the gel in airand in water.

An intriguing introduction to refraction and refractive index

light 33

PHYSICS ON STAGE 3

You will need... two glass rods two small beakers Clinique body oil (or a

cheaper alternative, ifyou can find one)

BackgroundSolids and liquids can havethe same refractive index.

Follow these steps1 Put water into one of thebeakers and the body oil intothe other.2 Slowly lower a glass rodinto each and record whathappens.

So what happened?The rod is visible in the waterbecause glass and waterhave different refractiveindices. The light rays arerefracted as they pass fromone to the other, so thejunction between the two iseasily seen. The rod seems todisappear as it enters thebody oil, because this hasthe same refractive index asthe glass.

What next?This can lead to anexplanation of how thistechnique is widely used inforensics to establish theprecise source of fragmentsof glass found at a crimescene (e.g. from a particulartype of car headlamp orwindow). The glass fragmentsare placed in standard oils ofknown refractive index. Theoil in which the fragments“disappear” has the samerefractive index as the glass.

Disappearing glass rodBulgariaAnother illustration of refraction and refractive index

34 light

PHYSICS ON STAGE 3

How to make a wooden mirrorSlovakiaAn unusual way to illustrate total internal reflection

You will need... a thin piece of wood a polythene pocket a container of water

BackgroundTotal internal reflection canoccur when a ray of lightpasses from water to air.

Follow these steps1 Pose the question: howcan you make a mirror from apiece of wood?2 Place the wood in thepolythene pocket and theninto the water, making surethat no water is able to enter.3 Vary the angle of the woodin the water. At a particularangle the part of the woodthat is submerged will actlike a mirror.

So what happened?At a particular angle, totalinternal reflection occurs inthe layer of air, between thewood and the polythene, sothat it behaves like a mirror.

What next?See what happens when youput your finger into a glasstest tube and then lower itinto a beaker of water.

light 35

PHYSICS ON STAGE 3

Images in a shiny canThe Czech RepublicYou can’t trust images reflected from a curved surface

You will need... a shiny can some cocktail sticks

BackgroundThis exercise will give thestudents an understanding ofhow images are formed atcurved surfaces.

Follow these steps1 Ask the students to formpatterns with the cocktailsticks while not looking at thesticks themselves but usingonly the reflection in the canto guide them.

So what happened?Curved mirrors producedistorted images.

What next?A similar exercise can becarried out using concavemirrors.

36 lightInvestigating colourIrelandHow are colours made using stage filters?

PHYSICS ON STAGE 3

You will need... some red, green and

blue stage filter paper a pair of safety

spectacles a colour table to view

with the spectacles andfill in (see table). Youcan create the tableusing colour markerpens or by printing oneout (Paint Shop Pro orsimilar software may beuseful)

BackgroundAll colours can be made fromthe three primary colours:red, green and blue.

Follow these steps1 Cut the filter paper so thatit can be inserted inside thesafety spectacles.2 Ask the students to vieweach of the colours in thetable through each of thecolour filters and with nofilter, then record whethereach colour appears light,medium or dark.3 Review with the studentshow the colours appear.

So what happened?Coloured light is made up ofred, green and blue light.When you look at a yellowobject through a primary bluefilter, only blue light isallowed through, so that isthe colour that the objectappears to be. When youlook through a primary redfilter at a primary blue object,the object will appear blackor at least very dark, becausethe red filter does not allowthe blue light from the objectthrough. (Primary colourfilters work best.)

What next?Put the students into teamsof three, each studentwearing a red, green or bluepair of spectacles. Then holdup objects of differentcolours and ask them toidentify the colour of eachobject.

This exercise leads nicelyinto looking at how the eyesees colour; examining colourblindness; and investigatinghow colour is created on atelevision screen.

Colour Red Green Blue White

light 37

PHYSICS ON STAGE 3

Laser showGreeceSimple apparatus for a laser light introduces reflection

You will need... two dental mirrors two DC electric motors two plastic pipette

droppers two 1.5 V batteries and

holders two DPDT switches two 2.2 kΩ

potentiometers a piece of white paper

to act as a screen

BackgroundA simple laser light show canbe created by shining apocket laser onto a set ofrotating mirrors.

Follow these steps1 Mount a mirror on the endof the axle of each electricmotor. The plane of themirror must be tilted slightlywith respect to the axle of themotor. To achieve this, usethe end of a plastic dropperpipette cut at the requiredangle and glued to the backof the mirror with epoxy glue.The tip of the pipette shouldfit snugly onto the axle.2 Wire each motor to a 1.5 Vbattery. The motor speed canbe controlled by a 2.2 kΩpotentiometer in the circuit.

Varying the speed of themotor will allow you tochange the laser patternproduced. A switch to reversethe direction of each motorshould also be included.3 Arrange the mirrors andlaser so that the laser lightreflects from one mirror backonto the second one beforehitting the screen. Arrangingthe apparatus for the firsttime may take a bit offiddling about. Once youhave found an arrangementthat works, attach thecomponents to a base.

So what happened?As you vary the speed of themotors and the direction ofthe laser light, differentpatterns are created on thescreen.

38 lightHot headlampsThe Czech RepublicAn everyday application of reflection of light in concave mirrors

You will need... two car headlamps (the

older round ones areprobably the best – notthe sealed units –available reasonablyfrom scrapyards)

some matches four panes of glass some waterproof

sealant in a gun a dilute solution of

copper sulphate(~0.2 Molar)

a solution of iodine incyclohexane (~1 Molar)

a power pack two retort stands

Follow these steps1 Remove the bulb and thereflector in front of the bulbfrom one of the lamps.2 Clamp this lamp.3 Clamp the second lampdirectly facing the first about0.5 m away from it.4 Connect the power pack tothe back of the second lampto illuminate the bulb.5 Hold a match in throughthe hole at the back of thefirst lamp so that the matchhead is roughly at the focalpoint of the mirror and watchwhat happens.6 Place a line of sealantalong three sides of twopanes of glass and place asecond pane of glass oneach, thus making twonarrow containers that canhold liquids. Allow them todry.7 Fill one of the containerswith dilute copper sulphatesolution.8 Grind iodine crystals with apestle and mortar and addcyclohexane to them to makea dark purple solution. Thenpour this solution into thesecond container.9 Repeat step 5, first withthe copper sulphate solutionand then with the iodinesolution between the twoheadlamps.

Safety notes When connecting the

power pack, take carenot to exceed thevoltage of the bulb(usually 12 V).

Copper sulphate andiodine are both toxic, souse gloves.

Don’t put your finger inthe hole at the back ofthe headlamp to feel ifit is hot – it is.

PHYSICS ON STAGE 3

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PHYSICS ON STAGE 3

What next?This can lead on to solarpower, the visible spectrumand infrared radiation.

So what happened?When the match head is heldat the focal point of the lampit smoulders then bursts intoflame. The bulb at the focalpoint of one mirror producesvisible light and infraredradiation, which are reflectedoff the mirror in a parallelbeam to the other mirror.Here they are reflected intothe focal point, where thematch head is.

This is the principle behindsome types of solar powerstations. The copper sulphatesolution absorbs infrared butonly a little of the visible

light. The light is reflected tothe focal point of the mirrorbut the match doesn’t ignite.

The iodine absorbs most ofthe visible light but not theinfrared. Only a little lightpasses through to bereflected off the mirror butthe match still lights, showingthat the infrared passesthrough the solution and isreflected into the focal point.

After a few “ignitions” thesecond mirror can get verysooty and doesn’t work aswell, so clean both mirrorsperiodically with a soft cloth.

40 lightWords in a tubeThe Czech RepublicRefraction and lenses

PHYSICS ON STAGE 3

You will need... a test tube or boiling

tube and bung water words written on a

sheet (see diagram)

So what happened?It will appear that one wordin each pair is inverted whilethe other is not. In fact, bothwords are inverted but thewords are chosen so that oneis formed entirely fromsymmetrical letters, so that itappears the same wheninverted.

So many schoolexperiments are verypredictable, whereas thissimple activity will reallysurprise and stimulatethought and discussion todetermine what is happening.

What next?An alternative means ofillustrating the same point isto write CARBON DIOXIDE onone side of a bottle and thenrotate it so that you areviewing the words throughthe bottle.

Follow these steps1 Fill the tube with water andinsert the bung securely.2 Hold the tube horizontally,over the words.3 Move the tube up anddown (towards and awayfrom your eyes) until you cansee the words.4 Describe what you see.

BOOK WAY

HIDE PRAM

BED MUST

DOCK SAY

DEED START

CODEX LAMP

light 41How the eye works: introductionBulgariaHow the different parts of the eye and brain contribute to vision

PHYSICS ON STAGE 3

BackgroundThe following experiments areequally relevant to physicsstudents studying therefraction of light by lensesand to biology studentsstudying vision. They havethe enormous dualadvantages of involvingstudents actively in theexperiments yet requiringonly very simple equipment.

The nine experiments showin turn how the followingparts of the visual systemcontribute to human vision:the aqueous humour, the

pupil, the lens, the retina, therods, the cones and thebrain.

The retina is the layer ofcells in the eye that aresensitive to light, includingthe rods (black-and-whitevision) and the cones (colourvision). The fovea is theplace on the retina where thecones are concentrated andwhere the image is focusedfor the best vision when youlook directly at something.The brain also plays a vitalrole in the amazing sense ofvision.

You will need... an overhead projector

and screen small circles of red and

green filter (diameter~2 cm)

green and red filters(106 and 139 fromsuppliers of filters forstage lighting)

an image from theInternet (www.ebaumsworld.com/manillus.html)

for each person, asquare of black card(~4 ×4 cm), a pin, arubber band, twomatches and a longmeasuring tape

some plain white paper a thick black marker

pen an optician’s Snellan

chart

42 lightHow the eye works, continued...

Pupil: why have a narrow entrance to the eye?

So what happened?The pin remains in focusmuch closer to the eye whenyou look through a smallhole. A much greater depthof field is in focus when theentrance to the eye issmaller. The small entranceto the eye means that thelight coming from bothobjects (the distant objectand the pin) is much moresimilar, so the depth of focusis greater (see figure).

Follow these steps1 Make a small pinhole atthe centre of the card.2 Without the card, focus onan object at the front of theroom and hold the pin infront of you at arm’s length.3 Still focusing on thedistant object, bring the pincloser and closer until it goesout of focus.4 Note how far away the pinis when this happens.5 Now focus on the samedistant object as before butview it through the hole in thecard.6 Move the pin closer andcloser as before, noting thedistance at which it becomesout of focus.

PHYSICS ON STAGE 3

light 43How the eye works, continued...

PHYSICS ON STAGE 3

Aqueous humour: isthe eye full of fluid?

So what happened?You will see “floaters” –strands and particles movingaround in the fluid of theaqueous humour. This fluidplays an important role inmaintaining the shape of theeyeball, which is essential ifthe image is to be focusedprecisely by the lens onto theretina.

The pressure of the fluid isaccurately maintained by aduct, but it is important inmiddle age to get thispressure checked to screenfor a condition calledglaucoma, which can dopermanent damage to sight ifleft untreated.

Follow these steps1 Illuminate the screen withthe overhead projector.2 Use the pin to make asmall hole in the centre ofthe card.3 Hold the card in front ofone eye and look through thehole at the brightly lit screen.4 Keep looking for at least1 min.5 Describe what you see.

Lens: what kind is it?

So what happened?The shadow of the pin isinverted. This can only beexplained if the lens in theeye is a convex lens and thepin is blocking some of therays of light entering the eye(see figure). Images on theretina are normally invertedand the brain turns themupright. The shadow of thepin is really the same way upas the pin, so it looksinverted.

Follow these steps1 Look through the pinholein the card at the brightly litscreen in front of theoverhead projector.2 Hold the pin between oneeye and the card.3 Move the pin around untilyou can see the shadow(image) of the pin.4 Move it up and downslightly and record what younotice.

44 lightHow the eye works, continued...

Retina: how far apart are the rods and cones?

So what happened?You should calculate thedistance between the light-sensitive cells to beapproximately 8–10 µm.

This simple activityinvolving the whole class hasthe added bonus of showingthat geometry does actuallyhave its uses.

What next?If you can get hold of anoptician’s Snellan chart, thestudents can measure theirown visual acuity, which theyalways enjoy.

Follow these steps1 Place a sheet with twoparallel black lines 2 mmapart at the front of the room2 Ask who can distinguishthe two separate lines.3 Measure the distance fromthe sheet to the fartheststudents who can distinguishthe two lines.4 Take eyeball length to beapproximately 20 mm anduse geometry (see diagramand formula) to determinethe distance between thelight-sensitive cells.

L = lD d

where l = 20 mm, D = 2 mmand L = distance measured.

PHYSICS ON STAGE 3

So what happened?The retina is made of livingcells, so it has blood vesselsto supply these cells with thefood and oxygen that theyneed to survive. Normallythe image of these bloodvessels is there all of thetime and the brain ignores it,but in this experiment themessages are so confusedthat this does not occur andyou can see them.

Follow these steps1 Hold the card in front ofone eye with the other eyeclosed.2 Look through the hole atthe brightly lit screen.3 Vibrate the card quicklywhile still looking through thehole.4 Note what you see.

Retina: where arethe blood vessels?

light 45How the eye works, continued...

Rods: how do they work?

So what happened?You will clearly see an after-image of Jesus Christ.

The rods – the mostnumerous of the light-sensitive cells in the retina –contain a pigment,rhodopsin, which is sensitiveto light. When the pigment isexposed to light it breaksdown, causing an impulse topass along the optic nerve tothe brain.

When you stare at theoriginal image, the brightparts cause the rhodopsin inthe corresponding cones inyour retina to break downand send messages to thebrain. The dark parts of theimage emit no light, so thecorresponding rods send no

messages.When the image is

removed, the rods that werereceiving no light(corresponding to the darkareas of the original image)start receiving light, causingrhodopsin to break down andmessages to be sent to thebrain, so these areas lookbright.

However, the rhodopsin inthe rods that were originallyreceiving light has beenbroken down and has to bereformed before it can bebroken down again andmessages can be sent to thebrain. When the image isremoved, these areas thusappear darker. The result isan after-image of the original.

Follow these steps1 View the Internet image onthe screen for 1 min (seewww.eyetricks.com/jesus.htm).2 Remove the image andstare at the white screen.3 Note what you see (tryblinking).

PHYSICS ON STAGE 3

46 light

Cones: how do they work?

So what happened?Most people see red where itwas green and green where itwas red.

There are three types ofcone in the retina – sensitiveto each of the three primarycolours. After staring at redfor a while, the red-sensitivecones are less sensitivebecause some of thepigments in them have beenbroken down, so when thecolour is removed and theretina is bombarded withwhite light (containing allthree primary colours), themessages sent from thegreen- and blue-sensitive

cones are stronger for awhile. Similarly, after staringat green, it is the red andblue cones that send mostmessages.

Some people claim insteadthat they see thecomplementary colours (i.e.cyan where there was redand magenta where therewas green), but they mayhave been physics teachersused to looking at theirresults with the eye of faith.There is also likely to besome individual variation,depending on which part ofthe retina the image wasfocused on.

Follow these steps1 Project two small circles ofred and green filter placedside by side.2 View the screen for 1 min.3 Remove the colours.4 Record what you see.Note: If filters are notavailable, this also workswith squares of vividlycoloured paper held in stronglight for 30–60 s.

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How the eye works, continued...

light 47How the eye works, continued...

The brain: what is its role in vision?

So what happened?It appears as if you arerotating one match throughthe other even when youwatch quite closely. Thewound-up elastic in factrotates the match back to its

original position each time itis let go, but this movementis too quick for the eye torespond to, so the brainmakes the logicalinterpretation that the matchis being rotated in a circle.

Follow these steps1 Place a rubber band overthe thumb and first finger ofone hand in a “C” shape.2 Place a match betweenthe strands of the band andtwist it round many times.3 Place a second matchbetween the thumb andforefinger so that it holds thefirst match in place andprevents it from turning.4 Half rotate the first matchand then let go.

Fovea: what is it for?

Follow these steps1 Repeat the experiment onp42 but look more carefully.2 Look for a clear area in thecentre of the retina with noblood vessels – the fovea.3 View the letters projectedonto the screen from an OHP.4 Stare at the centre letter.4 Note which letters eitherside of the centre letter is outof focus.

E M C L I A G D F H J K B N F X R

So what happened?The fovea is at the centre ofthe retina where there is thehighest concentration of light-sensitive cells, especially thecones for colour vision. Thisis why the best vision is at

the centre of the field of view(the central letter) whilevision deteriorates fartherfrom the centre as the imagelies farther and farther fromthe fovea.

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48 electricity & magnetismThe electrostatic shoelaceThe Czech RepublicAn everyday example of electrostatics

PHYSICS ON STAGE 3

You will need... a synthetic shoelace

BackgroundElectrostatic charges can beproduced by friction. Likecharges repel.

Follow these stepsStroke a shoelace betweenyour thumb and forefingerseveral times, then hold it afew centimetres from theend.

So what happened?The shoelace becomeselectrostatically charged andstands upright. Several typesof shoelace may need to betested to find one that workswell.

What next?Try charging differentmaterials.

Like charges repel whereas unlike charges attract

electricity & magnetism 49Repelling strawsSpain

You will need... a wooden frame (about

20 ×30 cm) six screws some fishing line (1 m) three plastic straws different cloths (e.g.

silk, fur, wool)

BackgroundThere are two kinds ofcharge: positive andnegative. Forces of repulsionand attraction appear withlike and unlike charges.Charging by friction is usefulfor charging insulators:rubbing a plastic straw withfur gives the straw a negativecharge; rubbing the strawwith silk gives it a positivecharge.

Follow these steps1 Fix the screws near thelong sides of the frame (seepicture). 2 Attach the fishing linebetween the pairs of screwsto make taut supports for thestraws (preferably use three).3 Charge the straws byfriction, rubbing one with furand one with silk.4 Rest one of the strawsacross the wires.5 Bring the second strawclose to the first one and seewhat happens.6 Charge a third straw byrubbing it with silk and placeit between the other two.7 Note what happens to thefirst two straws.

So what happened?Rubbing one straw with furand one with silk means thatthey have opposite chargesso they attract each other.

Charging two straws withthe same material meansthat they will have likecharges and will thereforerepel each other.

What next?Try using different colouredstraws to distinguish betweenpositive and negativecharges. Also try differenttypes of material and thendetermine what type ofcharge is on each of thestraws.

This exercise can also beextended to show chargingby induction.

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50 electricity & magnetism

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Seeing alternating current in a bulbBelgiumMains current is alternating

You will need... a clear light bulb a powerful magnet a mains source

BackgroundMains supply electricity is analternating current with afrequency of 50 Hz.

Follow these steps1 Connect the light bulb tothe mains supply.2 Bring the magnet close tothe glowing bulb.

So what happened?The filament vibrates whenthe magnetic field is present.

In the presence of themagnet, the filament of thebulb acts as a conductor in amagnetic field and thereforeexperiences a force.

The alternating nature ofthe current produces avarying force, resulting in thevibration of the filament.

The effect is best seen in abulb that has a long filament,which can be found in certainornamental bulbs. Anordinary candle bulb can alsobe used.

What next?You can use a convex lens toproject a magnified image ofthe filament onto a suitablescreen or wall, therebymaking the vibrations easierto see.

electricity & magnetism 51

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Making a simple induction motorHungaryHow does an induction motor work?

You will need... wire coiled round an

iron core (12,000 turns;use a demountabletransformer kit)

an aluminium can a 1 mm thick

aluminium ring a 6–12 V AC power

supply a pencil a piece of BluTack

BackgroundThis is a simple inductionmotor that can be made ifyou have a demountabletransformer kit.

Follow these steps1 Mount the coil on one armof the core.2 Remove the top of the can.3 Position the pencil pointingupwards with the BluTack.4 Place the inverted can overthe pencil so that it balanceson the pencil tip and canrotate freely.5 Position the can so thatone end of the transformercore faces it.6 Shield half of the core thatis facing the can with thealuminium ring.7 Connect the coil to thepower supply and see whathappens.

So what happened?The can rotates owing toelectric currents in the caninduced by the magnet.

What next?For an explanation of theprinciple behind this type ofinduction motor, see any texton shaded-pole inductionmotors. These motors arecommon – they are used todrive washing-machinepumps and the cooling fan ofoverhead projectors.

52 electricity & magnetismThe Darlington trioThe Czech RepublicA simple circuit demonstrates several electrical phenomena

You will need... three transistors

(BC337 or BC547) resistors (R1 = l0 MΩ,

R2 = l MΩ andR3 = 15 kΩ, but theirvalues aren’t critical)

a.35 V/O.2 A light bulb a 4.5 V battery a wooden skewer

BackgroundThe configuration of thecircuit is known as aDarlington trio.

Follow these steps1 Set up the circuit shown inthe diagram.2 Place the leads of A and Bacross the wooden skewer.3 Hold lead A (let lead Bhang in air) then rub yourfoot on the ground.4 Move a charged objectclose to lead A.

So what happened?The amplified output of thefirst transistor is fed onto thebase of the secondtransistor, so there is acascade effect. The circuitcan detect currents in theorder of nanoamperes.

When the leads are placedon the wood, the bulb lightsup, showing that wood is nota perfect insulator.

When you hold lead A, letlead B dangle and rub yourfoot on the ground, the bulblights because the circuit isable to detect the staticelectricity generated.

When you move a chargedobject close to lead A, thebulb lights only when thecharge moves.

What next?Sockets can be added to thecircuit so that it can beconnected to a digitalvoltmeter and a cathode-rayoscilloscope to measurecurrents and view signals.

PHYSICS ON STAGE 3

A

B T1

T2

T3

R2

R1

R3

electricity & magnetism 53

PHYSICS ON STAGE 3

Making a simple electric motorSlovakiaHow does an electric motor work?

You will need... some lacquered copper

wire two bicycle/head torch

batteries (4.5 V) a strong magnet some sandpaper a piece of cable

BackgroundUsing a few inexpensivecomponents, it is easy toconstruct a simple electricmotor.

Follow these steps1 Using the lacqueredcopper wire, make a coil ofabout 10 turns (the ends ofthe wire will form the axles ofthe coil).2 Remove all of the lacquercoating from one of the axlesusing the sandpaper.3 Remove the lacquer fromonly one side of the otheraxle.4 Bend the battery terminals

so that they can support theends of the coil.5 Place the coil so that oneaxle sits in the positiveterminal of one battery andthe other sits in the negativeterminal of the other battery.6 Complete the circuit byconnecting the two batterieswith a piece of cable.7 Place the magnet near the coil and give the coil a singleturn by hand.

So what happened?The coil receives torque foronly half of each rotation andits angular momentum isenough to keep it turning.

54 miscellaneousBottle raceBelgiumThe effect of a vortex in a liquid

You will need: two 250 ml drinks

bottles some parafilm or duct

tape some water

BackgroundSee what happens to waterwhen you empty the bath.

Follow these steps1 Pour water into one bottleso that it is three-quarters full.2 Tape the two bottlestogether (see picture).3 Ask the students to timehow long it takes the water toflow from one bottle to theother when the bottles areinverted.4 Repeat the experiment butthis time give the water someangular velocity by movingthe bottles in a circularmotion before inverting them.5 Again, get the students totime how long it takes for thewater to run through to thesecond bottle.

So what happened?The water is transferred fasterfrom one bottle to the otherafter you generate someangular velocity.

The angular momentum ofthe water creates a vortex atthe neck of the bottles, thus

allowing air to move up easilythrough the centre of thevortex and the water to movedown the sides into theempty bottle. The lesschaotic motion at the neck ofthe bottles therefore allowsthe water to flow faster.

What next?If you make two sets of thisapparatus you can have arace. Let the students figureout how to get the water totransfer faster.PHYSICS ON STAGE 3

miscellaneous 55Broken hardware experimentsThe Czech RepublicMaking useful physics demonstrations from discarded equipment

You will need: the remains of two old

CD drives a 6 V bulb some old CDs some CD labels some coloured stickers

(you can use CD labelprinting software tomake these)

2

BackgroundThere are many experimentsthat can be devised using thebits and pieces that can besalvaged from old computersand other hardware. Here weshow three uses for theremains of an old CD drive.

Follow these steps1 Remove the gears andmotor that operate a CD drive(pictures 1 and 2).2 Wire the motor up to a 6 Vbulb and add a handle sothat the motor can be turnedeasily.3 Turn the handle and notewhat happens.4 Next, connect together themotors of two CD drive trays(picture 3).5 Open one drive manuallyand see what happens to theother.6 Apply different colouredstickers to a CD and place iton the motor that spins a CDwhen it is playing (picture 4).Note that these motors arecomplicated stepper motorsthat require a 6–12 V ACsupply to get them to spin forthis experiment unless theirown electronics are adaptedto operate them.7 See what happens to thecolours when the motor isturned on.

So what happened?When the handle is turnedthe gears turn the motor athigh speed and electricity isgenerated, thereby lightingthe bulb. This simpleapparatus is useful todemonstrate the conversionof mechanical energy toelectrical energy.

Depending on how the twoCD drive motors are wiredtogether, opening one traymanually generates enoughelectricity to open or closethe other. This againdemonstrates the conversionof mechanical energy toelectrical energy.

In the final set-up themotor and CD allow you tomake an electrically drivenNewton’s disk. The coloursshould appear white whenthe motor is turned on.

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1

4

3

56 miscellaneousThe Coriolis effect

BackgroundAll places on the Earth havethe same rotation rate of onerevolution per day. However,the linear speed of a placedepends on its latitude:places on the equator moveat 1670 km/h while thosenear the pole have nearlyzero speeds (see red arrowsin Figure 1).

When an object movesnorth or south and is notfirmly connected to theground (air, artillery shells,etc), it keeps its initialeastward speed as it moves.

If the object is launchednorth from the equator, it willeventually be going fastereast than the groundbeneath it and will seemfrom the ground to be movedeast by a mysterious “force”(see black arrow).

Objects moving towardsthe equator will eventually bemoving slower than theground and will appear todeviate to the west (seeyellow arrow). In reality thereis no force involved – theground is simply moving at adifferent speed than at thestarting point.

Thus air moving towardsthe poles appears to curve to

the east and air movingtowards the equator appearsto curve to the west. Thismeans that, for a low-pressure system in thenorthern hemisphere, motionis inwards andcounterclockwise (figure 2).The Coriolis effect isstrongest at mid-latitudes.

Important for an understanding of the movement of large-scaleweather systems

PHYSICS ON STAGE 3

west east

lowpressure

Figure 1: Apparentdeflection of objects onnortherly trajectories.

Figure 2: Vortex created in alow-pressure system.

What next?In small-scale systems, likewater draining from a sink ora bath, the Coriolis effect isusually overwhelmed by localvariations, such as residualcurrents or irregularities inthe shape of the container. Itis only by very carefulelimination of these factorsthat the Coriolis effect can beobserved in the laboratory.

The rotation of the Earthdoes, however, influence thedirection of rotation of largeweather systems and largevortices in the oceans,because these are very long-lived phenomena and soallow the very weak Coriolisforce to produce a significanteffect with time.

Gustave-Gaspard Coriolisof Paris published hisexplanation of the effect in1835. For more information,see http://www.physics.ohio-state.edu/~dvandom/Edu/newcor.html.

See pvii for a picture of thewinning Coriolisdemonstration from Ireland.

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