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Preliminary Physics Topic 2 Electricity in the HomeCopyright 2005-2009 keep it simple sciencewww.keepitsimplescience.com.au

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1

but first, lets revise...

Preliminary Physics Topic 2

ELECTRICAL ENERGY in the HOME

What is this topic about?To keep it as simple as possible, (K.I.S.S.) this topic involves the study of:1. SOCIETY NEEDS ELECTRICITY

2. ELECTRIC FIELDS & CURRENTS

3. ELECTRICAL CIRCUITS... SERIES & PARALLEL

4. ELECTRICAL POWER & ENERGY

5. MAGNETIC EFFECTS OF ELECTRICITY

6. ELECTRICAL SAFETY in the home

WHAT IS ELECTRICITY?

To answer that, you need to be reminded aboutatoms:

Atoms consist of a cent ral nucleus in which are:

Protons (+ve) & Neutrons (neutral).

In orbit are tiny Electrons(-ve).

Both protons and electrons have a property wecall Electric Charge, which is responsible forall the things we know as Electricity and

Magnetism.Normally, every atom has exactly the samenumber of protons and electrons, and thereforethe same amount of (+ve) and (-ve) electriccharge.

However, itis relatively easyto separateelectronsfromatoms,

STATIC ELECTRICITY(Static = not moving)

If different materials are rubbed together, frictioncan often remove electrons from one and depositthem on the other. The result is that eachsubstance is left with an electric charge.

Chargedobjects canattract orrepel eacheach otherand causeall sorts ofweird thingsto happen.

CURRENT ELECTRICITY(Currentmeans moving or flowing)

Electrons canalso flowthrough

Conductors,such as metal

wires.

e

-

and thats whenelectricity gets

very useful...

electron(-)

NucleuscontainsPROTONS(+)andNEUTRONS(0)

Structure

ofanATOM

-

++ + + + +

Electronsrubbedoffperspexrod.Rodbecomespositively charged.

and then things getinteresting...


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2

History of

EnergySources

ElectricalResistance

Ohms Law

Voltage(Potential

Difference)

Ammeters&

Voltmeters

Series&

Parallel

MagneticFields

Earthing&

DoubleInsulation

Fuses,Circuit

Breakers&

ELDs

Magnetic Fieldaround a wire

Carrying Current

Solenoids&

Electromagnets

ELECTRICALENERGY

in the HOME

SocietyNeeds

Electricity

Electric Fields&

Currents

ElectricalCircuits

ElectricalPower &Energy

ElectricalSafety Magnetic Effects

of

Electric Current

CONCEPT DIAGRAM (Mind Map ) OF TOPICSome students find that memorising the OUTLINE of a topic helps them learn and remember

the concepts and important facts. As you proceed through the topic,come back to this page regularly to see how each bit fi ts the whole.

At the end of the notes you wil l find a blank version of th is Mind Map to pract ise on.

Definition &Measurement

ofElectric Field Electric

Current


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4

Back in the 18th century there was greatdisagreement about what electricity actuallywas, and where it came from.

From 1780 Luigi Galvani carried out a series ofexperiments inwhich freshlydissected frogs legs twitched when touchedby d ifferent metalhooks and wires.Galvani believed thatelectric ity came from thefrog as animal electricity,

a sort of fluid that wasconnected with the lifeforce.

(This scared somepeople who thought Science

was mucking around with the soul . Mary Shelley was

prompted to wri te a warningnovel about a mis-guided

scientist who used electricityto bring l ife to a

spare-parts monster...you know the rest)

Many did not agree with Galvani. AlessandroVolta suggested that the electricity making thefrog muscles jump was produced by chemicalreactions in the metals and fluids present. Hisexperiments of 1794 supported his idea.

The debate raged between the supporters ofeach theory, until 1800 when Volta made hugeamounts (for that time) of electricity from aseries of metal plates with paper soaked in saltwater in between.. and not a frog in sight.

This settled the debate!

Frankie

Volta v Galvani

THE DEBATE ABOUT ELECTRICITY

Power Supply in Remote Places

Despite our societys massive usage ofelectric ity, there are still many remote locations(e.g. in central Australia) where it is impracticalto link small communities to the main powergrid.

These places must use things like: Diesel Generators. A diesel powered enginedrives an electric generator. Solar Cells. A solar cell converts sunlightdirectly to electricity which can be stored inbatteries for night use. Wind turbines generate electricity from thepower of the wind.

Although he turned out to be wrong, Galvani sidea sparked tremendous interest in the study ofelectricity, so he did contribute to scientificprogress.

The basic electricmeter fordetecting andmeasuringelectric ity is calleda galvanometerin his honour.

Voltas Pile wasthe forerunner ofmodern batteries,and for many yearsthe best way forthe scientists to produce electricity in thelaboratory for further study.

In recognition of his great contribution, wename the electrical unit, the volt in honour ofAlessandro Volta.

VoltasPile

Aremotecommunity

equippedwithsolarcellsandwindturbinesforelectricity.

Arguably,thisishowallhomesshouldbepowered,

remoteornot

+

_

Plates ofZinc & Silver,with papersoaked

in brinemakeelectricity...a chemicalbattery

(brine is concentratedsalt water)


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Electrostatic Charges and Fields

Electric charges push or pull each other... thereare forces between them:

SAME CHARGES REPEL

DIFFERENTCHARGES ATTRACT

The forces are best explained by imagining thateach electric charge is surrounded by a FORCE FIELD . Any electric charge that isplaced within the field will experience a force.

By definition, the direction of the force fieldlines is the direction a positive (+ve) chargewould move if placed in the field.

SHAPES OF FIELDSAROUND POINT CHARGES

POSITIVE NEGATIVE

FIELD SHAPEBETWEENTWO OPPOSITECHARGES.(attracting each other)

ELECTRIC FIELD BETWEENTWO IDENTICAL CHARGES.

(repelling each other)

These fields areirregular and the

strength of thefield varies from

place to place.

The only electric field that is quite regular andhas the same strength at each point is the

FIELD BETWEENTWO CHARGED PLATES

5


2. ELECTRIC FIELDS & CURRENTS

MEASUREMENT OF ELECTRIC CHARGE & FIELD

The unit of electric charge is theCoulomb (C).

1 coulomb of charge is a very large

amount, so microcoulombs(C)are commonly used.

1 C = 1 x 10-6 C

(The coulomb is named in honour ofa French scientist.)

TRY THE WORKSHEET QUESTIONS

at the end of th is section

The Electric Field strength is defined andmeasured as the Force per unit of Charge:

Since force is measured in newtons (N), andcharge is in coulombs (C), it follows that theunit of electric field strength is the newtonper coulomb (NC-1)

This means if a charge Q experiences anelectric force F , then there must be an

electric field present, and its strength is F/Q.

ELECTRIC FIELD = FORCE

CHARGEE = F

Q

Electric Field is aVECTOR.

It has a direction aswell as a value.

The direction is theway a +ve charge

would move

Positively (+ve)charged plate

+

Negatively (-ve)charged plate

Uniform FieldBetween Plates


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6

Electric CurrentIf electric charges are located on insulators(e.g. plastic) then the charge cannot move andyou have static electricity. This can attractthings, or repel other same-type charges, suchas when your hair stands on end from static.

If, however, electric charges are located in aconductor (e.g. a metal wire) AND there is anelectric field present, then the charges willFLOW THROUGH THE CONDUCTOR because ofthe force applied to them by the field... this isELECTRIC CURRENT.

Electric current (symbol I) is defined as the rateof flow of charge, and can be measured inCoulombs per second. (C.s-1)

However, we call this unit the Ampere ( Amp

for short, symbol A ) in honour of yet anothergreat scientist .

Direct Current &Al ternating CurrentIf the electric field is constant, then the chargewill flow steadily in one direction. This is calledDIRECT CURRENT (DC). Batteries produce DC.

If the field keeps reversing i ts direction , so doesthe current. The charges will move back-and-forth. This is called ALTERNATING CURRENT(AC). Generators produce AC. Our mainselectricity supply is 50 Hz AC... it moves back-and-forth 50 times per second.

Real & Conventional CurrentIn the mid 19th century, when scientists figuredout that electric current was a flow of electriccharges, the obvious question was is itpositives going this way, or negatives going theother way? Back then they couldnt tell, butrealised that in terms of energy flow it was all

the same anyhow, as long as everyone wasconsistent about it.

So, they decided that current is a stream of (+ve)charges flowing with the electric field directionlines.

They had a 50-50 chance and got it wrong! Laterit was discovered that electric current in a wireis always the flow of (-ve) electrons in theopposite direction.

We still use both descriptions . You must acceptthat:

Conventional current is a flow of(+ve) charge from (+ve) terminal to (-ve)

AND Real current is a flow of (-ve) electrons the

other way.

VoltageSo what makes the charges flow?An electr ic field provides a force that acts oneach charge. (remember E=F/Q ?) This electromot ive force (emf) acts on each charge,

giving it ENERGY (measured in Joules (J)). Voltage is a measure of how much energy isgiven to each unit of charge, so...

1 Volt (V) = 1 Joule (J) of energy per Coulomb (C)

So, a 9 volt battery gives 9 Joules of energy toeach Coulomb of charge. A 12 V battery gives12 J to each coulomb of electrons, and so on.

The bigger the voltage, the more energy isavailable to an electric circui t, and

...the MORE CURRENT FLOWS.



CURRENT = CHARGETIME

I = Qt

1 Amp = 1 coulomb per second

TRY THE WORKSHEETat the end of th is section

+ -Electric Field lines

CONVENTIONAL CURRENTa flow of (+ve) charges along the field lines

-Electric Field lines

REAL CURRENTa flow of (-ve) electrons

up the field lines

+


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7

Prac WorkYou may have carried out a 1st handinvestigation in which you set up a simpleelectric circui t and meaured the current flow (inAmps) at various Vol tage settings.

When your data was graphed, it may havelooked like this:

The straight linegraph shows a directrelationship

between voltage& current.

About 1830, Georg Ohm discov ered th isrelationship and established that the gradient ofthe graph is a constant value for any givenresist or. This value is now called the

RESISTANCE, and may be thought of as avalue for how the current is being retarded as itflows in the circuit.

The relationship is now called Ohms Law

The unit of resistance is called the Ohm . The

symbol used is the greek letter omiga ( ).

Non-Ohmic Resistance

If you tried the same activity using a light bulbas your resistor, the graph will come out ratherdifferently:

The curve indicates that the resistance of thebulb keeps changing and does not have a singlevalue. The bulb does not follow Ohms Law(straight line, single gradient value) and is saidto be NON-OHMIC

Conductors and Insulators

A conducto r can now be understood as asubstance with a very low resistance value, sothat current flows through it easily. An insulator

as a substance with a very high resistance valuewhich impedes current flow.

Generally, metals are good conductors. Silver &gold are excellent conductors, but we mostlyuse copper and aluminium for electrical wiringbecause they are nearly as good as conductors,and a lot cheaper.

Good insulators include glass, plastic andpaper. Although their resistance is very high,its all a matter of Ohms Law. If a large enoughvoltage is applied, even a good insulator can break down and allow current to flow.

Contrary to general belief, water itself is NOT agood conductor... the resistance of pure water i svery high. However, sea water, bath water oreven tap water may have enough dissolvedchemicals in it to increase the conductivity(decrease the resistance) to dangerous levelswhen mains electricity (240 V) is i nvolved.



Transformer-Rectifier unit.(Power pack)

Ammetermeasures

current

Voltmetermeasuresvoltage

Current(A)

V

t

a

V

gradien

t=

V/I

gradient = VOLTAGE = RESISTANCEof graph CURRENT

V = R or V = I RI

Worksheet on Ohms Lawat end of section

Current

V

t

a

Experimental

datapoints

lin

eofb

estfit

Ohms Law

A

V

AC

off

on

DC

Solid-stateresistor,orresistancecoil.


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8

Factors Affecting theResistance of a Wire

LengthEverything else being equal, the LONGERconductor has MORE RESISTANCE.

Cross-sectional AreaThe LARGER the cross-sectional area, theLESS RESISTANCE.

TemperatureGenerally in metals, the HOTTER they get, theMORE RESISTANCE they develop.

Type of MaterialAs al ready mentioned, metals are most ly goodconductors while glass & plastic are poor.

2wires,samethickness

Longerwire=MoreResistance

Shorterwire=LessResistance

2wires,samelength

Thickerwire=LessResistance

Thinnerwire=MoreResistance

PlasticInsulation

Conducting

Wire

Conductingwiresandglassinsulatorsonapowerpole

The voltage or emf produced by a powersource is a measure of how much energy perunit of charge ( J.C-1) is given to the charges bythe electric field.

However, when you measure the voltage across

a resistor you are measuring the ENERGYDIFFERENCE (per charge) from one side of theresistor to the other. So, instead of measuringthe energy gained by the electrons, you aremeasuring the energy LOST by the electrons asthey push through the resistor. (Energy per unitcharge)

You may have measured thi s POTENTIALDIFFERENCE (P.D.) (or Voltage Drop ) acrossdifferent resistors in a circuit similar to thatshown in the diagram.

R=3R=2 R=1

6V 4V2V

12VTotal

V V

V

V

More About Voltage... Potential Difference

You wil l have found that:

the higher the Resistance, the greater the P.D,because more energy is los t by the charges.

the sum of the P.D.s around the circuit is equal

tothe total voltage for the entire circuit.


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For most of human history, energy was providedby burn ing of a)......................... or came from themuscle power of people or b).......................Abou t 3,000BC c)...... ........... .... .. and

......................... power began to be used, but itwas not until the d)............................. Revolution(about 1750AD) that large amounts of energystarted to be used. The burning of e)....................to power f)...................... engines and machinery,resulted in large scale manufacturing industry.Electric ity began to be investigated scientificallyabout g)...................... (year) but it was 100 yearsbefore large scale use of electricity began. Thedemand for electric ity was created by inventionsof new devices such as h)..................................,.............................. and ..........................................

The increased usage of energy has totallychanged the way humans live. Society hasbecome i)....................................... (=most peoplelive in cities)

Real Current actually is a stream ofw).............................. flowing from x)..........................towards y)..............................

Vol tage (or z)....................................... Force (emf))

is a measure of how much aa).......................... is givento each coulomb of charge by the electric field. Theunit is the ab)................................, which is equivalentto the number of ac)............................ of energy perad).................................. o f charge.

If the current through a resistor is measured atvarious voltages, the data forms aae).................................... (shape) graph. Theaf)...................... of the graph is equal to theag)................................ of the resistor. The relationshipis known as ah)........................ Law . Resistance ismeasured in the unit called the ai)...............................

When you measure the voltage across a resistor, you

are really measuring the energy aj)............................by the charges as they push their way through theresistor. The higher the resistance, the greater theak)........................ ......................... (P.D.)

The resistance of a conducting material is affectedby:Length: the longer the wire, theal)................................... the resistance.Cross-sectional Area: the thicker the wire, theam).................................. the resi stance.Temperature: at higher temperatures most metalshave an)................................. resis tance.Type of Material: generally, ao)............................. aregood conductors. Plastic, glass, paper etc are goodap)..............................

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Worksheet 1 Society & ElectricityFill in the blank spaces. Student Name...........................................

Worksheet 2 Electric Fields & CurrentsFill in the blank spaces. Student Name...........................................

People now enjoy much more j)..........................time. Much of this time is spent beingentertained by technology powered byk)............................., such as l)..........................

and .............................. In the home are manylabour-saving devices such asm)................................ and .....................................

Despite our widespread use o f electricity, thereare still remote communities who must usen).............................. or ................................... toget electricity.

Luigi Galvani believed that electrici ty wascreated by o)..............................................., butAlessandro p)............................. showed that itcame from chemical reactions. He invented the q)............................... pile which was a pr imi tiver)......................................

Two electric charges of the same type willa)..................... each other, while b)...................charges will attract. Electric charge is measuredin units called c)........................... The forces

between charges are explained by the conceptof the d)................ ................................ which isthought to surround each charge. Any chargewhich is in this field will experience ae).........................

The direction of the field is defined as thedirection that a f )................... charge would movedue to the field. The strength of the field isdefined as the g)....................... per unit ofh).................... The measurement unit for electricfield is i).............................................

If an electric field acts on charges in a

j).............................., they wi ll flow. This is anelectric k)................................, which is definedas the l)................. of f low of m)....................... Theunit of current is the n).........................., which isequivalent to the number of o)................... ofcharge flowing per p)....................

Direct Current (DC) is when the current flowsq)............................................................................r)............................................... (AC) is when thecharges flow s)....................................................... Conventional Current is imagined to be a flowof t)................. charges from u)........................ to

v)....................................


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Example Problem 1When an electric charge Q = 6.50x10-4 C isplaced in an electric field, it experiences a forceof 8.15x10-2 N. What is the field strength at that

point?Answer: E = F / Q

= 8.15x10-2 / 6.50x10-4

E = 125 NC-1

Example Problem 2What force would be experienced by a charge of4.68x10-6 C, when placed in an electric field withstrength 3.65x103 NC-1?Answer: E = F/Q, so F = E.Q

= 3.65x103 x 4.68x10-6

= 0.017082= 1.71 x 10-2 N

TRY THESE...1. A charge of 45.0 C is placed in an electric fieldand experiences a force of 100 N.What is the field strength?

2. What is the strength of a field if a force of 458N is experienced by a charge of 0.245 C?

TRY THESE...

1. If 25.0 C of charge passed through a 0.500 W

resistor in 10.0 s:a) what current is flowing?b) What is the voltage across the resistor?

2. Across a 20.0 resistor the P.D. is 12.0 V.a) Find the current which would f low.b) How much electric charge would passthrough the resistor in 30.0 seconds?

3. Across a resistor there is a P.D. = 15.0 V, anda current of 3.00 A flows.a) What is the value of the resistance?b) The voltage is reduced to 6.00 V across the

same resistor. What current will f low?c) What voltage would be required to cause acurrent of 4.50 A to flow in this resistor?

4. Find the resistance of a circuit if 9.00 V causesa total of 120 C of charge to pass through in 3.00minutes.

5. What voltage is needed to make a total of360C of charge to pass through a 22.5 resistorin 3.00 minutes.

10



Rememberthatforfullmarksincalculations,youneedtoshow

FORMULA,NUMERICAL SUBSTITUTION,APPROPRIATE PRECISIONandUNITS

Worksheet 3 Practice ProblemsElectric Charges & Fields Student Name...........................................

Worksheet 4 Practice ProblemsOhms Law Student Name...........................................

3. A charge of 1.06x10-6 C is placed in an electricfield. It experiences a force of 25.0N.Calculate the strength of the electric field.

4. What force would be applied to a charge of1.25C if placed in a field with strength 250 NC-1?

5. A charge is placed in an electric field withstrength of 5.00x10-2 NC-1. A force of 8.20x10-3 Nis experienced. What is the value of the charge?

6. Calculate the force that would be felt by a0.0465C charge if placed in a 3.48x102 NC-1 field.

7. a) What is the value of the electric field if a3.00x10-4 C charge experiences a force of0.0255N?

b) What force would be experienced by a 8.22 Ccharge when placed in the field from (a)?

Careful!You must use charge in coulombs, NOT C.

Example Problem 1What current would flow through a 4.0 resistorif the voltage across the resistor is 10 V ?

Answer: V = IR10 = I x 4.0I = 10 / 4.0 = 2.5 A.

Example Problem 2In an electr ic c ircuit, a 5.00 resistor is found tohave 2.50A of current flowing through it.a) What is the potential difference (PD) acrossthe resistor?b) How much electric charge is passing throughthe resistor per second?c) How much charge would pass through in 1.00

hour?

Answera) V = IR

= 2.50 x 5.00= 12.5 V

b) Since 1 Amp = 1 coulomb per second, theremust be 2.50 C of charge per sec. ( 2.50 C.s-1)

c) I = Q / t2.50 = Q / (60x60)

Q = 2.50 x 60 x 60 = 9,000 C= 9.00x103 C


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Multiple Choice

1. In the 17th century debate about the origin ofelectricity, and the meaning of the famous frogs legs experiments:

A. Vol ta thought it came from within the f rog.B. Galvani believed it came from the metals &fluids reacting chemically.

C. Volta believed i t came from the metals &fluids reacting chemically.

D. Galvani & Volta supported the same ideas.

2. The diagram shows 2 electric charges. Thedirection of the electric field at point P would be:A. to the right.B. to the left.C. upwards.D. downwards.

3. An electric charge of Q coulombsexperienced a force of F newtons due to anelectric field with magnitude of E units. Thevalue of Q could be calculated as follows:A. Q = F / E B. Q = F x EC. Q = E / F D. Q = F + E

4. An electric current composed of positivecharges flowing steadily in one direction is:A. real, AC B. conventional, ACC. real, DC D. conventional, DC

5. If 5.00 x 102 C of charge flowed past in 2.50

minutes, then the electric cur rent would be:A. 200 A B. 333 AC. 3.33 A D. 0.3 A

6. The circui t shown was used totest Ohms Law on theresistor R .The results gathered were:

Voltage(V) Current (A)2.0 0.54.0 0.86.0 1.010 1.25

From these results, it would be true tosay that:A. Ohms Law was obeyed.B. R is a non-ohmic resistor.C. R has a resistance value of 5.75 ohmD. the meter readings must have been

inaccurate.

7. Voltage or potential difference measures:A. difference in amount of energy per unit of charge.B. the force pushing a charge through a circu it.C. the force per unit of charge in an electric field.D. the potential energy at a point i n an electric field.

8. Several conduct ing wires made of the samematerial were tested for their electricalresistance. The one with the highest resistancewould most likely be:A. th in, long and high temperature.B. thick, long and high temperature.C. thick, shor t and cool.D. thin, short and cool.

Longer Response QuestionsMark values shown are suggestions only, and are togive you an idea of how detailed an answer isappropriate. Answer on reverse if insuff icient space.

9. (3 marks)Give a brief outline of how the main sources ofdomestic energy have changed over time.

10. (3 marks)Calculate the force on a charge of 3.95x10-3Cplaced in an electric field of magnitude7.55x104NC-1.

11. (6 marks)Using the circui t shown, the voltage and currentreadings were recorded for a variety of powersettings.

Results:Voltage(V) Current (A)

6.5 1.58.2 2.0

10.4 2.412.6 2.8

c) Would you describeR as ohmic or non-ohmic? Explain youranswer.

11



Worksheet 5 Test Questions sections 1 & 2 Student Name...........................................

P- +

AR

V

AR

V

a) Graphthese resultsappropriately.

b) Use thegraph to finda resistance

value for R.


12/28



In your home, electricity is supplied to lightsand appliances by a number of electricalcircui ts. Each circuit usually supplies power toseveral lights or power points. For example alight circuit might have 6 lights connected,

each able to be switched on/off separately.

How are these lights in one circui t connected?

There are 2 basic ways to connect multiplecomponents into a single circuit...in SERIES, or in PARALLEL.

12


3. ELECTRICAL CIRCUITS... SERIES & PARALLEL

Series CircuitsIn a series circuit the components are

connected one after the other, in a singlepathway for the current..

The electricity has no choice. All the currentmust flow in the single path through all thebulbs.

The light bulbs are either all on, or all off.They CANNOT be switched independently.If one bulb burns out the circuit is brokenand they all go out.

You will have done laboratory work to

measure the voltages and currents indifferent parts of a series circuit:

Below is a circuit for measuring voltages andcurrent in different parts of a series circuit.

What you may have found: Current is the same throughout the circuit.

(i.e. in the circuit above IT = I1 = I2 )

Voltages are different across differentresistors., BUT they add up to the total forthe circuit.(i.e. in the circuit above VT = V1 + V2 )

Ohms Law is obeyed for each resistor,AND for the entire circuit.

V

T

T

A

1

A

2

V

1

V

2

R

2

1

VoltmetersmeasureP.D.acrosseachresistor

Voltmeter measurestotal for the circuit

Ammetersmeasurecurrentindifferentpartsofthecircuit

3lightbulbsinSERIES

-ve

DCPowerSource

+ve

Allthecurrentmustflowthroughallthebulbs

AC

off

on

DC


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13

Parallel CircuitsIn a parallelcircuit thecomponentsare arrangedin separatebranchesof the circuit.

At each branch the current dividesand flows through ONE bulb only.

Each bulb can be switched on/offseparately, and if one burns out , theothers continue to work.

You will have done laboratory work to measurevoltages and currents in different parts of aparallel circuit :

What you would have found: Voltages are all the same across each resisto r.

(in the circuit above VT = V1 = V2 = V3)

Currents are different in each branch , butadd up to the total current. ( IT = I1 + I2 + I3 )

Ohms Law is obeyed in each branch , AND

for the entire circuit.

3light

bulbsinPARALLEL

Ammeters & Voltmeters

Ammeters measure the current (flow of charge)and so they must be placed in SERIES with thecomponent you wish to measure current flowthrough. They have very low resistance, toallow current th rough easily.

Voltmeters measure the Potential Differenceacross a component, and must be placed inPARALLEL with it. Voltmeters have extremely

high resistance and must NEVER be placed inseries.

Electrical Circuits in the HomeA typical modern home is wired to contain anumber of separate circuits. Each circuit maycontain many lights or power outlets and all ofthem are wired in parallel.

WHY ALWAYS PARALLEL CIRCUITS?If you have several components on the sameparallel circuit :

they can be switched on/off independently.

if one burns out, the others keep operatingnormally. (In series circuits, it s one off - all off) The total resistance of the parallel circuit isless, and more usable POWER can be deliveredto each light or appliance.

WHY NOT JUST ONE CIRCUIT?If all the lights and appliances in your homewere on just one parallel circuit , and everythingwas switched on at the same time, the currentflow in the main circuit line would be enormous.

This would be very dangerous. High currentlevels can cause wires to get hot, melt their

insulation and perhaps start a f ire.

MainCircuit

Parallelbranch

Ammeterin Series with

bulb

Voltmeterin parallelwith bulb

V

T

A

T

V

1

V

2

A

1

A

2

V

3

A

3

Total CurrentTotal Voltagemeasurement

A

m

m

e

e

s

m

e

u

e

c

u

e

t

h

o

e

h

r

e

s

o

V

t

m

e

e

s

m

e

u

e

P

D

a

o

e

h

r

e

s

o

AC

off

on

DC


14/28

There are 2 different ways to arrange electricalcomponents in a circuit: a).....................................or .....................................

In a series circuit the electricity must flowthrough b)............... the components. Thecurrent is c)......................... in every part of thecircuit. The voltages (P.D.) across each resistormay be d)..............................., but the sum ofindividual voltages is equal to e)...........................for the whole circuit. Ohms Law f)................(is/is not) obeyed for each resistor and forg).....................................................

In a parallel circuit the electricityh)............................. at each branch of thecircuit. The voltage in each branch isi)................................

14



Worksheet 6 Series & Parallel CircuitsFill in the blank spaces. Student Name...........................................

The current in each branch can bej).............................., and wi ll add up equal to thecurrent for k).............................. Ohms Lawl).............. (is/is not) obeyed for each resisto r andfor m)......................................................

Circuits in a home are always wired inn)...........................This allows each light orappliance to be switched on/offo)..............................., and if one bulb burns outthe others on the same circuit willp)...................................... It also allows maximumq).................... to be delivered to each light orappliance without requiring dangerous levels ofr)................................

Am met ers must be connect ed ins)............................., while Voltmeters must be

connected in t).................................

Worksheet 7 Test Questions section 3 Student Name...........................................

Multiple Choice

1. In the series circuitshown, the 3 resistorsall have differentresistance values.

It would be true to say:A. The vol tages V1, V2 & V3 would be all the same.B. The currents through R1, R2 & R3 would add up to

the total current in the circuit.C. The same current flows through each resistor.D. For each resistor, the higher the resistance,

the less current would flow through it.

2. Which statement about electrical meters i s co rrect?

A. Ammeters have low resistance andmust be connected in parallel.

B. Voltmeters have low resistance and

must be connected in series.C. Ammeters have high resistance andmust be connected in series.

D. Voltmeters have high resistance andmust be connected in parallel.

3. Which of the following is NOT a reason for ahousehold circu it containing 6 lights to be connectedin parallel? (as compared to the same lights in series)

A. Each light can be swi tched on/of f i ndependently.B. Parallel will allow less current t o flow, and be safer.C. Total resistance is less, and more power delivered.D. If one light burns out , the others will keep going.


4.(8 marks)In the circuit shown, thebattery provides12.0 V e.m.f.

a) What reading would youexpect on voltmeter V3?

b) Calculate the resistanceof resistance R.

c)Find the current flow in:i) A1 ii) A2 iii) AT

5. (5 marks)Sketch a circui t diagram containing 2 light bulbspowered by a D.C. battery. The lights must beable to be switched on & off independently. Anammeter is positioned to record the total currentof the circuit, and a voltmeter must bepositioned to measure the P.D. for one of thebulbs.

8.00

2.50

0.25 A

AT

VT

V1

A1

A2

A3

V2

V3

R


15/28

TRY THESE:1. In the series circuit shown, the current ismeasured to be 1.25 A.

a) Find the resistances of R1 & R3.b) Find the potential difference across R2.

c) What is the voltage of the power source?d) What would happen in this circuit if R3 burnedout?

2.a) Sketch an electric circuit containing 2 resistors (R1& R2) in parallel and a DC power source. Show theposition of an ammeter ready to measure the currentthrough R1, and a voltmeter to measure potentialdifference across R2.

b) If the voltmeter reads 8.00 V, and R2=6.00W,calculate the current through R2.

c) The ammeter reads 2.50 A. Calculate the resistance

of R1.

d) What is the total current flowing out of the powersource?

3. In this circuit are 3identical light bulbs(A,B & C) and 4 switcheslabelled S1, S2, etc.The battery is 6.0 V.

a) Which switch(es)must be closed tolight up bulb B ONLY?

b) When all bulbs are lit,the current through Cis 2.0 A. Find the resistance of the bulb.

c) What is the total current flow from the batterywhen all the bulbs are lit? (hint: bulbs are identical)

d) Later, with ALL switches closed, bulb A burnsout. What happens to the other bulbs? (On or off?)

e) When A bu rns out, it is found by ammeter thatthe current through C becomes 1.50 A.Calculate the resistance of bulb C.

f) Considering the answer to (b), is bulb C an ohmic or non-ohmic resistor? Explain.

15



V

1

V

2

V

3

R

1

R

3

2

=10.0

6.50V

2.40V

6.0V

A

B

C

S1

S2

S3

S

4

Worksheet 8 Practice ProblemsSeries & Parallel Circuits Student Name...........................................

Examp le Prob lem 1 Ser ies Ci rcui ta) Ammeter A1 reads8.00A.What current flowsthrough A

2and A

3?

b) Find the resistanceof each resistorR1, R2 & R3.

c) Predict the readingon voltmeter VT.

Solution:a) 8.00 A flows through both.(current is the same in every part of a series circu it)

b ) Us ing Ohms Law: V = IRin R1: 12.0=8.00xR1, R1=12.0/8.00 = 1.50

in R2: 4.00=8.00xR2, R2=4.00/8.00 = 0.50 in R3: 16.0=8.00xR3, R3=16.0/8.00 = 2.00

c) VT = V1 + V2 + V3 (in a series ci rcui t, P.D.s= 12.0 + 4.00 + 16.0 add up to the total)= 32.0 V

Examp le Prob lem 2 Paral lel Ci rcui tTotal voltage = 12.0 V.The total cu rrent at

AT= 5.20 A.Ammeter A1 reads0.800A.Resistor R3 has

resistance of 4.50.

a) What would be thereadings on theother 3 voltmeters?b) Find the current at A3c) Find the current at A2d) Find the resistance of

R1 & R2.

Solution:a) 12.0 V.(Voltages are the same in every branch of aparallel circui t)

b) V=IR12.0= A3 x 4.50A3=12.0/4.50 = 2.67 A

c) Since (in any pl lel crt.) AT = A1 + A2 + A35.20 = 0.800 + A2 + 2.67

A2 = 1.73 Ad) V=IRR1: 12.0 = 0.800 x R1

R1=12.0/0.800= 15.0

R2: 12.0 = 1.73 x R2R2=12.0/1.73 = 6.94

8.00 A

12.0 V 4.00 V 16.0 V

VT

R1

R2

R3

AT

A2

A3

V1

V2

V3

R3

=4.50

5.20 A

12.0V

0.800 A

VT

R1

R2

A1

AT

V1

A2

V2

A3

V3


16/28



Electrical Power Power is defined as the rate at whichenergy is transformed.

Mathematically:

The unit of power should (therefore) be

the Joule per sec ( J.s

-1

) but this unitis called a Watt (W), in honour ofJames Watt who engineered steamengines and discovered much about theconcept of power.

It can be shown that, in the case ofelectrical energy:

Electrical EnergyIf you combine the equations P = VI and P = E

tThen, it follows that VI = E

tand therefore,

16


4. ELECTRICAL POWER & ENERGY

Power = Energytime

P = E or E = P.tt

Power = Voltage x Current(Watts) (Volts) (Amps)

P = VI

E = V.I.t

Electrical = Voltage x Current x timeEnergy(Joules) (Volts) (Amps) (sec)

The Kilowatt-Hour(kW.h)

Measuring energy in joules can be quiteinconvenient because 1 joule is a verytiny amount.

For this reason, in everyday life,electrical energy is measured in

kilowatt-hours (kW.h)

An appliance wi th a power rating of1,000 W (=1 kW) if allowed to run for 1hour will consume 1 kW.h of energy.

On a domestic electricity bill, yourhomes electricity consumption ismeasured in kW.h and you pay per kW.hused. Currently youll pay about 20

cents per kW.h.

Typical Power Consumption Values

Appl iance Power Time to use 1 kW.hLight bulb 100 W 10 hoursTV set 400 W 2.5 hoursElect. Heater 2,000 W 30 min.Oven (large) 8,000 W 7.5 min.

This toaster

is rated at800 watts.

How muchenergy is

consumed tomake some

toast?

Time to cook toast = 1.5 min = 90 s.

P = V I = 800 W and E = P.t= 800 x 90= 72,000 J (72 kJ)

(This is about 0.02 kW.h... cost about 0.4 cents)

Example ProblemIn an electric circuit, a 240 V source causes aflow of cur rent of 8.50 A.a) What is the resistance of the circuit?

b) What power does the circuit use?c) How much energy is consumed if this c ircuitis left on for 3.00 minutes?

Solution:a)Ohms Law V= IR

240= 8.50 x RR = 240 / 8.50

= 28.2 b) P= VI

= 240 x 8.50= 2,040 = 2.04x103 W (2.04 kW)

c) E= V.I.t

= 240 x 8.50 x (3 x 60) (time must be in sec.)= 367,200 = 3.67x105 J


17/28


17



An Experiment You May Have Done:

Energy Conversion in an Electric Heating Coil

The equipment set-up for a typicalexperiment is shown:

The heating coil is a resistance wirewhich gets hot when electric ity is forcedthrough it.

The energy change is:ELECTRICITY HEAT

A measured quantity of water is heated

for a measured period of time. Thecurrent and voltage in the circuit arerecorded, as is the temperature changeof the water.

From these measurements the amountof electrical energy used by the electriccircuit can be compared to the amountof heat energy gained by the water.

Typical results:

Electrical data Heat data

Voltage = 12V mass of water=100gCurrent = 2.0A start Temp. water

= 15oCTime circuit end Temp. water

ON= 300 s. = 32oCchange in Temp.

= 17oCHow to analyse these Results:Electrical Energy Used

E = V.I.tE = 12 x 2.0 x 300 = 7,200 J

Heat Energy ProducedHeat = mass of x Temp x 4.2Energy water change

H = 100 x 17 x 4.2= 7,140 J

Therefore, (within experimental error)the electrical energy consumed by theelectric circuit is equal to the amount of

heat energy produced (and absorbed bythe water).

A

V

Wires toPowerPack

Measuredquantity of

water.

Insulatedcontainer

Electricalheating

coil

Thermometermeasures

temp. changeof water

Also need astop-watch tomeasure exact

time ofheating.

Metres measurevoltage & current

Thisisbecauseittakes4.2Joulesofenergytoraisethetemp.of1gram

ofwaterby1oC

Manycommonelectrical

appliancesare used for

heating water.

Energy Usage in a Typical HomeStudy Your Electricity AccountYou should have a good look at the electricityaccount for your home.

If your home has an electric hot water systemand uses electric ity for cook ing and heating/air-con then it probably uses at least 20 kW.h ofenergy per day. If you have gas cooking and/orsolar or gas for hot water, your electrical energyusage may be less than this. Your usage may be

higher if there are more than 4 people.

Save Energy, Save $$,Save the Planet

Every kW.h of energy you use, releases approx.1 kg of CO2 gas from coal burned to generatethe electricity.

If your household (assumed average) reducedelectricity usage by (say) 10%, you would saveabout $150 per year. More importantly, it wouldalso reduce greenhouse gas emissions by over

1 tonne.


18/28

a)............................. is the rate at whichenergy is used or converted. The S.I.uni t of power is the b)............................

Electrical power is equal to thec).......................... multiplied by thed).............................

The amount of energy used in anelectric c ircuit is equal toe)............... x ................... x ....................

5.A 12.0 V car battery is rated at 100 Amp-hoursof energy. This means it is capable of delivering1 Amp of current for 100 hours (or 2A for 50hours, or 4A for 25 hrs, and so on). The batteryis connected to a light bulb with resistance of16.0 ohm.

a) What current will flow?b) For how long can the battery run this lightbulb? (hours)c) What is the power of the bulb?d) How much energy (in joules) will beconsumed in the time calculated in (b)? (hint:time must be in seconds!)e) Therefore, how many joules of energy areequivalent to 1 Amp-hour?

6.An electric toaster is described as 240 V, 1.2 kW

a) What current flows through it, under normaloperation?b) What is the resistance of the toaster?c) How much energy (in joules) is consumed inthe time it takes (1.50 min) to make toast?

7.An electr ic heater is rated as 250V, 1000Wa) What current will the heater draw?b) If this heater was switched on for 1.00 hour,how much energy would it consume,

i) measured in kW.h?ii) measured in Joules?

c) Therefore, how many Joules is equivalent to1 kW.h?

18



HINT: In many of these problems you will need to convert the

time into seconds, before using it in calculation.


FORMULA,NUMERICAL SUBSTITUTION,APPROPRIATE PRECISIONandUNITS

Worksheet 9 Electrical Power & EnergyFill in the blank spaces. Student Name...........................................

1. a) Find the power rating of a 12.0 V circuit,drawing 1.50A of current.

b) How much energy will it consume in 1.00hour? (Remember, time must be in seconds!)

2. A 240 V circui t used 1.65x103 J of energywhen left running for 30.0 minutes.a) What is its power rating?b) What current was flowing?c) How much electric charge (in coulombs)flowed in the 30 minutes?

d) What is the resistance of the circuit?3. In an electric circuit, 7.50 A of current isflowing through a load (an appliance usingpower) with resistance of 16.0 ohm.a) What is the voltage across the load?b) Find the power developed by the load.c) How much energy (in Joules) will be used ifthe appliance is run for 45.0 min?d) If the same appliance (i.e. same resistance)was pluggedinto a 240 V supply, what current would flow?Why might this be dangerous?

4. In an experiment, a 240 V electric kettle wasused to boil 1.00 litre (= 1,000g mass) of water.The water temperature went from 20o to 100oC in3.50 minutes.

a) How much heat energy was added to thewater?(Heat energy = mass water x t emp. change x 4.18)b) Assuming perfect energy conversion, howmuch electrical energy was consumed?c) What power was developed by the kettle?d) What cur rent flowed?e) What was the resistance of the heating

element?

The SI unit of energy is thef)...................... but in everyday l ife this isnot convenient becauseg)...............................................................The unit used instead is theh).........................................

Worksheet 10 Practice ProblemsPower & Energy Student Name...........................................


19/28

Magnetic FieldsJust as every electric charge acts as if it issurrounded by an invisible force field , so toofor magnets.

Magnets have 2 different poles , and can eitherattract or repel each other.

The Earth has a magnetic field, and that is howthe poles of any magnet have come to be called

north & south.

Each magnet can beimagined to be

surrounded by magneticlines of force...

a magnetic field .The magnetic field direction

is defined as thedirection that a small

NORTH pole would moveif placed in the field.

(But you cant ever get

an isolated north pole!They always come in

north-south pairs)

When 2 magnets are brought near each otherthe attraction or repulsion is due to the way theirfields interact:

IT TURNS OUT THAT MAGNETISM IS

CAUSED BY ELECTRIC CURRENTS...

Electric CurrentCreates Magnetic Fields

It is now known that all magnetic fields areproduced by moving electric charges.

In a bar magnet, the charged particles within theatoms move in such a way to produce apermanent magnetic field.

In the Earths liquid-iron core there are electriccurrents flowing and creating the huge magneticfield that causes small magnets to point north-south... the magnetic compass.

You may have used small magnetic compassesto map various magnetic fields, including thefield produced by an electric current flowingalong a straight wire:

To predict the shape of such a field, use the Right-Hand Grip Rule . Pretend you aregripping the wire with your thumb pointing thedirection of the flow ofConventional current(+ve towards -ve).The curling fingersshow thedirection of the field.

To more easily draw and understand diagramsyou must also learn the arrow technique torepresent currents or field-lines that areperpendicular to the page.

Imagine an arrow coming straight out of thepage at you... all you see is its point ( . ). If thearrow is going down into the page, you only seeits feathers ( x ). Use the R.H. Grip Rule on thesediagrams to get the idea.

19


5. MAGNETIC EFFECTS OF ELECTRICITY



FieldsAttracting

FieldsRepelling

ConventionalCurrent

Flow

I

Magnetic Field lines

wire

Iwire

magnetic field into page

magnetic field out of page

N

N

N

N

s

s

s

s

Opposite poles attract

Same poles repel

N

S

magneticfields

aroundwires

wire with currentINTO page

wire with current

OUT OF page

I

RightHand

N S N

S N N

Notice that field lines never cross each other


20/28


20

Solenoids & ElectromagnetsThe magnetic field around a straight wirecarrying current, is quite weak. However, if thewire is wrapped into a helix o r coil , the magneticfield in each loop adds to its neighbours tointensify the field.

The magnetic field of a solenoid is exactly the

same shape as a bar magnet. To determine thepolarity of the solenoid (i.e. which end is northand south) once again use a Right-Hand Rule .

You may have carried out a laboratory exercisein which you made an electromagnet. This issimply a coil w ith a bar of iron in the middle. Theiron intensifies the solenoid field so that evenwith quite low currents (e.g. 2 Amps) themagnetic effect is as strong as a small barmagnet.

The big difference, of course, is that themagnetic field of an electromagnet can beswitched on and off with the electric cur rent.

AS YOU WILL LEARN IN THE HSCCOURSE, ELECTROMAGNETS ARE

THE BASIS OF ELECTRIC MOTORS.

An Appl ication of Electromagnets...MOVING COIL SPEAKERS

Al l the electrical devices in your home that youlisten to (radio, TV, music system, etc) producesounds from a speaker . HOW?

Electromagnets!

The electrical cur rent from the radio/TV tuner ormusic system is modulated according to thesignal involved. This means the currentfluctuates in a way corresponding to the music,or persons voice, or whatever.

Since the current fluctuates, so does themagnetic field of the electromagnet.

Since there is another magnetic field c lose by to

interact with, the electromagnet vibrates backand forth as its field varies, and the attraction /repulsion of the other magnet varies.

The electromagnet is attached to a cone of stiffplastic which also vibrates, sendingcompression waves into the air.

As you wi ll remember from the previous topic,compression waves in air are SOUND WAVES.

An electromagnet has convertedelectrical current into the sounds of a

human voice, music, or whatever

you want to listen to.



SolenoidCoil

Conventionalcurrent flow

+-

Solenoid coil

Electricitysource

switch

ASIMPLEELECTROMAGNET

Circular,permanent

magnet

Electromagnet vibratesas its fluctuating field

interacts withother magnet.

Sound waves

Iron core

N

S

N

I

magneticfield lines

Coiled wire carryingelectric current

Magnetic field of a Solenoid

...then your thumb pointsto the North pole

Conevibrates

Current fluctuatesaccording to signalfrom radio, TV ormusic system.

If you curl your fingers in the samedirection as the flow of

conventional current in the coil...

RightHand


21/28

The Dangers of Electricity

Electrocut ion is very dangerous. Even a smallelectrical current (say, 0.1 Amp) from a voltagesource as lit tle as 50 V can disrupt nerve signalsand send your muscles into spasms. If themuscle involved is your heart, it can go into fibrillation where it quivers uncontrollably anddoes not pump blood properly... a potentiallylethal situation.

Our mains electr ici ty, at 240V, is well able to k ill .

As wel l as that, badly designed or faulty wiringsystems can cause an electrical circuit tooverheat, or create sparks which can start a f ire.Many house fires are started by electrical faults.

Safety DevicesFusesA fuse is merely a short piece of wire with a verylow melting point. If an excess of current flowthrough it , it gets hot, melts and thereby breaksthe circuit.

Fuses are designed to be 5 Amp or 8 Amp or15 Amp, etc, according to the maximumcurrent they will allow through, before they blow . It is vital to replace a burnt-out fuse withthe correct one, to avoid a circuit becomingoverloaded, and creating a fire risk.

Fuses in house circui ts are now old-fashionedand have been replaced by more efficientdevices:-

Circuit Breakers do the same job as afuse, but can be re-set after a circui t overloadcauses them to trip . Therefore, they are muchmore convenient, as well as more efficient andreliable for interrupting a faulty circuit.

Circuit breakers can work in different ways, but

one design invol ves an electromagnet. Ifexcessive current flows, the magnetic fieldbecomes strong enough to attract an ironswitch, which turns the circui t off. Once thefault is fixed, the system can be re-set bypressing a button.

Earth Leakage Devices (ELDs) areelectronic circuit breakers which monitor thecurrent going into, and out of, a circuit. If thecurrent in both directions is the same, no problem.

If there is even slightly less current coming outthan going in, it means some is leaking out ,maybe in the process of electrocuting a person.In this case the ELD shuts the circuit off soquickly that the person at risk is not hurt.

Al though expensive, ELDs save l ives, and theyare now compulsory in all new buildings in mostcities and towns.

EarthingEver wonder why a power point and most plugshave 3 slo ts/pins? Only 2 are needed for the

electric circuit, the 3rd is for the earth wire.

So long as nothing goeswrong, the earth wi recarries no current anddoes nothing.

However, if a loosewire or faultyinsulation allows anappliance to become live with electricity,the current is conductedsafely by the earth wire down

into the ground, rather than through a persontouching the appliance.

This flow of current to Earth will usually burn-out the fuse, or trip the circuit-breaker or ELD,as well.

Double InsulationSo why do some appliances only have 2-pinplugs, with NO earth connection?

If a fault occurred in a small hand-held appliance(e.g. power drill, hair-drier), even with an earth

wire the person holding the appliance would geta shock. So, these appliances are designed sothat the electrical circuits within are shieldedfrom human contact by TWO layers ofinsulation, one being the moulded plastic bodyof the appliance.

Even if something goes wrong inside, thedouble layer of insulation ensures thatelectric ity cannot make contact with the person.

For larger appliances, or those in which normaloperation does not involve human contact,double-insulation is not practical, so the earth-

wire system is used.

21


6. ELECTRICAL SAFETY in the home



Earthslot

Circuit slots

Fuse-Boxinamodernhome.(nofusesatall..)

ELDs&CircuitBreakersThesearethe

re-setswitches


22/28

Magnets (like electric charges) are surroundedby a force a)........................ which has 2 oppos iteb).................. called c).................... and .................

Poles of the same type d)...................................each other, while opposite polese)................................... The direction of themagnetic field is defined as the direction thatf).................................................. if placed in thefield. Magnetic fields are produced byg)............................ that are h).................................

This means that any wire carrying ani)........................ will produce a j)..........................field. The field around a straight wire carryingcur rent is k)........................... (shape) Thedirection of this field can be determined byusing the l)...................................... Grip Rule.

4. ELDs. This stands for l)............................................................................ An ELDmonitors the current going in, andcoming out, of an electric circuit. If thecurrents are the same m)...................

............................... If they are different, itmeans that electricity isn).............................. of the circuit andsomething is wrong. The ELD willinstantly o)..............................................

5. Earthing. All power points, and mostplugs have p)................ (number) slotsor pins. Only q)...................... are neededfor the circuit, the other is ther)........................................... If there is a

fault and an appliance becomess)................................., the current willflow safely to the t)...................................

6. Double Insulat ion is used onu).................................... appliances,such as v).................................. Theappliance has 2 layers ofw)............................................... betweenthe user and the electric circuits inside.Even if a fault occurs, the electricitycannot x)...............................................

22



Worksheet 11 Magnetic EffectsFill in the blank spaces. Student Name...........................................

Worksheet 12 Electrical SafetyFill in the blank spaces. Student Name...........................................

If a wire is wound into a coil, it is called am).......................... If current is passed throughthis, the magnetic field is intensified and has thesame shape as n)........................................ Its

polarity can also be determined by the R.H. Griprule: if the fingers wrap in the direction of theo)................................ current, then the thumbpoints to the p)........................ pole.

If an q)...................... core is placed inside thesolenoid, it becomes an r)....................................These are the basis of many useful devices suchas electr ic s)....................................The Moving Coil Speakers in TVs, radios, etcuse an electromagnet too. The electric signal ist).............................. and this causes the field ofthe electromagnet to u)...........................................

This interacts wi th a circular, permanent magnetto cause a cone to v).................................. andproduce w).................... waves from the speaker.

Electricity is dangerous because it cancause a)....................... in your muscles.If this happens to your b)....................., itcan be c)......................., even at quite lowvoltages (about d).................. volts) andcurrent (about e)............... Amps)

The main electrical safety devices in thehome are:1. Common Sense!

2. Fuses. A fuse is a piece off)...................... with a very lowg)....................................... If excessivecurrent flows in the circuit, the fuse wireh)..................... and thereby

i)......................... ........................................

3. Circuit Breakers. These are electro-mechanical devices that j)..................................... a switch if excessivecurrent flows. They are more convenientthan a fuse becausek)..........................................................

COMPLETED WORKSHEETSBECOME SECTION SUMMARIES


23/28

Multiple Choice1. Which machine delivers the most power?

Energy conversion (J) Time(s)A. 500 10B. 2,000 100C. 10 0.1D. 800 20

2. The unit of electrical energy, the kW.h iscommonly used because:A. one Joule is too small an amount to

be practical.B. the kW.h is more accurate for measuring

energy usage.C. most electrical meters are scaled in kW.h.D. the kW.h is the standard S.I. unit .

3. Which diagram shows correctly the direction

of conventional current in the wire, and themagnetic f ield around it?(in each case the wire is vertically in /out of page)

4. Although only 2 wires are needed for anelectric circuit, most household wiring contains3 wires. The 3rd wire is for:A. extra power to be suppl ied if needed.B. connection to earth in case of a fault.C. a fuse wire, to protect against overload.D. connection to a circuit breaker to

prevent fires.


5. (8 marks)a) What is the resistance of a circuit if 4.20 Aflows across a potential difference of 240 V?

b) If this circuit is left running for 30 s how muchelectric charge flows through it?

5. (cont)c) Calculate the power developed by the c ircuit .

d) In 2 minutes, how much energy would thecircuit consume?

6. (4 marks)How long (time) would i t take for a 240 V toaster,with resistance of 64.0 ohm to consume2.00x104joules of energy?

7. (3 marks)The diagram shows a simple electromagnet inan electric circuit.

a) Mark on the diagram with an arrow, thedirection of flow of conventional current if theswitch was closed.b) Sketch the shape of the magnetic fieldproduced.c) Indicate clearly the polarity of the magneticfield.

8. (3 marks)Outline one application of electromagnets in ahousehold appliance.

9. (2 marks)Explain the purpose & operation of a fuse.

23



Worksheet 13 Test Questions sections 4, 5 & 6 Student Name..................................

A

B

C

D

+ -


24/28




24

CONCEPT DIAGRAM ( Mind Map ) OF TOPICSome students find that memorising the OUTLINE of a topic

helps them learn and remember the concepts and important facts.Practise on this blank version.

ELECTRICALENERGY

in the HOME


25/28


25

Answer Section

Worksheet 1a) wood b) animalsc) wind & water d) Industriale) coal f) steamg) 1780-1800h) light bu lb, telephone & radio

i) urbanized j) leisurek) electricity l) TV, movies, CD music, etcm) washers, microwave oven, dishwasher, etcn) diesel generators or solar cellso) living things p) Voltaq) Voltaic r) battery

Worksheet 2a) repel b) likec) Coulomb (C) d) electric fielde) force f) positiveg) force h) chargei) newton per coulomb (NC-1)

j) conductor k) cur rentl) rate m) chargen) Ampere (Amp) (A) o) cou lombsp) second q) steadily, in one directionr) Alternating Current s) back-and-fortht) positive u) positivev) negative w) electrons (negative)x) negative y) positivez) Electromotive aa) energyab) Volt (V) ac) Joulesad) coulomb ae) straight lineaf) gradient ag) resistanceah) Ohms ai) Ohm (W)aj) lost ak) Potential Differenceal) higher am) lower

an) higher ao) metalsap) insulators

Worksheet 3Electric Charges & Fields

1. E= F/Q = 100/45.0 = 2.22 NC-1

2. E= F/Q = 458/0.245 = 1.87x103 NC-1.

3. E= F/Q = 25.0/ 1.06x10-6 = 2.36x107 NC-1.

4. E = F/Q, so F = E.Q = 250 x 1.25 = 313 N.

5. E=F/Q, so Q=F/E = 8.20x10-3/5.00x10-2

= 0.164 (1.64x10-1) C.

6. F=E.Q = 3.48x102 x 0.0465 = 16.2 N.

7. a) E = F/Q = 0.0255 / 3.00x10-4 = 85.0 NC-1.

b) (8.22 C = 8.22x10-6C)F= E.Q = 85.0 x 8.22x10-6 = 6.99x10-4 N.

Worksheet 4Current & Ohms Law1. a) I= Q/t = 25.0/ 10.0 = 2.50 A.b) V=IR = 2.50 x 0.500 = 1.25 V.

2. a) V=IR, so I= V/R = 12.0/ 20.0 = 0.600 A.b) I = Q/t, so Q = I.t = 0.600 x 30.0 = 18.0 C.

3. a) V=IR, so R= V/I = 15.0/3.00 = 5.00 b) V=IR, so I = V/R = 6.00/5.00 = 1.20 A.c) V=IR = 4.50 x 5.00 = 22.5 V.

4. I=Q/t = 120/(3x60) = 0.667 Athen, V=IR, so R= V/I = 9.00/0.667 = 13.5 .

5. First find current: I=Q/t = 360/(3x60) = 2.00 A.then, V=IR = 2.00 x 22.5 = 45.0.

Worksheet 51. C 2. B 3. A 4. D 5. C 6. B 7. A 8. A

9.Up until about 1750, the sources of domestic powerwere wood burning for heat, animals for carryingthings and human muscle. Later coal replaced woodfor heating. In the late 19th-early 20th centurieselectricity came into general use for li ghting, heating,cooking and began to be used for many labour-saving devices.

10.E = F/Q, so F = E.Q

= 7.55x104 x 3.95x10-3

= 298 N.11.

a) graph (3 marks) mustshow labelled axes, evenscales, accurate plottingof points, line of best fit.(Note (0,0) IS a point onthis graph)

b) must show that youused gradient of graph.(example shown, dottedlines)

gradient = V/I = R

= 9/2.1 = 4.3 (approx)

c) Ohmic, because it has a constant resistanceover the range of voltages tested.i.e. Ohms Law is obeyed.



9

2.1

VOLTAGE(V)

0

2

4

6

8

10

12

0 1.0 2.0 3.0CURRENT (A)


FORMULA,NUMERICAL SUBSTITUTION,

APPROPRIATE PRECISIONandUNITS


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Worksheet 9a) Power b) Watt (W)c) voltage d) currente) voltage x current x timef) Jouleg) the joule is a very small quantityh) kilowatt-hour (kW.h)

Worksheet 101.a) P= VI = 12.0 x 1.50 = 18.0 W.b) E= P.t = 18.0 x (60 x60) = 6.48x104 J.

2.a) P = E/t = 1.65x103/(30x60) = 0.917

= 9.17x10-1 W.b) P=VI, so I=P/V = 0.917/240 = 3.82x10-3A.c) I=Q/t, so Q= I.t = 3.82x10-3 / (30x60)

= 2.12x10-6 C (2.12mC).d) R= V/I = 240/ 3.82x10-3 = 6.28x104

3.

a) V=IR = 7.50 x 16.0 = 120 V.b) P= VI = 120 x 7.50 = 900 W.c) E = P.t = 900 x (45x60) = 2.43x106 J.d) I= V/R. = 240 / 16 = 15.0 AThis is double the original current. Could cause wiresto become hot & s tart a fire, or melt insulation so thatperson is electrocuted.

4.a ) H = m x T x 4.18 = 1000 x 80 x 4.18

= 3.34x105 J.b) Same: 3.34x105 J.c) P= E/t = 3.34x105 / (3.50x60) = 1590 W.

= 1.59 kW.d) P=VI, so I= P/V = 1590/240 = 6.63 A.

e) R=V/I = 240/6.63 = 36.2

5.a) I=V/R = 12.0/16.0 = 0.750 A.b) 100 / 0.75 = 133 hr.c) P=V.I = 12.0 x 0.750 = 9.00 W.d) E = V.I.t = 12x0.75x(133x60x60) = 4.31x106 J.e) 4.31x106 / 100 = 4.31x104 J.

6.a) P=VI, so I=P/V = 1200/240 = 5.00 A.b) R = V/I = 240/ 5.00 = 48.0 c) E = V.I.t = 240 x 5.00 x (1.5x60) = 108,000 J

= 1.08x105 J.

7.a) P=VI, so I=P/V = 1,000/250 = 4.00 Ab) i) 1 kW power, for 1 hour = 1.00 kW.h

ii ) E=V.I.t = 250 x 4.00 x (1x60x60)= 3,600,000 J= 3.60x106 J (3.60 kJ)

c) 1 kW.h = 3.60x106 J



Worksheet 6a) series or parallel b) all of c) the same d) differente) the total f) isg) the entire circuit h) dividesi) the same j) differentk) the entire circuit l) ism) the en ti re c ircui t n ) paral lel

o) independently p) remain onq) power r) currents) series t) parallel

Worksheet 71. C 2. D 3. B

4.a) 12.0 V.(all voltages the same in parallel branches)b) V=IR, so R=V/I = 12.0/0.25 = 48.0.c) i) I= V/R = 12/8 = 1.50 A.

ii ) I=V/R = 12/2.5 = 4.80 A.iii) IT = I1 + I2 + I3 = 1.50+4.80+0.25

= 6.55 A.5.Sketch must show- correct symbols used.- bulbs in parallel.- a switch in each branch.- ammeter in series with battery.- voltmeter in parallel with one bulb

Worksheet 81.Series circui t, so current is the same at every point.a) R = V/IR

1

= 6.50/1.25 = 5.20

R3 = 2.40/1.25 = 1.92 b) V=IR2 = 1.25 x 10.0 = 12.5 V.c) series circuit, soVT = V1 + V2 + V3 = 6.50 +12.5 + 2.40 = 21.4 V.d) Circuit broken. Current flow throughou t circuitwould cease.

2.a) sketch must show resistors

in parallel, ammeter in series wi th R1,and voltmeter in parallel with R2.b) V=IRso I = V/R = 8/6 = 1.33 Ac) Each branch o f parallel circuit hassame voltage, so V= 8.00 V.and V= IR, so R= V/I = 8/2.50 = 3.20

d) IT = I1 + I2 = 2.50 + 1.33 = 3.83 A.

3.a) S1 and S3.b) Rc=V/I = 6.0 / 2.0 = 3.0 ohm.c) Since all bulbs identical, and voltage across eachis the same (parallel circuit) then same current f lowsin each branch.IT = I1 + I2 + I3 = 2 + 2 + 2 = 6.0 A.d) They continue to glow.e) Rc= V/I = 6.0/1.50 = 4.0 f) Non-ohmic, because its resistance has changed.(Ohmic resistors show constant resistance over a

wide range of voltage & current values.)

A

V

A

V

R1

R2


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27

Worksheet 11a) field b) polesc) north & south d) repele) attract f) a north pole would moveg) electric charges h) movingi) electric current j) magnetick) circular l) Right Handm) solenoid n) bar magnet

o) conventional p) northq) iron (steel) r) electromagnets) motors t) modulatedu) fluctuate v) vibratew) sound

Worksheet 12a) spasms b) heartc) fatal d) 50 Ve) 0.1 A f) wireg) melting point h) meltsi ) b reaks the c ircui t. j ) t ri p o r openk) they can be reset by pushing a buttonl) Earth-Leakage Devices

m) nothing happens / all OKn) leaking out o) break the circuitp) 3 q) 2r) earth connection s) livet) earth / ground u) small, hand-heldv) power-drill, hair dryer w) insulationx) get out

Worksheet 131. C 2. A 3. D 4. B

5.a) V=IR, so R= V/I = 240/4.20 = 52.1 b) I=Q/t, so Q=I.t = 4.20 x 30 = 126 C.

c) P=VI = 240 x 4.20 = 1008 W = 1.01x103 W.(or 1.01 kW)

d) E = P.t (or E = V.I.t)= 1.01x103 x (2x60) = 1.21x105 J.

6.First find current flow: I=V/R = 240/64 = 3.75 Athen, E = V.I.t, so t = E/(V.I) = 2x104 /(240 x 3.75)

t = 22.2 s.

7.

8.Speakers in radio/TV or music systems use anelectromagnet attached to the speaker cone. As theelectric current varies with the signal, the strength ofthe magnetic field varies too. There is anothermagnet nearby, so the electromagnet vibrates back-and-forth. This causes the cone to vibrate, and thiscreates the sound waves of the speaker.

9.The purpose of a fuse is to break an electric circuit ifthe current goes above the safe level, due to a fault.

It operates by getting hot & melting if the currentexceeds the design maximum.

Conventionalcurrent

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