The Professional Development Service for Teachers is funded by theDepartment of Education and Skills under the National Development Plan

Electricity

M.D.

ElectricityElectricityDid you know?Did you know?

7 percent of power generated at large central stations is lost during transmission to the user over high-voltage lines!

1 lightning bolt has enough power to service 200,000 homes!

An electric eel can produce a voltage of up to 650 Volts!!

20 mA of current running through your body can stop your heart!!

Potential Difference (Potential Difference (V V ))

This is the work done per unit charge to transfer a charge from one point to another. (Also Voltage)

i.e V = W Q

Unit: Volt (V) or JC-1

Volt = The P.d between two points is one volt if one joule of work is done bringing one coulomb from one point to another.

Potential at a point = This is the p.d between a point and the Earth, where the Earth is at zero potential.

Capacitance (Capacitance (C C )) Discovered

independently in 1745 by von Kliest and van Musschenbroek using the Leyden Jar while studying electrostatics.

http://micro.magnet.fsu.edu/electromag/java/lightning/

Capacitance (C)Capacitance (C) This is the ratio of the charge on a conductor to its p.d

i.e. C = QV

Unit: Farad (F) or C V-1

Capacitor: This stores charge

Parallel Plate Capacitor: C = A

dA = area of overlap of platesd = distance between plates = permittivity of dielectric (insulator between

plates)

EnergyEnergy Stored in a Charged CapacitorStored in a Charged Capacitor

To charge a capacitor one plate is connected to + terminal and the other to – terminal and the power supply is turned on.

An equal – charge builds up on one plate and a + charge on the other.

This charge remains even when disconnected from the power supply.

It can be discharged by connecting it to a conductor.

W = ½ CV 2

http://lectureonline.cl.msu.edu/~mmp/kap23/RC/app.htm

http://www.thephysicsteacher.ie/lcphysicscapacitance.html

CapacitorsCapacitors

Allow a.c. to flow but block d.c.

Tune in radio stations (variable capacitor)

Smooth out variations in d.c.

Camera flash Filtering: allow certain

frequencies of an alternating signal to pass but block others

Electric Current Electric Current (I)(I) This is the flow of electric

charge. {In a metal conductor it is

the flow of electrons} Size of current in a

conductor is the amount of charge passing any point of that conductor per second.

I = Qt

Unit: Amp (A) or C s -1

Electric Current SummaryElectric Current Summary

+_e-

e-

Electrons flow from – to + Conventional current flows from + to – i.e. flow of

positive charge. (This the defined direction of an electric current).

d.c. = direct current flows in one direction caused by a power supply.

a.c. = alternating current is when the current reverses direction every so often e.g. mains is 100 times per sec.

Current is the same at every point in a series circuit. Sum of current flowing into a junction equals sum of

current flowing out of junction Ammeter = used to measure current and is always

connected in series in the circuit. {Galvanometer = sensitive

ammeter/microammeter}

Current is the same at every point in a series circuit.

Sum of current flowing into a junction equals sum of current flowing out of junction

Ammeter = used to measure current and is always connected in series in the circuit.

Galvanometer = sensitive ammeter/microammeter

Electric Current SummaryElectric Current Summary

Potential Difference (V)Potential Difference (V)

This can also be said to be the energy lost by 1 coulomb as it moves between 2 points in a circuit.

i.e. V = WQ

Note:Note: W = VQ Divide both sides by t (time)

W = VQ t tP = VI (P = W and I = Q)

t t

Voltage (V)Voltage (V) Voltages in series:

V = V1 + V2 + V3

Voltages in parallel: V1 = V2 = V3

Voltmeter is used to measure voltage and is always connected in parallel with the part of the circuit to be measured.

Voltages in Series and ParallelVoltages in Series and Parallel

Electromotive ForceElectromotive Force (e.m.f.)(e.m.f.)

E.M.F. (E): a voltage applied to a circuit.

Unit: Volt

http://video.google.com/videoplay?docid=-6226504780579469841

Electric cell: device that converts chemical energy into electrical energy and is a source of E.M.F.Sources:

Simple CellPrimary CellSecondary CellThermocoupleMains

Simple CellSimple Cell Copper and zinc plates are

electrodes Dilute sulfuric acid and copper

sulfate is the electrolyte Plates chemically react with

the acid leaving the plates charged

Copper electrode is a positive anode

Zinc electrode is a negative cathode

This simple cell can’t be recharged as the chemicals are used up as a current flows

e.m.f. ≈ 1 V

Primary CellPrimary Cell

This type of cell can’t be recharged.Also known as a dry cell because the

electrolyte is generally a chemical paste.

Secondary CellSecondary Cell

This is a cell that can be recharged.Also known as an accumulator.E.g. Car battery is a lead-acid

accumulator.

Resistance (Resistance (R R )) This is the ratio of the

p.d. across a conductor to the current flowing through it.– i.e. R = V

I Unit: ohm ()

http://micro.magnet.fsu.edu/electromag/java/filamentresistance/

Ohm’s LawOhm’s Law This states that for certain

conductors (mainly metals) the current flowing through them is directly proportional to the p.d. across them at a constant temperature.

– i.e. V = IR

http://micro.magnet.fsu.edu/electromag/java/ohmslaw/

Series Vs ParallelSeries Vs Parallel

+_

BulbBulb

+_

Resistors in Resistors in Series and ParallelSeries and Parallel

In series the total resistance is:

R = R1 + R2 + R3

R1

R2

R3

R1 R2 R3

In parallel the total resistance is:

1 = 1 + 1 + 1R R1 R2 R3

http://lectureonline.cl.msu.edu/~mmp/kap20/RR506a.htm

Factors affecting resistance of a conductorFactors affecting resistance of a conductor

Resistance depends on;– Temperature– Material of conductor– Length – Cross-sectional area

TemperatureThe resistance of a metallic

conductor increases as the temperature increases. e.g. Copper.The resistance of a

semiconductor/insulator decreases as the temperature increases. E.g. Thermistor.

Length:Resistance of a uniform conductor is directly proportional to its length.

i.e. R L

Factors affecting Factors affecting Resistance of a Resistance of a

conductorconductor

Cross-sectional area:Resistance of a uniform conductor is inversely proportional to its cross-sectional area.

i.e. R 1 A

Factors affectingFactors affecting Resistance of a conductor Resistance of a conductor

Material:– The material also affects the resistance of a

conductor by a fixed amount for different materials. This is known as resistivity ().

R = L = constant of proportionality

A Unit: ohm meter (m)

= Rd2 (For a wire with circular cross-sectional

area) 4L

Wheatstone bridgeWheatstone bridgeUses:

– Temperature control– Fail-safe device (automatic

switch circuit off)– Measure an unknown

resistance– R1 = R3 (When it’s

balanced)R2 R4

Metre Bridge: R1 = (|AB|) R2 |BC|

http://www.magnet.fsu.edu/education/tutorials/java/wheatstonebridge/index.htmlhttp://www.electronics2000.co.uk/calc/calcwstn.htm

I

r 1

r2

r 4

r3

AA CC

BB

DD

Potential DividerPotential Divider This is connected directly across

the voltage and divides voltage into the ratio of the resistances.

E.g A rheostat (variable resistor, two fixed resistors.

The greater voltage is across the greater resistor.

The sum of the voltages is the voltage supply.

If one of the resistances is extremely large then the voltage across it is almost the same as the voltage supply.

Effects of an Electric CurrentEffects of an Electric Current 1. Heat2. Chemical3. Magnetic

Joule’s Law:– States that the rate at which heat produced

in a conductor is directly proportional to the square of the current provided its resistance is constant i.e. P = I 2R

In order to prevent power lines from overheating, electricity is transmitted at a very high voltage (EHT: Extra High Tension).

From Joule’s law the larger the current the more heat produced hence a transformer is used to increase voltage and lower current i.e. (P = VI).

Effects of an Electric CurrentEffects of an Electric Current Electrolysis = the chemical effect of an electric

current.

Voltameter = electrodes, electrolyte and container.

Inactive electrodes = electrodes that don’t take part in the chemical reaction e.g. platinum in H2SO4

Active electrodes = electrodes that take part in the chemical reaction e.g. copper in CuSO4

Effects of an Electric CurrentEffects of an Electric Current Ion = an atom or molecule

that has lost or gained 1 or more electrons.

Charge carriers = In an electrolyte the charge carriers are + and – ions carriers.

Uses: Electroplating to make

metal look better, prevent corrosion Purifying metals Making electrolytic

capacitors

Relationship between Relationship between V V and and II for conductorsfor conductors

Metallic conductor:Negative electrons are the charge carriers

I

V

I

V

I

V

Filament bulb: Negative electrons are the charge carriers

Semiconductor: Negative electrons and positive holes are the charge carriers

Relationship between Relationship between VV and and II for conductorsfor conductors

Active electrodes:Positive and negative ions are the charge carriers

I

VI

VI

V

I

V

Inactive(Inert) electrodes: Positive and negative ions are the charge carriers

Gas: Positive and negative ions and electrons are the charge carriers

Vacuum: Electrons are the charge carriers

Domestic electric circuitsDomestic electric circuits

Electricity entering the home is supplied at 230V a.c.

2 wires enter the house from the mains: Live + neutral and pass through the meter box.

These 2 wires pass into a distribution box with fuses.

Domestic Electric CircuitsDomestic Electric CircuitsRadial circuit: for appliances that take a large current. Each circuit has their own live + neutral wires and fuse e.g. cooker, electric shower.

Ring circuit: for connections to sockets. Live terminals are connected together as are the neutral terminals.

Lights: connected in parallel and a number of them are connected to the same fuse.

Domestic Electric CircuitsDomestic Electric Circuits Safety in house circuits:

– Switch: should always be connected in the live wire.

Fuse: piece of wire that will melt when a current of a certain size passes though it. Connected to the live wire.

Domestic Electric CircuitsDomestic Electric Circuits Safety in house circuits

MCBs: miniature circuit breakers are found in the distribution box. They are bimetallic strips(for small currents) and electromagnets (for large currents). Can be reset when the switch trips, faster than fuse.

RCDs: residual current devices protect sockets and people against electrocution by detecting a difference between current in live and neutral wire (30mA).

Domestic Electric CircuitsDomestic Electric Circuits Safety in house circuits:

– Bonding: All metal taps, pipes, water tanks etc are connected to the earth

– Earthing: Earth wire prevents electrocution from touching metal parts of appliances by providing a path of least resistance when faults occur.

E.S.BE.S.B

Kilowatt-hour (kW h)• This is the amount of

energy used by a 1000 W appliance in one hour.

• The E.S.B charge bills based on the no. of units (kW h) used in the home.

CreditsCreditsSlide 2: Slide 2: Lightning Bolt Image

http://www.msha.gov/Accident_Prevention/Tips/lightning.htm

Electric eel image ~ Amy Lebeauwww.nfpa.org/riskwatch/teach_eslp_pkk_04.html

Slide 3:Slide 3: Animation ~ Irina Nelson and Johnny Ericksonwww.slcc.edu/schools/hum_sci/physics/tutor/2220/e_potential

Slide 4:Slide 4: None

Slide 5:Slide 5: First capacitor image

www.mainlinegroup.co.uk/jacksonbrothewww.mainlinegroup.co.uk/jacksonbrothers/5250.htmrs/5250.htm

Slide 6:Slide 6: Capacitor image ~ Christopher Borghttp://qarnita.tripod.com/comp.htm

Slide 7:Slide 7: Bulb and battery animation ~ David Chase Edventures.com

http://discover.edventures.com/functions/termlib.php?action=&termid=153&alpha=c&searchstring

= Electric Motor animation

~ UK Motion Gallerywww.bbc.co.uk/science/robots/techlab/v_rollerbots.shtml

Slide 8:Slide 8: NoneSlide Slide 9: 9: NoneSlide 10:Slide 10:NoneSlide 11:Slide 11: Voltages in series image ~ Andrew Turner

Primary School Sciencewww.primaryschoolscience.com/about/about_assessment.php

Slide 12:Slide 12: Voltages in series and parallel image ~ Graham Knot

http://ourworld.compuserve.com/homepages/g_knott/elect27.htm

Slide 13:Slide 13: Lemon battery image and video link ~ Carol and Wayne Campbell

www.hilaroad.com/camp/projects/lemon/lemon_battery.htmlNote: google video player needs to be

downloaded from the web page to play video clip

Slide 14:Slide 14: NoneSlide 15:Slide 15: Battery image ~ EDF Energy

www.edfenergy.com/powerup/keystage3/in/page2.html

Slide 16:Slide 16: Lead-acid battery image ~ EUROBAT The Association of European Storage Battery Manufacturers.

www.mpoweruk.com/cell_construction.htm

CreditsCredits Slide 2:Slide 2: Resistors image

www.sffej.net/educational/resistor_Colour.htm

Resistor colour codes www.radiodaze.com/rescarbcomp.htm

Slide 3:Slide 3: George Ohm image~ www.past.dk/artefacts/photos/53/photo-1113908435-89551-5995.tkl?o

Slide 4:Slide 4: None (Note: Use P, for previous and N, for next on key board to go back and forth between photos if no remote control available. Both circuits are connected to a 12V power supply and can be compared in terms of how bright the 3 bulbs are)

Slide 5:Slide 5: None Slide 6:Slide 6: Temperature and resistance animation ~

Science Joy Wagon (www.sciencejoywagon.com) www.regentsprep.org/Regents/physics/

phys03/bresit/default.htm Slide 7:Slide 7: Cross sectional area and resistance

animation ~ Science Joy Wagon (www.sciencejoywagon.com)

www.regentsprep.org/Regents/physics/phys03/bresit/default.htm

Slide 8:Slide 8: Resistors image http://homepages.nildram.co.uk/

~vwlowen/radio/alarm/how2.htm Slide Slide 9:9: Sir Charles Wheatstone image ~ from

the BT Connected Earth Collection. See www.connected-earth.com

Slide 10:Slide 10: Sunset Power Lines www.tonyboon.co.uk/imgs/pages/

powerlines.htm Slide 11:Slide 11: Hoffman Voltameter image

www.dalefield.com/earth/hydrogen1.html Slide 12:Slide 12: Electroplating image ~

www.finishing.com/faqs/howworks.html Slide 13:Slide 13: None Slide 14:Slide 14: None Slide 15:Slide 15: Circuit Breaker image ~ Edfenergy

www.edfenergy.com/powerup/keystage3/in/page2.html

Slide 16:Slide 16: Circuit Breaker image ~ Edfenergy as above

Light Circuit image ~ www.buzzybee.org/diy/projects/electrical/lighting/wiring.html

Slide 17:Slide 17: None Slide 18:Slide 18: None Slide 19: Slide 19: None Slide 20: Slide 20: None