Current Current Electricity 2Electricity 2

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Resistance (R)Resistance (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 LawThis 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 I or 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 + 1 R R1 R2 R3

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

Factors affecting Factors affecting Resistance of a Resistance of a

conductorconductorResistance depends on

TemperatureMaterial of conductorLength Cross-sectional area

TemperatureThe resistance of a metallic

conductor increases as the temperature increases e.g. copperThe resistance of a

semiconductor/insulator decreases as the temperature increases e.g. thermistor.

LengthResistance 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 areaResistance of a uniform conductor is inversely proportional to its cross-sectional area.

i.e. R 1 A

Factors affectingFactors affecting Resistance of a Resistance of a

conductorconductor

MaterialThe 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

= Rd 2 (For a wire with circular cross-sectional area) 4L

Wheatstone Wheatstone BridgeBridge

UsesTemperature controlFail-Safe Device (switch circuit off)Measure an unknown resistance

R1 = R3 (When it’s balanced)

R2 R4

Metre Bridge R1 = R2 (|AB|)

|BC| www.dwiarda.com/scientific/Bridge.htmlI

r 1

r2

r 4

r3

AA CC

BB

DD

Effects of an Electric Effects of an Electric

CurrentCurrent 1. Heat2. Chemical3. Magnetic

Joule’s LawStates 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 = V I

Effects of an Electric Effects of an Electric CurrentCurrent

Electrolysis is the chemical effect of an electric current

Voltameter consists of electrodes, an electrolyte and a container

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

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

Effects of an Electric Effects of an Electric CurrentCurrent

Ion is an atom or molecule that has lost or gained 1 or more electronsCharge Carriers in an electrolyte are + and – carriers

Uses Electroplating to make

metal look better, prevent corrosion Purifying metals Making electrolytic

capacitors

Relationship between Relationship between V and I for conductorsV and I for conductors

Metallic conductorNegative 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 V and I for conductorsV and I for conductors

Active electrodesPositive 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 Domestic Electric CircuitsCircuits

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 Domestic Electric CircuitsCircuits

Radial circuit are used for appliances that take a large current. Each closed circuit has its own live wire, neutral wire and fuse e.g. cooker, electric shower

Ring circuit are used for connections to sockets.

Live terminals are connected together as are the neutral terminals

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

Domestic Electric Domestic Electric CircuitsCircuits

Safety in house circuitsSwitch: 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 Domestic Electric CircuitsCircuits

Safety in house circuitsMCBs: 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 (30 mA).

Domestic Electric Domestic Electric CircuitsCircuits

Safety in house circuitsBonding: 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 kWh• This is the amount of

energy used by a 1000 W appliance in one hour

• The ESB charge bills based on the no. of units, kWh, used in the home

CreditsCredits Slide 2:Slide 2: Resistors image

www.sffej.net/educational/resistor_Colour.htmResistor 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