Current ElectricityCurrent Current ElectricityElectricity

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Prepared in Dec 1998Second editing in March 2000

At the end of this unit you should be able to :

1. show understanding that potential difference across a circuit component is measured in volt.

2. use the concept of e.m.f. is measured by energy dissipated by a source in driving a charge round a complete circuit.

3. show appreciation that volt is given by J/C.4. use the equation V = E/Q.5. use and describe the use of a voltmeter with

different ranges.

Learning objectivesLearning objectives

Potential DifferencePotential DifferencePotential Difference• Potential difference also called as voltage or as

p.d.• The voltage (p.d.) across the bulb is the amount

of energy by each coulomb of charge.• Thus

• The term potential difference is used for the voltage between two points in a circuit.

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V = p.d. E = energy Q = Charge

Potential DifferencePotential DifferencePotential Difference• The voltage (p.d.) across the bulb is the

amount of energy by each coulomb of charge.

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Electromotive ForceElectromotive Forcee.m.f.e.m.f.

• A cell (or a battery) pushes electrons round a circuit. It is a kind of electron pump.

• Different cells can exert different electrical pressures.

• For a higher electrical pressure, the electrons will flow out at a faster rate, i.e. a large current.

• This electrical pressure is called electromotive force ( e.m.f.).

• The SI Unit of e.m.f. is volt, V.

Electromotive ForceElectromotive Forcee.m.f.e.m.f.

• Hence,

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e.m.f. = Energy supplied by the cellcharges flow through the cell

ExampleExampleExample

• When 20 C of charge flows through a potential difference of 12 V, how much energy is involved ?

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– since E = VQ– = 12 x 20– = 240 J

– 240 J of energy is involved.

VoltmeterVoltmeterVoltmeter• Voltmeter is used to measure the amount

of voltage across two points in a circuit.• It is always connected in parallel in the

circuit.• Its internal resistance has very high value.

GCE OGCE O--LevelLevelPast Examination PaperPast Examination Paper

Science (Physics)

All rights go to University of Cambridge Examinations Syndicate and other sources

Nov 1995O’ level Physics

15. How could the unit of potential difference, the volt, also be written?

A A/sB C/AC C/JD J/C

DHint:

V = E/Q

GCE O Nov 1997

15. 50 J of energy are dissipated for every 10 C ofcharge passing through a resistance.Which instrument wold allow this to be measured ?

A an ammeter in series with the resistorB an ammeter in parallel with the resistorC a voltmeter in series with the resistorD a voltmeter in parallel with the resistor

DHint:V = E / Q

November 1992

7. The diagram shows acircuit containing two identical lamps and a resistor. Each lamp is marked 1.5 V 0.4 A. This refers to theconditions when the lamps are a normal brightness. The lamps can be operated at the normal brightness by using a 6 V supply and a resistor., R.(a) What is the potential difference between Y and

Z? ______________________________ [1]1.5 V

hint