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4.2.3Electricity and Magnetism

Electromotive Force AND Potential Difference — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 16 of 65 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

4.2.3 Electromotive Force and Potential Difference — IGCSE PHYSICS notes: Define e.m.f. and p.d., use voltmeters correctly and use E = W/Q and V = W/Q.Electromotive force (e.m.f.) = the electrical work done by a source in moving a unit charge around a complete circuit.Unit — e.m.f.

  • Potential difference (p.d.)
  • Compared with e.m.f
  • Cells in series
  • Cells in parallel
  • Series p.d
  • Unit — e.m.f. is measured in volts (V)

Part 1 of 7Electromotive force

Electromotive force
Electromotive force (e.m.f.)
Electromotive force (e.m.f.) the electrical work done by a source in moving a unit charge around a complete circuit.
Unit
  • Unit e.m.f. is measured in volts (V).
Meaning
  • Meaning a 12 V source gives 12 J of energy to each coulomb of charge that passes through it.

Part 2 of 7Potential difference

Potential difference
Potential difference (p.d.)
Potential difference (p.d.) the work done by a unit charge passing through a component.
Unit
  • Unit p.d. is measured in volts (V), where 1 V = 1 J/C.
Compared with e.m.f.
  • Compared with e.m.f. the e.m.f. is the energy given to each coulomb by the source, and the p.d. is the energy each coulomb transfers in a component.

Part 3 of 7Equations (Extended)

E
W / Q
  • e.m.f.
  • (V) equals energy transferred to the charge (J) divided by charge (C).
V
W / Q
  • p.d.
  • (V) equals energy transferred in the component (J) divided by charge (C).
  • Rearranged, the energy transferred is W = Q V.

Part 4 of 7Measuring p.d. with a voltmeter

Measuring p.d. with a voltmeter
Voltmeter
  • Voltmeter connected in parallel (across) the component or source, because p.d. is measured between two points.
Types
  • Types voltmeters can be analogue or digital; choose the correct range and check for zero error.
1
Build the circuit without the voltmeter first.
2
Connect the voltmeter in parallel across the component.
3
Connect the + terminal of the voltmeter to the side nearest the + of the supply.
4
Select a suitable range, switch on and record the reading.

Part 5 of 7P.d. in series and parallel circuits

CircuitRuleDetail
Seriesp.d.s add up to the supply e.m.f.The p.d. is shared between components, as energy is conserved.
Parallelp.d. across each branch equals the supply p.d.Each branch has the same p.d.

Part 6 of 7Combining cells

Cells in series
  • Cells in series in the same direction their e.m.f.s add.
Cells in parallel
  • Cells in parallel the e.m.f. stays that of one cell, but the cells last longer.

Part 7 of 7More from the specification

1Electromotive force
  • The energy transferred per unit charge by a source such as a cell when it drives charge round a complete circuit, measured in volts.
2Potential difference
  • The energy transferred per unit charge between two points in a circuit as charge passes through a component, V = W/Q.
3Current and resistance
  • Current is the rate of flow of charge, I = Q/t,
  • and resistance is R = V/I,
  • so a larger resistance gives a smaller current for the same voltage.
4Electrical power
  • P = IV = I2R, giving the rate of energy transfer in a component.
5Static charge
  • Rubbing a polythene rod gives it a negative charge by gaining electrons,
  • and an electroscope shows charge as its leaf is repelled;
    • conductors allow charge to flow while insulators do not.
6Insulator
  • Material that does not allow current to flow.
7Electric field
  • Region of space in which a charge experiences a force.
8Alternating current
  • Current that continually reverses direction.
9Field lines
  • Lines showing the direction and strength of a magnetic field.
10Gold-leaf electroscope
  • A device in which a charged gold leaf falls when photoelectrons discharge the plate.
11Mains supply
  • Domestic a.c. supply, rated 230 V rms at 50 Hz in the UK.
12Electrical power
  • Rate of energy transfer, P = I V.
    What the symbols mean: P = power (W) · I = current (A) · V = potential difference (V)
13Current
  • Flow of electrons in a circuit.
14Resistance
  • Opposition to current, R = V / I, in ohms (Ω).
15Electroscope
  • Instrument with a metal cap, stem and gold leaf used to detect charge.
16Conductor
  • Material that allows current to flow.

Key terms

TermMeaning
Electromotive force (e.m.f.)Electrical work done by a source in moving a unit charge around a complete circuit.
Potential difference (p.d.)Work done by a unit charge passing through a component.
Volt (V)One joule per coulomb.
VoltmeterInstrument that measures p.d., connected in parallel.
Series p.d.The p.d.s add up to the supply e.m.f.

Avoid these mistakes

1
Confusing e.m.f. with p.d. — e.m.f. is the energy given to charge by the source, p.d. is the energy taken from the charge by a component.
2
Connecting a voltmeter in series — it must be in parallel.
3
Saying e.m.f. is a force — it is energy per unit charge, measured in volts.
4
Adding the e.m.f.s of cells in parallel — the e.m.f. stays that of one cell.

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