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P4.2.3P4. Electricity and Magnetism

Electromotive force and potential difference — Combined Science 0653 - Physics Revision Notes & Topical Questions

5 min read 7 parts Lesson 5 of 19 1 quiz

📚 Combined Science 0653 - Physics Hub / Topical Revision
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Lesson summary

P4.2.3 Electromotive force and potential difference — COMBINED SCIENCE notes: Define e.m.f. and potential difference, use V = W/Q and measure potential difference with a voltmeter.E.m.f.

  • Across components
  • In the cell
  • In the components
  • Comparison
  • Analogue voltmeter
  • Digital voltmeter

Part 1 of 5Electromotive force (e.m.f.)

E.m.f.
E.m.f. the electrical work done by a source in moving a unit charge around a complete circuit.
Unit
  • Unit the volt (V).
Meaning
  • Meaning it is the potential difference of the power source, for example 12 V for a 12 V supply.
Equation
  • E = W ÷ Q, with E in V, W in J and Q in C.

Part 2 of 5Potential difference (p.d.)

Potential difference (p.d.)
Potential difference
Potential difference the work done by a unit charge passing through a component, which is the energy transferred per unit charge.
Unit
  • Unit the volt (V), where 1 V = 1 J/C.
Equation
  • V = W ÷ Q, with V in V, W in J and Q in C.
Across components
  • Across components the p.d. is the difference in electrical potential between two points, for example 5 V across a bulb and 7 V across a resistor.

Part 3 of 5Energy and charge in a circuit

Energy and charge in a circuit
In the cell
  • In the cell each coulomb of charge gains energy, for example 12 J per coulomb in a 12 V cell.
In the components
  • In the components each coulomb transfers energy to the components and wires as it flows round the circuit.
Comparison
  • Comparison e.m.f. is the energy given to each coulomb by the source and p.d. is the energy transferred from each coulomb by a component.

Part 4 of 5Measuring potential difference

Voltmeter
  • Voltmeter it measures p.d. and is connected in parallel across the component.
  • It must connect to two points because p.d. is a difference between two points.
Analogue voltmeter
  • Analogue voltmeter it has a needle and scale, with typical ranges of 0 to 1 V and 0 to 5 V.
  • View the scale at 90° to avoid parallax error.
  • Check for a zero error and subtract it from every reading.
Digital voltmeter
  • Digital voltmeter it shows digits, reads small values in mV or μV and is more precise.
  • The display may flicker between values, so decide which to record.

Part 5 of 5More from the specification

1
Electrostatic conduction in a metal
  • Electrostatic conduction in a metal delocalised valence electrons drift freely through a solid metal lattice when subjected to an external electric field.
2
Electromotive force
  • Electromotive force the energy supplied per unit charge by a source such as a cell,
  • while potential difference is the energy transferred per unit charge across a component.
3
Resistance
  • Resistance the opposition to current, R = V divided by I;
    • conductors have low resistance and insulators very high resistance.
4
Electric charge
  • Electric charge rubbing a polythene rod with cloth transfers electrons, leaving objects with opposite positive and negative charge.
5
Electroscope
  • Electroscope a gold-leaf device whose leaves repel when charged, showing the presence and sign of charge.
6
Electric field
  • Electric field the region where a charged object feels a force, with field lines pointing away from positive and towards negative plates.
7
Terminal
  • A connection point of a cell, marked + or -.
8
Voltage
  • A measure of the electrical energy supplied or changed in a circuit.
9
Insulator
  • A material that does not conduct heat or electricity.
10
Electric field
  • A region where a charge experiences a force.
11
Alternating current
  • A current that continuously changes direction.
12
Field line
  • A line showing the direction of the force on a north pole.
13
Direct current
  • A current that flows in one direction only.
14
Gold-leaf electroscope
  • A device that detects charge by the rising of a gold leaf.
15
Current
  • Flow of electrons in a circuit.
16
Resistance
  • How difficult it is for current to flow through a material.
17
Polythene
  • Material that becomes negative when rubbed with cloth.
18
Electroscope
  • Device for detecting charge using a gold leaf.
19
Conductor
  • Material that lets current flow.
20
Acetate
  • Material that becomes positive when rubbed with cloth.

Key terms

TermMeaning
Electromotive forceThe electrical work done by a source in moving a unit charge around a complete circuit.
Potential differenceThe work done by a unit charge passing through a component.
VoltThe unit of e.m.f. and p.d., equal to one joule per coulomb.
VoltmeterAn instrument connected in parallel that measures potential difference.
ChargeA quantity measured in coulombs that is moved round a circuit.

Avoid these mistakes

1
Connecting a voltmeter in series — it must be in parallel.
2
Saying e.m.f. and p.d. are the same thing — e.m.f. is the energy given by a source and p.d. is energy transferred by a component.
3
Saying a volt is a joule — one volt is one joule per coulomb.
4
Using kilojoules or millicoulombs without converting before using V = W ÷ Q.

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