Electricity and Magnetism
Cambridge O Level Physics 5054 Topic 4 revision chapter covering simple magnetism and magnetic fields, electrical charge, current, e.m.f. and potential difference, resistance, circuit diagrams, series and parallel circuits, sensors and potential dividers, practical electricity and safety, electromagnetic induction, the a.c. generator, the magnetic effect of a current, the motor effect, the d.c. motor, the transformer and uses of an oscilloscope.Show moreShow less
Core Revision Module
Revision & Practice Book
Interactive revision notes with exam tips and worked examples for this chapter.
Practice & Resources
2 toolsChapter overview
A summary of this Physics chapter — open a section to read it. The full notes, worked examples and practice questions are in the study modules above.
What is Electricity and Magnetism about?
Five quantities do almost all the work in Topic 4. Charge \((Q)\) is the property that is carried. Current \((I)\) is the rate at which charge passes a point. E.m.f. \((E)\) is the energy a source gives to each coulomb. Potential difference \((V)\) is the energy each coulomb delivers to a component. Resistance \((R)\) is the p.d. needed per unit current. Power and energy then follow. Magnetism adds fields and forces, and a changing magnetic flux adds induced e.m.f.
Like poles repel and unlike poles attract; a magnet also attracts an unmagnetised magnetic material because it induces magnetism in it. A magnetic field is a region in which a magnetic pole experiences a force. The field direction at a point is the direction of the force on an N pole placed there, so outside a bar magnet the lines run from N to S. Where the lines are closer together, the field is stronger.
Charge is the property carried; the coulomb is its unit. Current is the rate at which charge passes a point, \(I = Q/t\), measured in amperes where \(1\ \mathrm{A} = 1\ \mathrm{C/s}\). E.m.f. is the work a source does per unit charge; potential difference is the work done per unit charge in a component; both are measured in volts, where \(1\ \mathrm{V} = 1\ \mathrm{J/C}\). Resistance is the p.d. per unit current, \(R = V/I\), measured in ohms.
A circuit obeys two conservation rules. Charge is conserved, which gives the current rules: the current is the same everywhere in a series circuit, and the current into a junction equals the current out of it. Energy is conserved, which gives the p.d. rules: the supply p.d. equals the sum of the p.d.s across series components, and every branch of a parallel arrangement has the same p.d. across it.
Power is the rate of energy transfer, \(P = IV\), in watts. Total energy transferred is \(E = IVt\), in joules. For billing, energy is measured in kilowatt-hours, where one kilowatt-hour is the energy transferred by a \(1\ \mathrm{kW}\) device operating for one hour, equal to \(3.6 \times 10^{6}\ \mathrm{J}\). Safety rests on one idea: a fault must be disconnected quickly, and the disconnection must happen in the live wire.
This section is a two-way street with a strict division of labour. Current in a magnetic field produces a force — that is the motor effect, and its direction comes from Fleming's left-hand rule. Changing magnetic flux produces an e.m.f. — that is induction, and its direction comes from Lenz's law. Motors use the first; generators and transformers use the second. Keeping those two sentences apart prevents most of the errors in Topic 4.
Key ideas to remember
- Memory anchor — N-to-S outside, close means strong, never cross, only repulsion proves a magnet. Soft iron switches off; steel stays on.
- Memory anchor — Charge is carried, current is the rate, e.m.f. gives energy per coulomb, p.d. takes it, resistance is volts per amp. Ammeter in series (low \(R\)); voltmeter in parallel (high \(R\)).
- Memory anchor — Series shares p.d.; parallel shares current. Parallel resistance is always smaller than the smallest branch. In a divider, the bigger resistance takes the bigger share of the p.d.
- Memory anchor — \(P = IV\), \(E = IVt\), \(1\ \mathrm{kWh} = 3.6 \times 10^{6}\ \mathrm{J}\). Live wire holds the switch, the fuse and the breaker. Earth turns a fault into a big current that blows the fuse. Casing: earthed, or double-insulated.
- Memory anchor — Force from current in a field (motor, LEFT hand, split ring). E.m.f. from changing flux (generator and transformer, Lenz's law, slip rings). aNticlockwise face = N pole. Same-direction parallel currents attract.
- Memory anchor — Vertical is volts (Y-gain), horizontal is time (timebase). Count divisions, multiply by the setting. One full cycle is the period; \(f = 1/T\); peak is half of peak-to-peak.
Why Electricity and Magnetism matters
Why it matters: the oscilloscope is the instrument that lets you see the difference between d.c. and a.c., measure a peak voltage, and time an interval far too short for a stopwatch. The internal structure of the instrument is not required by the syllabus — only its use.
Key terms in Electricity and Magnetism
- Electric current
- The charge passing a point in a circuit per unit time, I = Q/t, measured in amperes; one ampere is one coulomb per second.
- Resistance and Ohm's law
- Resistance is the potential difference across a component divided by the current in it, R = V/I, measured in ohms; Ohm's law states that the current through a conductor is directly proportional to the potential difference across it, provided its temperature stays constant.
- Oscilloscope
- An instrument that draws a graph of potential difference against time on a screen ruled into divisions. The Y-gain setting gives the number of volts per vertical division, so a p.d. is the number of vertical divisions multiplied by the Y-gain; the timebase setting gives the number of seconds per horizontal division, so a time interval is the number of horizontal divisions multiplied by the timebase, and the frequency of a repeating trace is one divided by its period.
- Electric field
- A region in which an electric charge experiences a force; the direction of a field line at a point is the direction of the force on a positive charge placed there.
- The transformer equation
- For an ideal transformer the ratio of the primary to secondary potential difference equals the ratio of primary to secondary turns, Vp/Vs = Np/Ns; more turns on the secondary steps the p.d. up, fewer turns steps it down.
- Electrical power and energy
- Electrical power is the energy transferred per unit time in a circuit, P = IV, and the energy transferred is E = IVt; a kilowatt-hour is the energy transferred by a 1 kW device running for 1 hour, equal to 3.6 million joules.
- Electromagnetic induction and Lenz's law
- An e.m.f. is induced whenever the magnetic flux through a conductor or coil changes, for example when a magnet and a coil move relative to one another; Lenz's law states that the current produced by an induced e.m.f. opposes the change producing it.
- E.m.f. and potential difference
- E.m.f. is the electrical work done by a source in moving a unit charge around a complete circuit, while potential difference is the work done by a unit charge passing through a component; both are measured in volts, and one volt is one joule per coulomb.
- Magnetic field
- A region in which a magnetic pole experiences a force; the field direction at a point is the direction of the force on a north pole placed there, and the field is stronger where the field lines are closer together.
- Series and parallel circuit rules
- The two conservation rules that govern every circuit: charge is conserved, so the current is the same everywhere in a series circuit and the current into a junction equals the current out of it; energy is conserved, so the supply p.d. equals the sum of the p.d.s across series components while every parallel branch has the same p.d. across it. Combined resistance is R = R1 + R2 + R3 in series and R = R1R2/(R1 + R2) for two resistors in parallel.
Frequently asked questions
What is the difference between e.m.f. and potential difference?
E.m.f. is the electrical work a source, such as a cell, does in moving each unit of charge around a complete circuit; potential difference is the work done by each unit of charge as it passes through a component. Both are measured in volts, where \(1\ \mathrm{V} = 1\ \mathrm{J/C}\). E.m.f. describes energy supplied by the source; p.d. describes energy transferred in a component.
Why is the total resistance of resistors in parallel always less than the smallest individual resistance?
In parallel, each resistor gives the current an extra separate path, so the combined resistance is always lower than any one branch on its own. For two resistors, \(R = R_1R_2/(R_1+R_2)\), which is always smaller than the smaller of \(R_1\) and \(R_2\). This is the opposite of series, where resistances simply add and the total is always larger than the largest single resistor.
Does conventional current flow in the same direction as the electrons in a wire?
No. Conventional current is defined as running from positive to negative around the circuit, but the free electrons that actually carry the charge in a metal move the opposite way, from negative to positive. Both describe the same flow of charge; conventional current is simply the direction used consistently for circuit diagrams, circuit rules and left-hand-rule problems.
Why must the switch, fuse and circuit breaker be in the live wire, not the neutral wire?
The live wire is the one that is alternately at the dangerous potential. Opening a switch, fuse or circuit breaker in the live wire removes that potential from the appliance. Opening one in the neutral wire would only interrupt the return path, leaving the appliance's internal wiring still live even though the circuit no longer works — exactly the hazard the device was meant to remove.
How do you find the direction of the force on a current-carrying wire in a magnetic field?
This is the motor effect: a current in a magnetic field experiences a force. The relative directions of the force, the field and the current are found using Fleming's left-hand rule. Reversing either the current or the field reverses the direction of the force, which is exactly what the motor-effect experiment demonstrates.
How do you read a voltage and a time interval from an oscilloscope trace?
An oscilloscope draws a graph of potential difference against time on a screen ruled into divisions. The Y-gain gives the number of volts per vertical division, so count the vertical divisions and multiply by the Y-gain to get a p.d. The timebase gives the number of seconds per horizontal division, so count horizontal divisions and multiply by the timebase to get a time interval; the frequency of a repeating trace is one divided by its period.
Why does an induced current oppose the change that produced it?
This is Lenz's law: the current produced by an induced e.m.f. always acts in the direction that opposes the change producing it. An e.m.f. is induced whenever the magnetic flux through a conductor or coil changes, for example when a magnet and a coil move relative to one another. Generators and transformers rely on this changing-flux effect, which is the opposite mechanism to the motor effect.
Syllabus reference and sources
Written against: Cambridge O Level Physics (5054) 2026–2028 Syllabus (Subject Content, Topic 4: Electricity and Magnetism).
Written by: Academiq Edu Instructor Panel
Source documents
All educational content, structured explanations, diagrams, worked examples, and pedagogical materials contained within this chapter revision note are the exclusive intellectual property of Academiq Edu. Unauthorized reproduction, distribution, resale, or extraction of this content without prior written permission is strictly prohibited under international copyright laws. Cambridge Assessment International Education (CAIE) is a registered trademark of Cambridge University Press & Assessment. This revision guide is independently authored by the Academiq Edu Instructor Panel for educational purposes and is not affiliated with or endorsed by Cambridge Assessment International Education.
Every chapter note, MCQ explanation, and structured mark scheme is rigorously vetted by Cambridge curriculum specialists.