Electricity and magnetism (Topic 5 in IB Physics, with HL extensions in Topics 10 and 11) is one of the most concept-dense areas of the course. It's also one where students lose marks most easily — not because the mathematics is hard, but because the underlying ideas aren't fully understood.
This guide covers the key ideas in each sub-topic, the distinctions that matter for the exam, and the typical misconceptions to avoid.
1. Electric Fields and Potential
An electric field exists wherever a charged object would experience a force. The field strength E at a point is defined as the force per unit positive charge placed at that point: E = F/q. For a uniform field (between parallel plates): E = V/d.
Field lines and equipotentials
Electric field lines show the direction of force on a positive test charge — they run from positive to negative charge. Equipotential lines are perpendicular to field lines and connect points at the same electric potential. No work is done moving a charge along an equipotential.
Electric potential
Potential V at a point is the work done per unit charge in bringing a positive test charge from infinity to that point. For a point charge: V = kQ/r. Unlike field strength, potential is a scalar — you can add potentials algebraically.
E is a vector (has direction); V is a scalar (no direction). Both are tested, and many Paper 1 questions hinge on this difference. E = -dV/dr for HL students.
2. Electric Circuits
Circuit analysis is a major component of IB Physics and appears on all three papers. The core tools are Ohm's law, Kirchhoff's laws, and an understanding of series and parallel combinations.
Series and parallel rules
- Series: Same current through all components; voltages add; R_total = R₁ + R₂ + …
- Parallel: Same voltage across all branches; currents add; 1/R_total = 1/R₁ + 1/R₂ + …
Kirchhoff's laws
Kirchhoff's current law (KCL): The sum of currents entering a junction equals the sum leaving — conservation of charge. Kirchhoff's voltage law (KVL): The sum of EMFs around a loop equals the sum of potential drops — conservation of energy.
Internal resistance and EMF
Real batteries have internal resistance r. The terminal voltage is V = ε − Ir, where ε is the EMF and I is the current. This means terminal voltage drops as current increases — important for understanding why a battery "runs down" under heavy load.
3. Magnetic Fields and Forces
A magnetic field B exerts a force on moving charges and on current-carrying conductors.
Force on a moving charge
F = qvB sin θ, where θ is the angle between v and B. Use the right-hand rule (or left-hand rule for negative charges) to find the direction. When v is perpendicular to B, the force is centripetal — giving circular motion. This is how particle accelerators and mass spectrometers work.
Force on a current-carrying conductor
F = BIL sin θ. The direction is given by Fleming's left-hand rule: thumb = force (thrust), index = field, middle = current. Two parallel conductors carrying current in the same direction attract; opposite directions repel.
4. Electromagnetic Induction (HL — Topics 10 and 11)
EM induction is tested almost every year at HL. The key law: a changing magnetic flux through a circuit induces an EMF. Faraday's law: ε = −dΦ/dt, where Φ = BA cos θ is the magnetic flux.
Lenz's law
The induced current always flows in a direction that opposes the change in flux that caused it. This is the minus sign in Faraday's law. Lenz's law is a consequence of energy conservation — if the induced current helped the change, you'd get energy for free.
Transformers
An ideal transformer: Vs/Vp = Ns/Np and Ip/Is = Ns/Np. Power input equals power output in an ideal transformer. Real transformers lose energy through resistance in windings and eddy currents in the core — the IB may ask you to explain these losses.
AC generators and alternating current
A rotating coil in a magnetic field produces a sinusoidal EMF. The peak EMF: ε₀ = NBAω. For AC, the RMS values are used for power calculations: V_rms = V₀/√2 and I_rms = I₀/√2. The power dissipated is P = V_rms × I_rms (for resistors).
Confusing peak and RMS values in power calculations is extremely common. Only use V₀ and I₀ if the question explicitly asks for peak power. For average power, always use RMS values.
How Electricity Topics Are Examined
- Paper 1: Conceptual MCQs — field directions, circuit analysis, Lenz's law.
- Paper 2: Circuit problems with internal resistance, force calculations, induction questions. Show all steps.
- Paper 3 (HL): AC circuits, reactance, and power factor if studied as an option topic.
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