Physics
Magnetism & electromagnetic induction — quick study summary
A moving charge creates a magnetic field; a changing magnetic field induces a voltage in a nearby conductor — Faraday's law. The force on a current-carrying wire in a magnetic field: F = BIL (when perpendicular). Right-hand rule connects current direction, field direction, and force. Lenz's law: induced currents oppose the change that caused them. Together, these laws underpin motors, generators, transformers and wireless charging.
Key points
- F = BIL for a current-carrying wire perpendicular to a field B
- F = qvB for a charge q moving at velocity v through field B
- Faraday's law: EMF = −dΦ/dt (changing flux induces voltage)
- Lenz's law: induced current opposes the change in flux
- Right-hand rule connects current, field, and force directions
Practice quiz
Click each question to reveal the answer.
1. A wire of length 0.5 m carries 4 A perpendicular to a 0.2 T field. What force does it experience?
- 0.1 N
- 0.4 N
- 1 N
- 4 N
Answer: 0.4 N
F = BIL = 0.2 × 4 × 0.5 = 0.4 N.
2. What does Lenz's law tell you about the direction of an induced current?
Answer: It opposes the change in magnetic flux that produced it
This is conservation of energy — if the induced current didn't oppose the change, you could get free energy from a magnet.
3. How does a transformer change voltage?
Answer: Two coils share a magnetic core — voltage scales with the turns ratio: V_s / V_p = N_s / N_p
Changing current in the primary creates changing flux in the core, which induces voltage in the secondary proportional to its turn count.
Last reviewed: May 2026