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Magnetism & Induction
Moving charge makes magnetic fields, and magnetic fields push moving charge. You will apply the right-hand rule, analyze circular motion in a field, use the mass spectrometer and cyclotron, and predict induced emf with Faraday's and Lenz's laws.
Study this chapter in PrepMed (free)
What this chapter covers
- Magnetic Fields & Their Sources. Field direction from the right-hand rule and the 1/r falloff of a wire's field are tested again and again.
- Magnetic Force on Charges & Wires. The right-hand rule for F = qv × B is the most reliable direction question on the MCAT. Master it with signs.
- Circular Motion, Mass Spectrometer & Cyclotron. A single result, r = mv/(qB), drives the mass spectrometer, the cyclotron, and the velocity selector questions.
- Faraday's Law, Lenz's Law & Motional emf. Changing flux induces emf. The MCAT tests the magnitude with clean numbers and the direction with Lenz's law.
- Generators & Transformers. Rotating loops make AC and transformers change AC voltage. Expect ratio questions with turns and power losses.
Key points to know
- Field lines leave N and enter S; no isolated poles
- Straight wire: B = μ₀I/(2πr), direction by the right-hand rule
- F = qvB sinθ; zero when v ∥ B or v = 0
- Right-hand rule: fingers v, curl to B, thumb F (positive charge)
- r = mv/(qB); T = 2πm/(qB) is independent of speed
- Velocity selector passes v = E/B regardless of q and m
- Φ = BA cosθ; ε = −NΔΦ/Δt depends on the rate of change of flux
- Lenz: the induced current opposes the change in flux
Try a practice question
Which statement about the magnetic field lines of a bar magnet is correct?
- They start on the north pole and end on the south pole, never closing
- They leave the south pole and enter the north pole outside the magnet
- They can start and end on isolated poles
- Outside the magnet they run from north to south and they form closed loops
Show the answer
D. Field lines never end. Outside the magnet they run north to south and continue inside from south to north, so each line closes on itself. No monopoles exist to start or end a line.
Common questions
What does this chapter cover?
Magnetism & Induction covers Magnetic Fields & Their Sources, Magnetic Force on Charges & Wires, Circular Motion, Mass Spectrometer & Cyclotron, Faraday's Law, Lenz's Law & Motional emf, Generators & Transformers.
What should I know first?
Field lines leave N and enter S; no isolated poles Straight wire: B = μ₀I/(2πr), direction by the right-hand rule F = qvB sinθ; zero when v ∥ B or v = 0
25 original practice questions accompany this chapter in the app. Spaced review brings missed ideas back at the right time; it helps memory but cannot promise you will not miss an idea again.