Magnetism: every key term you need (+ practice quiz)
26 flashcard terms for AP Physics 2 Unit 5, written to match the course framework. Read them here, drill them as flashcards, or take the 25-question quiz. Free, no account needed.
∮B·dl = μ₀I_enclosed. Circular path: B = μ₀I/(2πr). Used to find B field from current.
Solenoid
Coiled wire. B inside = μ₀nI where n = turns per unit length. Uniform field inside.
Electromagnetic Induction
Changing B field induces E field. Faraday's law: ε = -N·dΦ/dt where Φ = magnetic flux.
Lenz's Law
Induced current opposes change that caused it. Direction ensures magnetic energy opposes flux change.
Transformer
Primary coil (input) induces secondary coil (output). V_s/V_p = N_s/N_p. Steps voltage up/down without energy loss.
Unit 5 Summary
Magnetic field exerts force on moving charges (Lorentz). Currents create B fields (Ampere). Changing B induces E (Faraday).
Radius of Circular Motion in B
qvB = mv²/r gives r = mv/(qB) = p/(qB). Momentum, not speed, sets the radius; a proton and an alpha particle with equal momentum trace radii in the ratio 2:1 because the alpha's charge is doubled.
Cyclotron Period
T = 2πm/(qB), independent of speed and radius. Faster particles make bigger circles in the same time — the basis of the cyclotron's constant-frequency accelerating voltage.
Velocity Selector
Crossed E and B: undeflected when qE = qvB, so v = E/B. Slower particles curve toward the electric force, faster ones toward the magnetic force. Works for any charge sign or magnitude.
Mass Spectrometer
After a velocity selector, ions curve with r = mv/(qB); heavier isotopes strike farther out. Landing separation ∝ Δm, so measuring positions gives mass ratios.
Magnetic Force Does No Work
F is always perpendicular to v, so kinetic energy of a free charge never changes in a pure magnetic field. Only direction changes; speed is constant.
Helical Motion
Velocity component parallel to B is unaffected while the perpendicular component circles; the path is a helix with pitch = v_∥ T. Explains charged-particle trapping along Earth's field lines (auroras).
Field of a Long Straight Wire
B = μ₀I/(2πr), circles around the wire by the right-hand rule (thumb along I). Two parallel currents in the same direction attract; opposite currents repel.
F/L = μ₀I₁I₂/(2πd). Doubling both currents quadruples the force; doubling separation halves it. Used to define the ampere historically.
Torque on a Current Loop
τ = NIAB sinθ, where θ is the angle between the loop's normal and B. Zero torque when the normal aligns with B (stable) or anti-aligns (unstable); maximum when the loop plane is parallel to B.
Motional EMF
A rod of length L moving at v perpendicular to B develops ε = BLv; charges separate until the electric force balances qvB. In a closed circuit this drives current and the rod feels a retarding force BIL.
Lenz's Law and Energy
Induced current opposes the change in flux; pulling a loop out of a field requires work equal to the heat dissipated. Lenz's law is energy conservation in disguise.
Faraday's Law Quantitatively
ε = −NΔΦ/Δt with Φ = BA cosθ. Flux changes via B, A, or orientation. A coil rotating at constant ω in uniform B gives sinusoidal EMF of amplitude NBAω.
Solenoid Field
B = μ₀nI inside a long solenoid (n = turns per length), nearly uniform, nearly zero outside. Doubling turns per unit length doubles B; total number of turns alone does not matter.
Eddy Currents
Changing flux in a bulk conductor drives circulating currents that heat the metal and oppose the motion — used in magnetic braking; slots in transformer cores minimize them.
Right-Hand Rule for Negative Charges
F = qv × B: for negatives, find the direction for a positive charge and reverse it. An electron beam moving right in a field into the page curves downward, not upward.