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Unit 4 · Magnetism & Electromagnetism Flashcards Cheat Sheet Essentials Visual Review MC Practice FRQ Practice

AP Physics 2 Unit 4 Essentials

The must-know terms and big ideas for Unit 4: Magnetism & Electromagnetism. Every vocabulary word and concept you need to master.

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Big Idea 1
Magnetic force only acts on moving charge, and never does work
Unlike electric force, which acts on any charge (moving or stationary), magnetic force acts only on charges in motion — F = qvB sinθ vanishes if v = 0. Even more surprising: because magnetic force is always perpendicular to velocity, it can never speed up or slow down a charge, only redirect it. This is why charged particles move in circles (not spirals inward or outward) in a uniform magnetic field — the force changes direction, never speed.
Magnetic Force Moving Charges Circular Motion
Big Idea 2
Electricity and magnetism are two faces of the same phenomenon
A moving charge creates a magnetic field; a magnetic field exerts force on a moving charge. This isn't a coincidence — magnetism IS a consequence of moving electric charge, whether that's current in a wire or the intrinsic motion (spin) of electrons in a permanent magnet. There is no magnetism without some form of moving charge underlying it, which is why studying circuits (Unit 3) was necessary preparation for this unit.
Magnetic Fields Right-Hand Rule Currents
Big Idea 3
Induction only responds to change — and always resists it
Faraday's law makes clear that a stationary loop in a constant magnetic field has zero induced EMF, no matter how strong that field is — only a CHANGING flux induces anything. And whenever flux does change, Lenz's law guarantees the induced current fights back against that change, never reinforces it. This resistance-to-change behavior is exactly what conservation of energy demands: getting "free" reinforcing currents would mean getting energy from nothing.
Faraday's Law Lenz's Law Energy Conservation
Magnetic field (B)
A region of space where a magnetic force would be exerted on a moving charge; field lines run from a magnet's north pole to its south pole outside the magnet.
Magnetic Fields
Magnetic pole
The two ends of a magnet (north and south) where field lines emerge or converge; like poles repel, opposite poles attract.
Magnetic Fields
Right-hand rule
A technique for determining the direction of magnetic force or magnetic field, using the orientation of the right hand's fingers and thumb.
Magnetic Fields
Magnetic force on a moving charge
F = qvB sinθ — the force on a charge q moving with speed v through field B, at angle θ to the field; always perpendicular to both v and B.
Force on Charges
Circular motion in a magnetic field
A charged particle moving perpendicular to a uniform magnetic field travels in a circle, since the magnetic force provides a constant centripetal force.
Force on Charges
Magnetic force on a current-carrying wire
F = BIL sinθ — the force on a length L of wire carrying current I through field B, at angle θ between current and field.
Force on Wires
Magnetic field from a current-carrying wire
A long straight wire carrying current creates a magnetic field that forms concentric circles around the wire, with strength decreasing with distance.
Fields from Currents
Magnetic flux (Φ)
A measure of how much magnetic field passes through a given area: Φ = B·A·cosθ, where θ is the angle between the field and the area's normal vector.
Induction
Faraday's law of induction
EMF = −ΔΦ/Δt — a changing magnetic flux through a loop induces an EMF proportional to the rate of that change.
Induction
Lenz's law
The direction of an induced current always opposes the change in magnetic flux that caused it — a consequence of conservation of energy.
Induction
Electromagnetic induction
The general process by which a changing magnetic flux induces an EMF (and current, if the circuit is closed).
Induction
Electric generator
A device that converts mechanical energy into electrical energy by rotating a loop in a magnetic field, continuously changing flux to induce an alternating EMF.
Induction