4.1 Magnets and Magnetic Force

Magnetic Poles and Fields

Magnets have two poles: north and south. Like poles repel, opposite poles attract. Magnetic field lines point from north to south outside the magnet. Unlike electric charge, magnetic poles always come in pairs — there is no isolated monopole; cutting a magnet in half just creates two smaller magnets, each with N and S poles.

Force on a Moving Charge

A magnetic field exerts a force on a moving charged particle given by F = qvB sin(θ), where θ is the angle between velocity and field. The direction is found with the right-hand rule: point fingers along v, curl toward B, thumb gives force direction for positive charges (reverse for negative). No force acts on charges at rest or moving parallel to B.

Force on a Current-Carrying Wire

Since current is moving charge, a wire in a magnetic field feels a force: F = BIL sin(θ), where L is wire length in the field and θ is the angle between current direction and B. This force is perpendicular to both the current and the field, again determined by the right-hand rule, and is maximized when the wire is perpendicular to B.

Key terms

Magnetic monopole

A hypothetical isolated magnetic pole; none has ever been observed — magnets always have paired N and S poles.

Force on a moving charge

F = qvB sin(θ); force is zero if the charge is at rest or moves parallel to the field.

Right-hand rule

Point fingers along velocity (or current), curl toward B, thumb points in the direction of force (for positive charges).

Force on a current-carrying wire

F = BIL sin(θ), where L is the wire length in the field and θ is the angle between current and B.

Like vs. unlike poles

Like poles (N-N or S-S) repel; unlike poles (N-S) attract.

Try an AP-style question

Outside a bar magnet, which way do magnetic field lines point?

1. In circles around the middle
2. Straight out of both poles
3. From south to north
4. From the north pole to the south pole