AP Physics 2: Magnetic Fields & Forces

« Back to AP Physics Guide / Unit 12: Magnetism

Neon-style visualization of magnetic field lines surrounding a North and South pole.

Visualization of the invisible vector field (B) that governs magnetic interactions.

Magnetism is all about the interaction between moving charges. Unlike electric fields which originate from stationary charges, magnetic fields ( $B$ ) are created by and exert forces on charges in motion.

1. Magnetic Fields (Topic 12.1)

Magnetic fields are vector fields. The direction of the field at any point is the direction a North pole of a compass needle would point.

Diagram of concentric magnetic field lines forming around a straight current-carrying conductor.

As current (I) flows through a conductor, it generates a circular magnetic field (B) according to the Right-Hand Grip Rule.

Field Lines: Always form closed loops (from North to South outside a magnet).
Unit: The SI unit for magnetic field strength is the Tesla (T).

2. Force on a Moving Charge (Topic 12.2)

A magnetic field exerts a force on a charge only if the charge is moving and moving non-parallel to the field lines.

$F_M = qvB \sin(\theta)$

Where $q$ is charge, $v$ is velocity, $B$ is field strength, and $\theta$ is the angle between $v$ and $B$ .

Educational diagram showing the Right-Hand Rule for determining the direction of magnetic force on a positive charge.

The Right-Hand Rule: A critical tool for predicting the direction of force in three-dimensional space.

The Right-Hand Rule (RHR): Use your right hand for positive charges! Point fingers in direction of $v$ , curl towards $B$ ; your thumb points in the direction of $F_M$ . (Use the left hand for electrons/negative charges).

3. Force on Current-Carrying Wires (Topic 12.3)

Since current is just a flow of moving charges, a wire placed in a magnetic field will also experience a force.

Diagram of a positive charge following a circular path inside a uniform magnetic field pointing into the page.

When v is perpendicular to B, the magnetic force acts as a centripetal force, causing the particle to move in a circle.

$F_M = ILB \sin(\theta)$

Where $I$ is current and $L$ is the length of the wire inside the field.

4. Quick AP Practice

📚 Unit 12 Mastery Challenge

1. An electron moves East through a magnetic field pointing North. In what direction is the magnetic force?

Check Answer

Using RHR: Fingers East, curl North, thumb points Up. However, since it is an electron, the force is in the opposite direction: Down (into the page).

2. If a particle moves parallel to the magnetic field lines, what is the magnitude of the magnetic force?