Key Concepts: Magnetic Fields & Forces

Magnetic Fields

• Produced by moving charges (currents) and permanent magnets.
• Field lines go from North to South outside the magnet.
• Unit: Tesla (T). Earth's field ≈ 5 × 10⁻⁵ T.

Force on a Moving Charge

• F = qvB sin θ — force is perpendicular to both velocity and field (use right-hand rule).
• A stationary charge experiences no magnetic force.
• The magnetic force does no work (it changes direction, not speed).

Force on a Current-Carrying Wire

• F = BIL sin θ — force on a wire of length L carrying current I in field B.

Right-Hand Rule: Point fingers in the direction of velocity (or current), curl toward B — thumb points in the direction of force (for positive charges).