Two pictures, one law. On the right a moving charge q feels a sideways force F = q·v×B and curves into a circle of radius r = m·v / (q·B).

On the left a loop of current is itself a source of field, B = µ₀·I / (2·R) at its centre — a charge in motion feels a field, and a charge in motion makes one.

The chapter turns these two facts into the cyclotron, the solenoid, and the torque that spins a motor.