Moving Charges and Magnetism — Class 12 Physics

A moving charge experiences a magnetic force , perpendicular to both velocity and field, doing no work and causing circular or helical motion. Combined with the electric force it gives the Lorentz force . The Biot-Savart law gives the field of a current element, , from which the field of a circular loop and a straight wire follow. Ampere's circuital law, , elegantly yields the field of a long solenoid and a toroid. A current-carrying conductor in a field feels , and two parallel currents attract or repel, defining the ampere. A current loop behaves as a magnetic dipole with moment , experiencing torque —the principle of the moving-coil galvanometer. With suitable shunt or series resistance, a galvanometer is converted into an ammeter or voltmeter.