The force a magnetic field puts on moving charges and on wires. The Lorentz force, circular and helical paths, motion in combined electric and magnetic fields, the cyclotron, and the force on a current-carrying wire.
3 lessons, about 2 h5 simulations11 problems9 practice questionsPython in every lesson
Created by StemPy LabA charge at rest feels no magnetic force. A charge moving across a magnetic field feels a force , at right angles to both the velocity and the field. This force does no work, so it can turn a charge but never speed it up. With an electric field as well, the force is .
A charge that enters a magnetic field at right angles goes round a circle of radius . The time for a turn is , which does not depend on the speed. At an angle to the field the path is a helix. With a magnetic and an electric field at right angles, only charges of speed go straight. The cyclotron uses the steady turning time to speed up particles.
A wire is a stream of moving charges, so a field pushes on it. A straight wire of length carrying a current feels , which in vector form is . A bent wire in a uniform field feels the force of a straight wire from its start to its end. A closed loop feels no net force.
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