Magnetic Fields & Magnetic Force — A/L Physics

Magnetic Field (B) A region of space where a moving charge or a magnetic material experiences a force. Magnetic fields are produced by moving charges (currents) and permanent magnets. They are vector fields, represented by field lines that form closed loops, always pointing from north to south outside a magnet. The direction of the field at a point is the direction of the force on a north pole placed there. Example: The Earth’s magnetic field deflects compass needles to point north.

Magnetic Force on a Moving Charge (Lorentz Force) A charge moving in a magnetic field experiences a force perpendicular to both its velocity and the field. The magnitude is given by F = qvBsinθ , where θ is the angle between velocity and field. The direction is found using Fleming’s Left-Hand Rule (for positive charges; reverse for electrons). This force does no work because it is always perpendicular to displacement, so it changes only the direction of motion, not speed. Example: A beam of electrons in a cathode ray tube is deflected by magnetic coils to scan the screen.