Magnetic Fields

Objective: Interpret field diagrams and magnetic interactions.

Magnetic fields are vector fields associated with magnets, electric currents, and changing electric fields. A compass needle aligns approximately with the local magnetic field direction. Magnets conventionally have north and south poles; like poles repel and unlike poles attract. Cutting an ordinary bar magnet makes smaller magnets with both poles rather than isolating one pole. Field-line diagrams indicate direction and relative concentration, not physical strings, and iron filings reveal a pattern but are not field lines themselves. Magnetic forces on moving charges depend on motion direction and field direction; a stationary charge in a steady magnetic field does not experience the ordinary magnetic Lorentz force.

Objective: Apply magnetic fields to calculations, evidence, and specified situations.

Magnetic fields exert a force on moving charge with magnitude |q|vB sinθ. A straight current segment has magnetic-force magnitude BIL sinθ in a uniform field. Direction follows the vector cross product and charge sign; the numerical exercises ask magnitude for perpendicular motion. A field-parallel charge has no magnetic force from that field. Ordinary magnets are dipoles; cutting one produces smaller dipoles rather than isolating a pole. Moving charge and current generate magnetic fields.