Electromagnetism

Magnetic Fields

A magnetic field is a region where a magnetic force acts on a magnetic material or a current-carrying conductor. Magnetic field lines run from north to south pole outside a magnet. Where lines are closer, the field is stronger.

Permanent magnets produce a magnetic field at all times. Electromagnets produce a field only when current flows; the field can be switched on/off and its strength varied by changing the current.

A solenoid (coil of wire) produces a magnetic field similar to a bar magnet when carrying current. The strength can be increased by: increasing the current; adding more turns; inserting a soft iron core.

The right-hand screw rule: if the right hand's thumb points in the direction of conventional current in a wire, the fingers curl in the direction of the magnetic field around the wire.

The Motor Effect (Force on a Current-Carrying Conductor)

When a current-carrying conductor is placed in a magnetic field (at an angle to the field), it experiences a force. This is the motor effect.

Fleming's left-hand rule (for conventional current):

• First finger: direction of magnetic Field (N to S)
• seCond finger: direction of conventional Current
• thuMb: direction of Motion (force on conductor)

Force is largest when current is perpendicular to the field; zero when parallel.

Force equation: F = BIL (N = T × A × m), where B is magnetic flux density (tesla, T), I is current A, L is length of conductor in the field (m).

The DC motor: a rectangular coil carrying current in a magnetic field. The two sides of the coil experience forces in opposite directions, creating a turning moment (torque). A commutator reverses the current direction every half-turn so the coil keeps rotating the same way. Brushes maintain electrical contact with the rotating commutator.

Electromagnetic Induction

When a conductor moves through a magnetic field (or when the field changes around a stationary conductor), an EMF (electromotive force) is induced. If the circuit is complete, this drives a current — electromagnetic induction.

Faraday's law: the magnitude of the induced EMF is proportional to the rate of change of magnetic flux.

Lenz's law: the induced current opposes the change that causes it (ensures energy conservation).

Ways to increase the induced EMF: move the conductor faster; use a stronger magnet; use more turns of wire; use a longer conductor.

Fleming's right-hand rule (for generators):

• First finger: Field
• seCond finger: induced Current
• thuMb: Motion of conductor

AC generator (alternator): rotating coil in a magnetic field. As the coil turns, it cuts field lines; a slip ring (not commutator) maintains contact, giving alternating current that peaks at 90° (coil moving fastest, perpendicular to field).

DC generator: uses a commutator (as in a motor) to produce pulsating DC.