Electromagnetic Effects

Wrap your right hand around a wire with the thumb pointing in the direction of the conventional current. The fingers curl in the direction of the magnetic field lines (the "right-hand grip rule"). Around a straight wire the field is a set of concentric circles; inside a solenoid (a coil), the fields from each turn add up to give a strong, near-uniform field down the middle — exactly like the field inside a bar magnet.

The poles of a solenoid can be worked out using the right-hand rule too: curl your fingers in the direction the current flows around the loops; the thumb points to the N pole.

Three ways to strengthen an electromagnet:

1. More current through the coil.
2. More turns of wire on the coil.
3. An iron core inside the coil (concentrates the field).

Switching the current off makes the magnetism vanish — perfect for a scrap-yard crane that needs to pick up and drop steel.

When a current-carrying wire sits inside a magnetic field, the wire's own magnetic field interacts with the external field and the wire experiences a force (Fleming's left-hand rule):

• First finger: Field direction (N → S)
• SeCond finger: Current direction (+ to −, conventional)
• ThuMb: Motion / force direction

Reversing either the current OR the field reverses the force. Reverse BOTH and the force is unchanged.

In a DC electric motor, a coil sits between magnetic poles. The motor effect makes one side of the coil go up and the other go down — the coil spins. A split-ring commutator flips the current direction every half turn so the rotation keeps going in the same sense.

If you move a magnet into or out of a coil, a voltage is induced across the coil's ends. Close the circuit and a current flows. This is electromagnetic induction, discovered by Faraday in 1831.

The size of the induced voltage increases with:

• the speed of relative motion,
• the strength of the magnetic field,
• the number of turns on the coil.

Reverse the motion and the voltage reverses too. The induced current always flows in a direction that opposes the change causing it (Lenz's idea).

This is the principle behind:

• AC generators (alternators) — a coil rotates between magnets, producing alternating voltage.
• Microphones — sound vibrates a diaphragm and coil near a magnet, inducing a tiny voltage that matches the sound.
• Transformers — a changing current in the primary coil creates a changing field that induces a voltage in the secondary coil.