Magnetic Effects of Electric Current
Magnetic Effects of Electric Current tests field patterns, the rules for force and induction, the motor and generator, and domestic circuit safety.
Magnetism and electromagnets
Field lines run from north to south outside a magnet and never cross. A current-carrying solenoid behaves like a bar magnet; inserting a soft iron core makes an electromagnet. Field strength rises with current and with turns per unit length.
Force and induction
Fleming's left-hand rule gives the force on a current-carrying conductor in a field: forefinger field, middle finger current, thumb force. This is the principle of the electric motor. Fleming's right-hand rule gives the direction of induced current, the principle of the generator. A DC generator uses a split-ring commutator to keep output one-way; in an AC generator the current reverses every half turn.
Domestic circuits
Live (red or brown), neutral (black or blue), earth (green). The fuse sits in the live wire and melts on overload; earthing protects a user from shock.
Worked example
An AC generator spins at 50 revolutions per second, giving 50 Hz supply. Each revolution contains two half turns, so the current reverses twice per revolution, that is 100 times per second. The frequency of the emf is therefore 50 Hz while the reversal rate is 100 per second.
Traps
Using the right-hand rule for the motor; saying field lines of two magnets can cross; putting the fuse in the neutral wire; confusing the commutators of AC and DC generators.