Magnetic fields and field lines
A compass needle is a small bar magnet. Near a magnet it turns, because the magnet has a magnetic field around it: a region in which its force can be felt.
The field is shown with magnetic field lines:
Outside a magnet they go from the north pole to the south pole; inside the magnet they go from south to north, so they are closed curves.
The direction at any point is the direction a compass needle's north pole would point.
Where lines are closer together, the field is stronger.
No two field lines ever cross. If they did, a compass at that point would have to point in two directions at once.
The field due to a current
Hans Christian Oersted discovered in 1820 that a current-carrying wire deflects a compass needle. Electricity and magnetism are linked.
Straight wire. The field lines are concentric circles around the wire. The field is stronger closer to the wire and when the current is larger.
Right-hand thumb rule: hold the wire in your right hand with the thumb pointing along the current. Your fingers curl in the direction of the field lines.
Circular loop. At the centre of a loop, the field lines are nearly straight. The field there increases with the current and with the number of turns.
Solenoid. A coil of many circular turns of insulated wire, wound like a cylinder, is a solenoid. Its field looks like that of a bar magnet: one end acts as a north pole and the other as a south pole. Inside the solenoid the field lines are parallel and evenly spaced, so the field is uniform.
Placing a soft iron core inside a solenoid makes a strong electromagnet, used in cranes, electric bells and relays.
Force on a current-carrying conductor
A wire carrying current in a magnetic field feels a force. The force is greatest when the current is at right angles to the field, and zero when parallel to it. Reversing either the current or the field reverses the force.
Fleming's left-hand rule: stretch the thumb, forefinger and middle finger of your left hand at right angles to each other.
Forefinger → direction of the magnetic Field
Middle finger → direction of the Current
ThuMb → direction of Motion (the force)
This is the principle of the electric motor, which converts electrical energy into mechanical energy. A coil in a magnetic field experiences forces on opposite sides in opposite directions, so it rotates. A split-ring commutator reverses the current every half turn so the coil keeps turning in the same direction. Motors run fans, mixers, washing machines and electric vehicles.
Medical MRI scanners use strong magnetic fields; weak currents in our nerves also create tiny magnetic fields.
Electromagnetic induction
Michael Faraday discovered that a changing magnetic field in a coil produces a current in it. This is electromagnetic induction, and the current is an induced current.
Move a magnet into a coil connected to a galvanometer: the needle deflects. Hold it still: no deflection. Pull it out: the needle deflects the opposite way.
Switching a current on or off in one coil induces a current in a nearby coil.
Fleming's right-hand rule gives the direction of the induced current: thumb = motion of the conductor, forefinger = field, middle finger = induced current.
Electric generators in power stations use this principle to convert mechanical energy into electrical energy.
AC and DC. Direct current (DC), from cells, always flows in one direction. Alternating current (AC) reverses direction periodically. In India, AC changes direction every 1/100 second, so its frequency is 50 Hz. AC can be transmitted over long distances with less energy loss.
Domestic electric circuits
Electricity reaches homes through three wires:
Wire | Insulation colour | Role |
|---|---|---|
Live (phase) | red (older) or brown | carries current at 220 V |
Neutral | black (older) or blue | return path, at 0 V |
Earth | green (or green and yellow) | safety: connected to a metal plate deep in the ground |
Appliances in a house are connected in parallel, so each gets the full voltage and can be switched independently.
Earthing. The metal body of appliances such as an iron, toaster or fridge is connected to the earth wire. If live wire touches the body because of a fault, current flows to the ground instead of through a person who touches it.
Overloading and short circuits. Too many appliances on one socket, or live and neutral wires touching directly (short circuit), cause a very large current. A fuse melts, or a miniature circuit breaker (MCB) trips, and cuts off the supply, preventing fires. Two separate circuits are often used: a 15 A circuit for high-power appliances such as geysers and air coolers, and a 5 A circuit for bulbs and fans.
Summary
Field lines run from N to S outside a magnet, never cross, and crowd where the field is strong.
A current produces a magnetic field; the right-hand thumb rule gives its direction; a solenoid's inner field is uniform.
A current in a field feels a force (Fleming's left-hand rule): the basis of motors.
A changing field induces a current (Fleming's right-hand rule): the basis of generators.
Earthing, fuses and MCBs keep household circuits safe.