What is a magnet? A magnet is a material that attracts iron, cobalt and nickel, and that always points in the north-south direction when suspended freely. Every magnet has two poles: a north pole (N) and a south pole (S). Like poles repel, unlike poles attract. You can never isolate a single pole — break a magnet and each piece becomes a complete magnet with its own N and S.
Magnetic field and field lines. The region around a magnet where its force can be detected is called the magnetic field. We picture it using magnetic field lines, drawn with a small compass. Their key properties are:
- Field lines emerge from the north pole and merge into the south pole outside the magnet (inside they run S to N, forming closed loops).
- They are closer together where the field is strong (near the poles) and farther apart where it is weak.
- No two field lines ever cross — if they did, the compass would point in two directions at one place, which is impossible.
- The tangent to a field line at any point gives the direction of the field there.
Oersted's discovery (1820). Hans Christian Oersted noticed that a compass needle placed near a current-carrying wire deflects. When the current was switched off, the needle returned to north-south. This proved that electricity and magnetism are linked — a current produces a magnetic field around it. Reversing the current reverses the deflection.
Field due to a straight conductor. Around a long straight wire the field lines are concentric circles centred on the wire. The field is stronger near the wire and weaker farther away (it weakens with distance). Its strength increases if the current increases.
Right-Hand Thumb Rule. If you hold the wire in your right hand so the thumb points along the current, the curled fingers show the direction of the field (the circular field lines). This is also called Maxwell's corkscrew rule.
Field due to a circular loop. At every point of a current loop the small circular fields add up. At the centre of the loop the field is nearly straight and perpendicular to the plane of the loop. A loop with n turns gives a field n times stronger because each turn contributes in the same direction.
Solenoid and electromagnet. A solenoid is a long coil of many circular turns. Its field is like that of a bar magnet — uniform and parallel inside, with a clear N and S end. Placing a soft-iron core inside makes a strong electromagnet whose strength can be controlled by the current and switched on or off.