Magnetic field of a current

In 1820, Hans Christian Oersted noticed that a compass needle moved when a current flowed in a wire near it. So an electric current makes a magnetic field: electricity and magnetism are linked. The unit of magnetic field strength, the oersted, is named after him. This is what the chapter's name means: the magnetic effect of a current.

Round a straight wire, the field lines are concentric circles around the wire. The field is stronger for a bigger current and weaker farther from the wire. Reverse the current and the field reverses too.

Right-hand thumb rule: imagine holding the wire in your right hand with your thumb pointing along the current. Your fingers curl around the wire in the direction of the field lines. (It is also called Maxwell's corkscrew rule: turn a corkscrew the way the current goes, and its rotation shows the field.)

To draw directions on paper, a dot (•) means a field coming out of the page towards you, and a cross (×) means a field going into the page. Practise on paper with the wire drawn in different directions: the answer is written on paper, so practise there.

A circular loop: bend the wire into a circle. Every part of the loop makes the field point the same way inside it, so the effects add up. Near the centre the lines are almost straight. A coil of n turns gives n times the field of a single turn.

A solenoid is a long coil of many turns. Its field looks just like a bar magnet's: one end acts as a north pole and the other as a south pole. Inside, the lines are parallel: the field is the same everywhere (uniform).

Put a piece of soft iron inside a solenoid and switch the current on: the iron becomes a strong magnet, an electromagnet. Switch the current off and it stops being a magnet.

Magnetism in medicine, from the book. Even the tiny currents of the nerve impulses in our body make magnetic fields, about a billionth of the earth's. They are strongest around the heart and the brain. A technique called MRI (Magnetic Resonance Imaging) uses magnetism to make pictures of the inside of the body, which helps doctors find what is wrong.