What are magnetic fields? - Cambridge OCR 21st CenturyMagnetic materials

Iron, steel, nickel and cobalt are magnetic materials. They are affected by magnets and are attracted to either pole of a magnet.

Permanent magnets

A permanent magnet is often made from a magnetic material such as steel. A permanent magnet always causes a force on other magnets, or on magnetic materials. Key features of a permanent magnet:

• it produces its own magnetic field
• the magnetic field cannot be turned on and off - it is there all the time

Bar magnets and horseshoe magnets are examples of permanent magnets.

Induced magnets

Unlike a permanent magnet, an induced magnet only becomes a magnet when it is placed in a magnetic field. The induced magnetism is quickly lost when the magnet is removed from the magnetic field.

An iron rod placed inside a coil carrying a current will become an induced magnet. If the current is dc, the ends of the iron will become a N-pole and a S-pole.

The iron filings in the image become induced magnets when they are near the bar magnet. They lose most or all of their magnetism when they are removed from the magnetic field.

Testing for magnetism

A magnet can:

• attract or repel another magnet
• attract a magnetic material (but not repel it)

This means that the only way to show if an object is a magnet is by checking if it repels another magnet.