Magnets around us
Figure 13.1 shows a large electromagnet being used to lift and move pieces of iron and steel. The steel pieces are pulled very strongly to this powerful electromagnet, so they can be moved away easily.
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Electromagnets
Magnetic only while a current flows through the coil.
Switched off, the scrap drops — which is why the crane can let its load go.
Permanent magnets
Magnetism is a property of the material itself.
It lasts a long time without fading.
Both kinds are used in many devices for many jobs: controlling household appliances and robots worked by electric motors, magnetic cards, and the MRI machines used in medicine, for example. Magnets hold a very important place in the modern world, so it is useful to know how they behave, how they work and how they are used.
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Where this chapter goes
The electromagnet in the scrap yard holds the whole chapter in one picture: an electric current can make a magnet. From there the chapter follows two roads. First, a current makes a magnetic field, and a magnetic field pushes on a current; that push is what turns every electric motor and moves the cone of every loudspeaker. Then the road is reversed: moving a magnet near a wire, or a wire near a magnet, makes a current. That is how almost all the world's electricity is generated, and how transformers step the mains voltage up and down on its way to your home.
Two kinds of magnet
An electromagnet is magnetic only while current flows in its coil; a permanent magnet's magnetism belongs to the material and lasts.