An electromagnet is a magnet that works with electricity. It can be switched on and off. The coils are nearly always made of copper wire because copper is such an excellent electrical conductor. Below are different sections of this e-source, for quick navigation.
Electromagnets have many uses. Here are some examples.
- An electric bell: The electromagnets make the hammer vibrate back and forth, ringing the bell.
- An electric lock: When an entry phone has been answered, the door can be unlocked from an upstairs flat. An electromagnet pulls the bolt open. Switch it off and the bolt springs back.
- A crane: A crane and lifting magnet can lift tonnes of steel without hooks and ropes.
- A surgeon's tool: An eye surgeon can pull scraps of steel out of a patient’s eye using an electromagnet by turning up the current until it pulls just enough to gently remove the metal.
- Microsurgery: Researchers are working on electromagnets that can move micro-robots around inside the body to carry out surgery without cutting a patient open.
Permanent Magnet Fields
Magnets have north and south poles. The names come from the way a magnet will point on Earth, which is a giant magnet. Like poles repel and unlike poles attract. Top of Page
'Seeing' a Magnetic Field
A magnet does not have to be touching another magnet to pull it or push it. The force from the magnet reaches out. It is an invisible force that works at a distance. We say that there is a magnetic field around the magnet. The magnetic field is the region in which a magnet's force works.
We can use iron filings to reveal the invisible magnetic field.
- Place a piece of cardboard over the magnet
- Gently sprinkle some iron filings onto the cardboard
- Tap the cardboard so that the iron filings line up with the magnetic field.
- Look for the pattern made by the iron filings
The iron filings tell us the shape of the magnetic field. However, it's also useful to know which way the field is going, i.e. will it attract or repel a north pole of another magnet?
We can find this out using a compass. The compass needle is itself a small magnet. Its arrow is a north pole, so the compass points away from the north pole of the magnet.
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The field is invisible, but with a card and some iron filings: sprinkle, tap, reveal!
A magnet will pull on some metals like iron. We say these metals are magnetic. We can also make a magnet from soft iron. This magnet will have two poles: a north and a south. Two bar magnets will attract or repel, depending on which way they are facing.
If we bring the north pole of one bar magnet up to the south pole of another one, they will attract each other. We say that opposite poles attract.
However, if we bring a north pole up to another north pole, they will be forced apart. We say that they repel. The same happens with two south poles. We say that like poles repel.
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Field Lines Around a Bar Magnet
Notice from the diagram showing the magnetic field lines around a bar magnet that they:
- Point away from the north pole
- Point towards the south pole
- Never cross over each other
- Only come out of the ends of the magnet
- Are closest together where the field is strongest, e.g. near the poles.
The arrows on the field lines tell us which way another north pole would move. A south pole would be pulled in the opposite direction to the arrows. Top of Page
An electromagnet is a coil of wire with an electric current flowing through it. When the wire is coiled around in a cylinder, we call this a solenoid. The solenoid becomes an electromagnet when a current flows through it.
Copper wire is used because it has a low electrical resistance. This means that it is easy for the current to flow through it. Also, copper wire can be easily shaped to make a coil.
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The magnetic field of an electromagnet is just like that of a permanent bar magnet. Increasing the current makes the field stronger, but a high current will heat up the coil, wasting energy as heat. Top of Page
Adding more turns will also increase the field strength. So we can add more and more turns to get a stronger and stronger magnet. This seems too good to be true... and it is. Adding more turns increases the coil resistance because the wire is longer. This reduces the current.
Designers have to decide how many turns and how much current they need to get the right field strength.
- Could a crane with a lifting magnet lift bronze, brass or copper pipes?
- A magnetic field acts at a distance. It exerts forces without touching. Gravitational fields also do this, but what does a magnetic field do that gravity does not?
- If you bring either pole of a magnet near to a piece of steel such as a nail or a paper clip, it always attracts. It never repels. Can you explain this?