Introduction

 
Using electromagnets

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 it is such an excellent electrical conductor (see conducting properties).

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 the entryphone 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 scrapyard crane can lift a whole car. Move it into position, and switch off to let go.
• A surgeon's tool - An eye surgeon can pull scraps of steel out of a patient's eye using an electromagnet. Turn up the current until it pulls just enough to gently remove the metal.

In this section, we'll look at how electromagnets work and how they can be made efficient.

Magnetic Fields

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.

What do the poles do?

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.

What is 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.

The magnetic field is invisible. We need to use some clever tricks to see its shape.

How can we see the magnetic field?

You can see the shape of the magnetic field using iron filings:

• 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

Which way is the magnetic field going?

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 small 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.

Magnetic field lines

We can show this on a diagram of the magnetic field using field lines. Notice that the magnetic field lines

• point away from the north pole and
• point towards the south pole and
• 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.

Solenoids

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.

Why use copper?

Copper is used because it has a low electrical resistance (see conducting properties). This means that it is easy for the current to flow through it. Also, copper wire can be easily shaped to make a coil.

What's the field like?

When the current flows through the wire, it makes the coil into a magnet. We call this an electromagnet. Field of the electromagnet is similar to the field of a bar magnet. The coil has a north pole at one end and a south pole at the other.

Remember, that we show the lines of force coming out of the north pole and going into the south pole.

How to remember the field

When a current passes through the solenoid, it becomes an electromagnet. So one end is a north pole and the other end is a south pole. There is a little trick for remembering which end becomes which pole.

It's a question of seeing whether the letter S or N will point in the same direction as the current.

When we look at one end of the coil, the current is going anti-clockwise. In this case, we can put in the letter N and it will point in the same direction as the current. An S will not. So this is the north pole.

If we look at the other end of the coil, the current is going clockwise. In this case we can fit in an S. So this is the south pole.

Using electromagnets

One use of an electromagnet is a simple door entry system for a block of flats. Someone on the fourth floor doesn't want to run downstairs to let people in. So they have a switch which operates an electromagnet in the lock. When they press the switch, the coil of copper wire becomes an electromagnet. So it attracts the armature and pulls back the lock. The door can now be opened. 

Making Them Stronger

We can make electromagnets stronger in a number of ways. Here are some of them:

• increase the current flowing
• use more turns of copper wire
• put in a soft iron core.

Let's look at each of these.

Increased current

An increased current will make a stronger magnet. However, there is a limit to how much current can flow in the wires before it gets too hot. Also, a bigger current means that more energy is wasted (as heat) in the coil and in the connecting wires. So it is often best to try to increase the strength by adding more turns rather than increasing the current.

More turns

Imagine you have an electromagnet made from a single turn of wire. You then add another turn. It's like putting another electromagnet next to the first one. So the strength of the magnet increases. The more turns, the stronger the magnet will become.

Iron core

Iron is a magnetic material. There are magnetic particles inside the iron. In soft iron, these particles will line up with an external magnetic field. In this way, the soft iron core behaves like a magnet itself. Once the external field is taken away, the core will return to normal.

Imagine we put a piece of soft iron in the middle of a coil of copper wire. When we switch on the current, the coil becomes an electromagnet. But also, the soft iron core becomes a magnet. It will add to the strength of the electromagnet.

The effect of the soft iron core is much more than doubling the current or the number of turns.