Physics
Cambridge IGCSE

TOPIC 4B: MAGNETISM

# 7.2 Electromagnetism

Have a look at figure 1 below. If you put a compass near to an electric circuit, the compass needle moves. What do you think causes this?

Figure 1: The effect of a current on a compass

The answer is that the current in the wire conductor is producing a magnetic field around it. This is producing a force on the compass. This effect is known as electromagnetism. The magnetic field depends on two factors; the strength of the current, and the distance from the wire:

• 1. The larger the current, the stronger the magnetic field.
• 2. The larger the distance from the wire, the weaker the magnetic field.

The magnetic field around a wire is circular as shown here in figure 2. To remember the direction of the field, we use the right-hand grip rule:

If you imagine gripping the wire with your right hand with your thumb pointing in the current direction, then your fingers will show the field direction, as shown here.

Figure 2: The field around a straight wire carrying a current
(wikimedia)

## Solenoids

A solenoid is basically a long coil of wire. When a current passes through it, a strong magnetic field is produced in the centre of the coil as shown in figure 3. Outside the coil, the field is much weaker.

Figure 3: The field produced by a solenoid

As you can see from the diagram, the field inside the solenoid is uniform - the lines are evenly spaced and straight. The field away from the coil (not shown in the diagram) is very similar in shape to a bar magnet. The larger the current, the stronger the field produced by the solenoid. By coiling the wire like this, the field can be made much stronger than in a single wire. As the distance from the solenoid increases, so dues the the strength of the field, just like a bar magnet.

How can you predict the direction of the field inside the solenoid? The right hand grip rule explained above works for all electromagnetic fields. If you imagine using your right hand to grip the downwards black wire on the left of figure 3, your fingers should be pointing to the right inside the coil. This shows that the magnetic field is pointing left to right inside the solenoid.

If you put a magnetic material like iron inside the coil, the magnetic field becomes even stronger. The iron becomes and induced magnet in the field produced by the coil. A solenoid with an iron core inside is called an electromagnet.

Required Practical: Describe an experiment to identify the pattern of the magnetic field (including direction) due to currents in straight wires and in solenoids

The simplest way to find the pattern and direction of a magnetic field is by placing a small compass in the field. For example, in figure 4 below, a small plotting compass has been put on a piece of card, with a straight wire threaded through the centre. The north pole of the compass shows the direction of the field, and by moving the compass around on the card, the shape of the field can be determined.

Figure 4. Investigating the magnetic field around a current carrying straight wire

The experiment can be repeated with a solenoid placed on a desk, and if a large enough coil is made, the compass can be placed inside the coil as well as around the outside, to deduce the shape of magnetic field shown in figure 3 above.

What would happen if the direction of current was reversed? In this case, the direction of the magnetic field would be reversed for both the straight wire and for the solenoid.

### Electromagnets

Any electromagnet is made with the following:

• A coil, like the solenoid, that will produce a concentrated field inside.
• A current flowing through the coil.
• A soft iron core inside the coil that will be magnetised when the current is turned on, and quickly lose its magnetism when switched off.

To make a powerful electromagnet, you need a large number of turns in the coil, a high current, and a highly magnetic material like a soft iron core. Did you notice the 3 'C's? Remember Coil, Current, Core. Electromagnets can be incredibly strong, and are used to pick up and release heavy cars in scrap yards. The advantage over permanent magnets is that the induced iron core can be switched on and off easily.

Figure 5: A powerful electromagnet picking up scrap metal

Questions:

1. A wire has a current flowing through it as shown. Sketch the magnetic field around this wire.

The magnetic field should have a circular shape as shown by the right-hand grip rule:

2. An iron bar is placed within a solenoid to create an electromagnet. This is used to pick up steel nails.

• a) Describe the construction of a solenoid.
• b) Describe the magnetic field at the centre of a solenoid.
• c) State 2 ways the electromagnet can be made stronger to pick up more nails.
• d) State one advantage of an electromagnet for this purpose compared to a permanent magnet.

a) A solenoid is a (long) coil made from a conductor / wire. (It has a current flowing through the wire, but this is not really part of the construction).

b) The field is uniform.

c) More coils / turns, or a larger current.

d) The electromagnet can be switched on / off. A permanent magnet is always 'on'.
(Electromagnets are - potentially - stronger magnets, although this depends on the construction).

## The motor effect

If a single wire can produce a magnetic field when a current flows through it, what will happen if we place the wire near to a magnet? The two magnetic fields will produce forces, and the wire and magnet will be pushed. The way the two magnetic fields interact is quite complicated, but the force produced can be used to produce movement.

This was a key discovery back in the 1820s and was the first method of converting electrical energy into kinetic energy. This led to the invention of the motor, and hence the name 'motor effect' given to the force on a wire in a magnetic field.

Required Practical: Describe an experiment to show that a force acts on a current-carrying conductor in a magnetic field, including the effect of reversing:

• (a) the current
• (b) the direction of the field

To do this experiment, pass a current through a wire, and place it between two strong bar magnets as shown here in figure 6:

Figure 6. Investigating the magnetic field around a current carrying straight wire

The wire in this case is pushed upwards, showing that a force is produced.

(a) if you reverse the current, the force also reverses direction, and the wire is pushed downwards.

(b) If you reverse the magnets so that the magnetic field reverses, then again the direction of the force reverses.

Note that for the foundation level exams, you do not need to know how to work out the initial direction of force, only that it reverses if the current or magnetic field direction is reversed.

For the higher level papers, you will need to be able to predict the direction of the force as explained by Fleming's left hand rule, as explained below:

### Fleming's Left-Hand Rule

Figure 7 below shows the experiment above again. It also shows an easy way to work out the direction of the force, called Fleming's Left-Hand Rule:

Figure 7: The motor effect and Fleming's left hand rule
(DougM wikimedia)

Fleming's left-hand rule (LHR) is a great way to work out the direction. You need to remember the following:

• The First finger indicates the magnetic field (remember field lines go from north to south).
• The seCond finger indicates the current.
• The THumb indicates the resulting thrust (a force) on the current carrying conductor.
• The magnetic field is always directed from north to south.

You will need to be able to apply this rule in a variety of situations. The best starting point is to line up your first finger with the magnetic field, and then rotate your hand appropriately to find the motion (or current) from there. Remember to use your left hand!

Note that all three fingers in the left-hand rule are perpendicular to each other. If there is a situation where the current is parallel to the magnetic field, then there is no force.

### What will happen if...?

The force on the conductor depends on three factors:

1. If you increase the current (I), the force increases.
2. If you increase the strength of the magnetic field, then the force increases.
3. If the length (l) of wire in the magnetic field is increased, the force increases. One way to increase the length is to use one side of a coil so that there are multiple sections of wire in the field, all with the current flowing in the same direction.

Have a go at the questions below to check your understanding of Fleming's LHR:

Questions:

3. A metal wire has a current flowing through it, and is placed between two permanent magnets as shown here:

• a) Use the left-hand rule to predict the force on the wire. Draw an arrow on the diagram to show the direction of this force. Label the arrow 'F'.
• b) Describe a method of increasing the size of the force F without replacing the magnets.

a) Your first finger on your left hand should point right, from N to S. Your second finger should point into the 'page'. This leaves your thumb pointing downwards. Your thumb indicates the motion and hence the force. Draw an arrow downwards as shown here:

b) You can increase the force by increasing the current. Alternatively, wrapping the wire into a loop or coil with only one edge inside the field will effectively increase the length of the wire. (Note that the magnets cannot be replaced in the question, so you cannot increase the field strength).

There are Grade Gorilla practice questions on this section after section 4.5b on the applications of the motor effect.

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