IGCSE Coordinated Sciences - Physics Student Book

Cambridge IGCSE™

Co-ordinated Sciences Physics STUDENT’S BOOK

Gurinder Chadha, Susan Gardner, Malcolm Bradley and Chris Sunley

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You will probably have experienced magnetism in simple toys and in bar magnets. However, it is the existence of electromagnetism that really makes a difference to your life.

Without electromagnetism, the generation of electricity would not happen in the way that it does. Without electromagnetism, the high voltages transmitted down power lines could not be transformed into the lower voltages that you need in your home. Electricity is something that many people use every day – for lighting, heating, cooking and to power many different items of equipment. However, it can be dangerous.

STARTING POINTS 1. How can you check that a piece of metal is a magnet?

2. Which parts of an atom are charged?

3. What must an electric circuit have for it to operate correctly?

4. Name one appliance that has an electric motor.

SYLLABUS SECTIONS COVERED 4.1 Simple phenomena of magnetism 4.2 Electrical quantities 4.3 Electrical circuits 4.4 Electrical safety 4.5 Electromagnetic effects

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4

Electricity and magnetism

∆ Electricity is an essential part of our lives – we rely on it to operate many devices.

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Simple phenomena of magnetism INTRODUCTION

The property of magnetism has been known for many centuries. Ancient travellers used naturally magnetic rocks such as lodestone to guide their journeys. Lodestone, which is magnetic, points towards the north when suspended. The development of the compass made it possible to make long sea voyages of discovery, whereas previously ships had stayed within sight of land. There ∆ Fig. 4.1 A compass. is evidence that some animals, such as birds, can sense the magnetic field of the Earth and that they have their own ‘in-built compass’, which may help navigation during long migrations.

The magnetic field of the Earth has a wider importance. It acts as a shield, protecting the surface of the Earth from many charged particles emitted from the Sun. The Earth would have been a much hotter and uninhabitable place without this protection from the Earth’s magnetic fields. A study of magnetism helps us to understand our Earth better and will be crucial when we start to learn about electromagnetism later in this section.

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ELECTRICITY AND MAGNETISM

KNOWLEDGE CHECK ✓ The ends of a bar magnet are called poles and that is where the magnetic field is strongest. ✓ Not all materials are magnetic.

LEARNING OBJECTIVES ✓ Describe the forces between magnetic poles and between magnets and magnetic materials, including the use of the terms north pole (N pole), south pole (S pole), attraction and repulsion, magnetised and unmagnetised. ✓ SUPPLEMENT Describe induced magnetism. ✓ State the differences between the properties of temporary magnets (made of soft iron) and the properties of permanent magnets (made of steel). ✓ State the difference between magnetic and non-magnetic materials. ✓ SUPPLEMENT Describe a magnetic field as a region in which a magnetic pole experiences a force. ✓ SUPPLEMENT State that the direction of a magnetic field at a point is the direction of the force on the N pole of a magnet at that point. ✓ Describe how a permanent magnet differs from an electromagnet.

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MAGNETS REPEL AND ATTRACT When a permanent magnet is suspended and allowed to swing, it will line up approximately north–south.

The two ends of a magnet, which are the most strongly magnetic parts, are called the north pole and the N south pole, often labelled N and S. For the suspended S magnet, the N pole points approximately towards the north, and the S pole points approximately towards the south.

There are forces between magnets, and between ∆ Fig. 4.2 A bar magnet aligns itself magnets and magnetic materials. with the Earth’s magnetic field when it is suspended – it is being used as a When two north poles from different magnets are compass. brought together, there will be a repulsion between them. This also happens when two south poles are used. Two like poles (N–N or S–S) repel. However, when a north pole and a south pole are brought together, there will be an attraction. Two unlike poles (N–S or S–N) attract. Magnets will also attract magnetic materials that can be magnetised by the magnet, such as iron, nickel, and cobalt.

QUESTIONS 1. You are provided with a bar magnet with its north pole marked. Describe how you can deduce the poles of another magnet.

2. The north pole of a compass points towards the Earth’s north pole. Suggest the

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Magnetically hard and soft materials Several elements are magnetic, the most important of which are iron, cobalt, and nickel.

Scientists have developed alloys and ceramics made from combinations of elements to get the exact properties that they want. Some of these materials are magnetically hard (such as steel, which is an alloy of iron and other elements such as carbon or tungsten).

This means that they stay magnetic once they have been magnetised. This is what you would want from a permanent magnet.

When we refer to a ‘magnet’, we mean a permanent magnet that is made of magnetically hard materials. Permanent magnets are used in computer hard drives, electric motors, phones, microphones, and loudspeakers.

Other materials are magnetically soft (such as pure iron), which means that they cannot be permanently magnetised. This is particularly useful in some electromagnetic devices such as the electromagnet. In these cases, magnetism is only needed under particular circumstances, such as when a switch is closed. Pure iron used in an electromagnet is an example of a temporary magnet.

MAGNETS REPEL AND ATTRACT

magnetic polarity of the Earth’s north pole.

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A magnetic material is defined as a material that can be magnetised, either temporarily or permanently. Examples of this include soft iron, steel, nickel, and cobalt. A nonmagnetic material is one that cannot be magnetised – basically the opposite of a magnetic material. A non-magnetic material will not be attracted to a strong magnet.

Examples of non-magnetic materials include copper, aluminium, gold, and silver.

∆ Fig. 4.3 Some microphones use permanent magnets.

QUESTIONS 1. Name a material that may be used to make temporary magnets.

2. Name a material that may be used to make permanent magnets.

3. State the difference between a magnetic material and a non-magnetic material.

SUPPLEMENT

permanent north pole

induced south pole

attraction

magnetic material

permanent south pole

induced north pole magnetic material

attraction

∆ Fig. 4.4 The pole of a permanent magnet always induces the opposite pole in an unmagnetised piece of magnetic material. The permanent magnet will always attract the magnetic material.

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ELECTRICITY AND MAGNETISM

Magnetic induction When a soft magnetic material is brought near to a magnet it will be attracted. It has had magnetism induced in it; it has become magnetised. When the magnet is taken away, the material loses its magnetism again – it becomes unmagnetised. The magnet will continue to attract the soft magnetic material even when the material is turned around.

This is the opposite behaviour to two magnets, as two magnets will repel each other in certain orientations. This simple method enables you to work out whether you are holding two magnets or one magnet and one piece of magnetic material.

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QUESTIONS 1. Describe how a permanent magnet can be used to identify an object made from:

a) non-magnetic material b) soft iron.

2. SUPPLEMENT The N pole of a magnet is brought close to the end of a soft iron rod. Explain why there is a magnetic force of attraction between the magnet and the rod.

SUPPLEMENT

Magnetic fields Magnets have a magnetic field around them.

A magnetic field is a region in which a magnetic pole experiences a force. We describe magnetic fields using magnetic field lines. Magnetic field lines come out from the N pole of a magnet and go into its S pole. The direction of the magnetic field is shown by arrows on these lines. The direction of the field at a point is the direction in which a free north pole would move.

Fig. 4.5 shows the magnetic field around a permanent bar magnet.

S

N

∆ Fig. 4.5 The magnetic field around a bar magnet.

QUESTIONS 1. SUPPLEMENT Define magnetic field.

2. SUPPLEMENT State the direction of a magnetic field.

3. An electromagnet is made using a copper wire wrapped around a soft-iron rod.

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Describe how you can use this arrangement to produce magnetism.

MAGNETS REPEL AND ATTRACT

Permanent magnets and electromagnets A permanent magnet never loses its magnetism. In contrast, an electromagnet is only magnetic when there is an electric current in the coil (see the topic on Electromagnetic effects). Electromagnets are useful in applications when magnetism is needed for a short period of time, such as in scrap yards, where they can be used to lift scrap metal that is magnetic.

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● SCIENCE IN CONTEXT

STRONG MAGNETS

The strongest magnets are made from alloys that contain rare earth elements such as neodymium (Nd), samarium (Sc), and yttrium (Y). For example, neodymium magnets are made from an alloy of neodymium, iron, and boron. Such magnets retain their magnetism for a very long time, and are found in electric motors in cordless tools and computer hard disk drives. Neodymium magnets can even be used at high temperatures of 200 °C. Our Earth has a magnetic field, and this can be detected using a simple compass.

Neodymium magnets are about 100 000 times stronger than the Earth’s magnetic field.

However, this is nothing compared with the magnetic fields that can be created by super-cooled electromagnets carrying large current. The strongest electromagnet in the world can provide a magnetic field that that is about one million times that of the Earth. The useful thing about electromagnets is that you can switch off the magnetic field by turning off the current.

Challenge Question: A space probe is being used to investigate the surface of Venus. The temperatures on Venus are extremely high. Why do you think it was sensible for the probe to use a drill machine made from rare-Earth element magnets?

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∆ Fig. 4.6 Neodymium magnets are strong.

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