PSLE Science · Primary 5 and 6
Magnets
Magnets belong to the Interactions theme in PSLE Science, because a magnet acts on other objects through magnetic force, sometimes without touching them. You need to know the properties of magnets, which materials are magnetic, how to tell a magnet from a piece of iron, and how to make a magnet by stroking or by using electricity. A favourite exam question gives the results of bringing bars together and asks which bars are magnets, so this guide works through that reasoning step by step.
What the syllabus expects
- State the properties of magnets: they attract magnetic materials, have two poles and point north to south when hung freely
- Classify materials as magnetic or non-magnetic
- Explain that like poles repel and unlike poles attract, and use repulsion to identify a magnet
- Describe how to make a magnet by the stroke method and by the electrical method
- Explain how the number of coils and the number of batteries affect the strength of an electromagnet
- Describe uses of magnets and electromagnets in daily life
Properties of magnets
A magnet attracts magnetic materials. Iron, steel, nickel and cobalt are magnetic. Most other materials, including copper, aluminium, plastic, wood and glass, are non-magnetic. Not all metals are magnetic, so a magnet cannot pick up an aluminium drink can. Every magnet has two poles, a north pole and a south pole, and its magnetic force is strongest at the poles. That is why more paper clips cling to the ends of a bar magnet than to its middle. A bar magnet that is hung freely so that it can turn comes to rest pointing in the north to south direction. A compass works in this way. Magnetic force can act through non-magnetic materials such as paper, glass, plastic and water, so a magnet can attract a paper clip without touching it.
Attraction and repulsion
When two magnets are brought near each other, like poles repel, so a north pole pushes away a north pole and a south pole pushes away a south pole. Unlike poles attract, so a north pole pulls a south pole. A magnet also attracts a magnetic material, such as an iron bar, at either of its poles. This means attraction cannot prove that an object is a magnet, because an iron bar is attracted to both poles of a magnet. Only repulsion proves that an object is a magnet. To test an unknown bar, bring one end of it near the north pole of a known magnet, then turn it round and bring the other end near. If one end is repelled, the bar is a magnet. If both ends are attracted, it is a magnetic material but not a magnet. If neither end is attracted, it is non-magnetic.
Making magnets
A magnet can be made from a magnetic material such as an iron nail or a steel bar. In the stroke method, stroke the bar from one end to the other many times with one pole of a magnet, always in the same direction, lifting the magnet well away from the bar at the end of each stroke. More strokes make a stronger magnet, up to a limit. In the electrical method, wind a coil of insulated wire around an iron nail and connect the ends of the wire to a battery. When electricity flows through the coil, the nail becomes an electromagnet. More coils of wire, or more batteries in series, make the electromagnet stronger, so it attracts more paper clips. When the electricity is switched off, an iron nail loses most of its magnetism, so it is a temporary magnet. Magnets can lose their magnetism if they are dropped many times, hammered or heated.
Uses of magnets
Magnets are useful in many everyday objects. Fridge doors have magnetic strips that keep the door tightly shut. Compasses use a small magnet that points north to south to help people find directions. Magnetic notice boards, pencil cases and bag clasps use magnets to hold things in place. At recycling centres, magnets separate steel cans from aluminium cans, because only the steel cans are attracted. Electromagnets are useful whenever the magnetism needs to be switched on and off. A crane in a scrapyard uses a large electromagnet to lift iron and steel scrap, then switches off the electricity to drop it in a new place.
Worked example
Ravi had three bars, X, Y and Z, that look the same. He recorded these results. End 1 of X and end 1 of Y pushed each other away. End 2 of X and end 1 of Y pulled towards each other. Both ends of Z were attracted to both ends of X. (a) Which bars are magnets? Explain. (b) Is Z a magnet? Explain. (c) Which of these could Z be made of: iron, copper or plastic?
- Look for repulsion first, because only repulsion proves that a bar is a magnet.
- End 1 of X and end 1 of Y repelled. Repulsion only happens between like poles of two magnets, so X and Y are both magnets.
- The second result fits: end 2 of X is the opposite pole to end 1 of X, so it is unlike end 1 of Y, and unlike poles attract.
- Z was attracted to both ends of magnet X and was never repelled. If Z were a magnet, one of its ends would have been repelled by one of the poles of X.
- So Z is a magnetic material but not a magnet. Copper and plastic are non-magnetic and would not be attracted at all, so Z must be iron.
Answer: (a) X and Y are magnets, because end 1 of X and end 1 of Y repelled. (b) Z is not a magnet: both its ends were attracted to both poles of X and none was repelled, so it is a magnetic material. (c) Iron.
Common mistakes
- Saying all metals are magnetic. Of the common materials, only iron, steel, nickel and cobalt are. Copper, aluminium, brass, gold and silver are not.
- Using attraction to prove that something is a magnet. A magnetic material is attracted too. Only repulsion proves it.
- Stroking a bar back and forth in the stroke method. Stroking in both directions undoes the effect. Stroke in one direction only, with the same pole, lifting the magnet away each time.
- Saying a magnet is strongest in the middle. It is strongest at the poles, which is why paper clips cluster at the ends.
- Saying magnetic force cannot pass through anything. It acts through non-magnetic materials such as paper, glass, plastic and water.
Exam tips
- For 'which bars are magnets' questions, look for the word 'repel' or 'pushed away' in the data first. Without repulsion you cannot conclude that an object is a magnet.
- In electromagnet experiments, name the variable changed (number of coils or number of batteries), the variables kept the same (the same nail, the same battery), and how strength was measured (the number of paper clips attracted).
- Use the words 'magnetic' and 'non-magnetic' to describe materials, not 'metal' and 'not metal'.
Practice questions
1. Easy: Which of these objects will a magnet attract: a steel paper clip, an aluminium can, an iron nail, a copper wire, a plastic ruler?
Show answer
The steel paper clip and the iron nail. Steel and iron are magnetic materials. Aluminium, copper and plastic are non-magnetic.
2. Medium: Lina made an electromagnet with an iron nail and one battery. With 10 coils it attracted 3 paper clips, with 20 coils 6 paper clips and with 30 coils 9 paper clips. What can she conclude, and how many paper clips would you expect with 40 coils?
Show answer
The more coils of wire, the stronger the electromagnet. Each extra 10 coils attracted 3 more paper clips, so 40 coils would attract about 9 + 3 = 12 paper clips.
3. Harder: Explain why a crane in a scrapyard uses an electromagnet instead of a permanent magnet.
Show answer
An electromagnet is magnetic only when electricity flows through its coil. The crane switches the electricity on to lift the iron and steel scrap, then switches it off so the electromagnet loses its magnetism and the scrap drops. A permanent magnet could not easily let go of the scrap.
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