Physics · 4 Electricity and magnetism
4.1 Simple phenomena of magnetism
Before you start
- A force is a push or a pull, measured in newtons.
- That some forces act without contact, such as gravity.
- The idea of a field as a region where a force is felt.
Syllabus objectives covered (11)
- 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
- 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
- Describe a magnetic field as a region in which a magnetic pole experiences a force
- Draw the pattern and direction of magnetic field lines around a bar magnet
- 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 the plotting of magnetic field lines with a compass or iron filings and the use of a compass to determine the direction of the magnetic field
- Describe the uses of permanent magnets and electromagnets
- Explain that magnetic forces are due to interactions between magnetic fields
- Know that the relative strength of a magnetic field is represented by the spacing of the magnetic field lines
Part 1 of 5
Poles, and the one rule that governs them
- Every magnet has two poles: a north pole (N pole) and a south pole (S pole). They always come in pairs — you cannot have one on its own.
- The rule: like poles repel, unlike poles attract. N and N push apart, S and S push apart, N and S pull together.
- But there is a second, quieter rule that questions are built on. A magnet also attracts an unmagnetised magnetic material — a paperclip, a nail, a piece of unmagnetised iron. Either pole does this.
- Put those two together and you get the standard exam reasoning: repulsion is the only sure test for a magnet. If two objects repel, both must be magnetised.
- Attraction proves nothing on its own, because a magnet attracts both an opposite pole and an unmagnetised piece of iron. That distinction is worth marks almost every year.
- So if you are asked how to tell whether a bar is magnetised, the answer is: bring a known pole near each end. If it repels at one end, the bar is a magnet.
Part 2 of 5
Magnetic and non-magnetic materials
- A magnetic material is one that is attracted by a magnet and can be magnetised. Iron, steel, cobalt and nickel are the ones to know.
- A non-magnetic material is not attracted and cannot be magnetised — copper, aluminium, plastic, wood, glass.
- Notice that most metals are non-magnetic. 'It is a metal, so it is magnetic' is a common and costly assumption; aluminium and copper are both metals and neither is attracted.
- Induced magnetism is what happens when a magnetic material is placed in a magnetic field: it becomes a magnet itself, temporarily, without ever touching the original magnet.
- The induced poles always arrange themselves to give attraction. The end nearest the magnet becomes the opposite pole, so the two pull together.
- This is why a chain of paperclips can hang from one magnet — each clip is magnetised by the one above it. Remove the magnet and they all fall apart, because the induced magnetism is temporary.
Part 3 of 5
Temporary and permanent magnets
- Soft iron is easy to magnetise and easy to demagnetise. It becomes strongly magnetic in a field and loses almost all of it when the field is removed — a temporary magnet.
- Steel is hard to magnetise, but once magnetised it keeps its magnetism. That makes it a permanent magnet.
- 'Soft' and 'hard' here describe magnetic behaviour, not how easily you could dent them. Soft iron is not physically soft.
- The two properties decide the uses, so learn them as a pair rather than as a list.
- Permanent magnets (steel) are used where the magnetism must stay: compass needles, fridge magnets, loudspeakers, and the magnets in motors and generators.
- Electromagnets (soft iron core) are used where you need to switch the magnetism on and off: a scrapyard crane, an electric bell, a relay, and the door lock on a washing machine.
- The scrapyard crane is the clearest example of why. It must pick up steel scrap and then let it go — a permanent magnet could do the first half of that job and never the second.
Part 4 of 5
Magnetic fields
- A magnetic field is a region in which a magnetic pole experiences a force. That is the definition to write — a region, and what happens in it.
- The field has a direction at every point, defined as the direction of the force on an N pole placed at that point.
- This is why field lines run from N to S outside the magnet: a free N pole would be pushed away from the magnet's N pole and pulled towards its S pole.
- Field lines are a way of drawing something invisible, and they carry two pieces of information. Their direction gives the direction of the field; their spacing gives its strength.
- Lines close together mean a strong field, lines far apart mean a weak one. That is why the lines bunch tightly at the poles — the field is strongest there.
- Field lines never cross. If they did, the field would have two directions at one point, which is meaningless.
- For the Extended explanation of why magnets push and pull without touching: each magnet has its own field, and the force arises from the interaction between the two fields. The magnets never need to touch because their fields overlap.
Part 5 of 5
Drawing and plotting the field
- The field around a bar magnet is a set of curved loops leaving the N pole, curving round the outside, and entering the S pole.
- Four things earn the marks when you draw it: arrows on every line, arrows pointing N to S outside the magnet, lines that never cross, and lines closest together at the poles.
Out of N, into S, tightest at the poles. - Plotting with iron filings. Place the magnet under a sheet of paper, sprinkle iron filings on top, and tap the paper gently. Each filing becomes induced-magnetised and lines up along the field.
- Iron filings show the shape of the field beautifully, and its strength through how densely they gather — but they show no direction. There are no arrows in a pile of filings.
- Plotting with a compass. Place a small plotting compass near the magnet, mark a dot at each end of the needle, move the compass so its tail sits on the previous head, and repeat. Join the dots into a smooth curve.
- The compass needle points along the field, so this method gives direction as well as shape. If a question asks how to find the direction of a field, the answer must be a compass.
- A compass works because its needle is itself a small magnet, free to turn, so it lines up with the field it sits in.
Worked examples
Example 1. A student has two identical steel bars. One is magnetised and one is not. Describe how the student could determine which is which, using only the two bars.
Show the working
- Start from the key fact: a magnet attracts unmagnetised steel, so simply putting the ends together gives attraction whichever way round they are. That test cannot separate them.
- Use the fact that a bar magnet's field is strong at its poles and weak at its middle.
- Hold the end of bar A against the middle of bar B, forming a T. If A is the magnet, its strong pole is touching B and you feel a clear attraction.
- Now swap them: hold the end of B against the middle of A. If A is the magnet, B's end is now touching A's weak centre, so there is little or no attraction.
- So the bar whose end produces strong attraction against the other's middle is the magnet. The unmagnetised bar gives a noticeably weaker result when tested the same way.
Answer: Touch the end of each bar in turn against the middle of the other. A magnet is strongest at its poles and weakest at its centre, so the bar whose end gives a strong attraction against the other's middle is the magnetised one.
Example 2. Explain why the core of an electromagnet in a scrapyard crane is made of soft iron rather than steel.
Show the working
- Say what the crane has to do: pick the scrap up, and then release it.
- Soft iron is easily magnetised, so it becomes strongly magnetic as soon as the current is switched on.
- Crucially, it loses its magnetism almost entirely when the current is switched off, so the scrap is released.
- Steel would retain its magnetism after the current stopped, so the crane could never let go of the load.
Answer: Soft iron is easily magnetised and easily demagnetised, so the crane becomes strongly magnetic when the current is on and loses its magnetism when it is switched off, releasing the load. Steel would stay magnetised and the scrap could not be dropped.
Example 3. Describe how to plot the magnetic field pattern around a bar magnet using a plotting compass, and state how the diagram shows where the field is strongest.
Show the working
- Place the magnet on a sheet of paper and draw round it so its position is recorded.
- Put a plotting compass near one pole and mark a dot at each end of the needle.
- Move the compass so that its tail sits on the dot the head just made, and mark the new head position. Repeat all the way round.
- Join the dots into a smooth curve and add an arrow showing the direction the compass needle pointed — from N to S outside the magnet.
- Repeat from several starting points to build the whole pattern. Where the resulting lines are closest together, the field is strongest, which is at the poles.
Answer: Mark the magnet's position, then step a plotting compass round it, marking each end of the needle and joining the dots into smooth lines with arrows pointing from N to S. The field is strongest where the lines are closest together, at the poles.
Where people go wrong
Mistake: Using attraction as proof that an object is a magnet.
Instead: Only repulsion proves it. A magnet attracts unmagnetised magnetic materials as well as opposite poles.
This is the single most examined point in the subtopic, and the reasoning is what earns the mark, not the answer.
Mistake: Assuming all metals are magnetic.
Instead: Only iron, steel, cobalt and nickel among common materials. Copper and aluminium are not.
Magnetism depends on the structure of the material, not on being a metal.
Mistake: Drawing field lines with no arrows, or with arrows pointing S to N outside the magnet.
Instead: Arrows on every line, running N to S outside the magnet.
The direction is defined by the force on an N pole, which is pushed away from N. A field line without an arrow is only half the information.
Mistake: Saying iron filings show the direction of the field.
Instead: Filings show the shape and relative strength, but not direction. Use a compass for direction.
A filing is magnetised along its length and has no way to indicate which way round it lies.
Mistake: Thinking 'soft' iron means physically soft.
Instead: It means magnetically soft — easily magnetised and easily demagnetised.
The words describe how readily the material gains and loses magnetism, nothing about hardness.
Check yourself
- State the rule for the force between magnetic poles, and explain why attraction is not a reliable test for a magnet.
Answer
Like poles repel, unlike poles attract. Attraction is unreliable because a magnet also attracts unmagnetised magnetic materials; only repulsion proves both objects are magnets. - What is induced magnetism?
Answer
A magnetic material becomes a magnet when placed in a magnetic field, with the nearer end becoming the opposite pole so that attraction results. - Give two differences between soft iron and steel as magnetic materials.
Answer
Soft iron is easily magnetised and easily demagnetised; steel is harder to magnetise but retains its magnetism. - Define a magnetic field and state how its direction is defined.
Answer
A region in which a magnetic pole experiences a force. Its direction at a point is the direction of the force on an N pole placed there. - How does a field-line diagram show where the field is strongest?
Answer
By the spacing of the lines — the closer together they are, the stronger the field. - Why can a plotting compass show the direction of a magnetic field when iron filings cannot?
Answer
The compass needle is itself a magnet free to rotate, so it turns to point along the field. Iron filings only line up along the field and have no fixed polarity to show. - Give one use of a permanent magnet and one use of an electromagnet, and say why each is suited to its job.
Answer
A compass needle must stay magnetised, so it is a permanent magnet. A scrapyard crane must pick up and release loads, so it uses an electromagnet that can be switched off. - Explain why two magnets can exert forces on each other without touching.
Answer
Each magnet produces a magnetic field around it, and the force arises from the interaction between the two fields where they overlap.
Exam questions
Try each part first. Open the mark scheme after you have written your answer.
Question 1. A student is given three identical-looking bars: one is a permanent magnet, one is unmagnetised steel, and one is copper.
(a) Describe how the student could identify the copper bar. 2 marks No calculator
Mark scheme
- B1 bring a known magnet close to each bar in turn
- B1 the copper bar is not attracted, because copper is not a magnetic material
(b) Explain why attraction between two of the bars does not prove that both are magnets. 2 marks No calculator
Mark scheme
- B1 a magnet attracts unmagnetised magnetic materials as well as opposite poles
- B1 so attraction could be between a magnet and unmagnetised steel; only repulsion proves both are magnets
Question 2. A bar magnet is placed on a sheet of paper. A student sprinkles iron filings around it and taps the paper gently.
(a) Sketch the magnetic field pattern around the bar magnet, including the direction of the field. 3 marks No calculator
Mark scheme
- B1 curved lines leaving one pole and entering the other, not crossing
- B1 arrows on the lines pointing from N to S outside the magnet
- B1 lines closest together at the poles
(b) State what the spacing of the field lines represents. 1 mark No calculator
Mark scheme
- B1 the relative strength of the field; closer lines mean a stronger field
(c) Explain why the student would need a plotting compass rather than iron filings to determine the direction of the field. 2 marks No calculator
Mark scheme
- B1 iron filings show only the shape of the field, not its direction
- B1 the compass needle is a small magnet that turns to point along the field, showing direction, owtte
Question 3. An electric bell uses an electromagnet with a soft-iron core.
(a) Explain why the core is made of soft iron and not steel. 2 marks No calculator
Mark scheme
- B1 soft iron is easily magnetised when the current flows
- B1 and loses its magnetism when the current stops, which is needed for the bell to work repeatedly; steel would stay magnetised
(b) Give one use of a permanent magnet in which its ability to retain magnetism is essential. 1 mark No calculator
Mark scheme
- B1 any one from: compass needle / fridge magnet / loudspeaker / magnet in a motor or generator
Words to know
- Magnetic field
- A region in which a magnetic pole experiences a force.
- Induced magnetism
- The temporary magnetism a magnetic material gains when placed in a magnetic field.
- Magnetically soft
- Easily magnetised and easily demagnetised, like soft iron.
- Magnetically hard
- Difficult to magnetise but retaining magnetism once magnetised, like steel.
- Field line
- A line showing the direction of a magnetic field, drawn from N to S outside a magnet, whose spacing shows the field's strength.
- Electromagnet
- A coil carrying a current, usually with a soft-iron core, which is magnetic only while the current flows.