Edexcel IGCSE Physics 6(b) Magnetism — Notes
Written for Pearson Edexcel International GCSE Physics (4PH1), Science Double Award (4SD0) and Science Single Award (4SS0), linear and modular. This page also covers the 6(a) units for Topic 6.
Which parts do I need?
Single Award candidates can skip every section tagged Double · Physics. On this page that is attraction and repulsion between magnets, magnetically hard and soft materials, and induced magnetism. Everyone needs the units, field lines, the field-pattern practical and the uniform field. Nothing on this page is Physics-only.
What this page covers
| Spec | Modular no. | You need to be able to… | Route |
|---|---|---|---|
| 6.1 | 7.1 | use the ampere, volt and watt as units | Single · Double · Physics |
| 6.2 | 7.2 | know that magnets attract or repel each other, and attract magnetic materials | Double · Physics |
| 6.3 | 7.3 | describe how magnetically hard and soft materials behave | Double · Physics |
| 6.4 | 7.4 | explain what a magnetic field line represents | Single · Double · Physics |
| 6.5 | 7.5 | know that some materials become magnets when placed in a magnetic field | Double · Physics |
| 6.6 | 7.6 | map the field around one bar magnet and between two bar magnets (practical) | Single · Double · Physics |
| 6.7 | 7.7 | describe how to set up a uniform field with two permanent magnets | Single · Double · Physics |
Modular candidates: magnetism is Topic 7 in the modular specifications, so these are statements 7.1–7.7 of 4XPH1 Unit 2 (4WPH2) and 4XSD1 Physics Unit 6 (4WSD6).
In one minute
- Topic 6 uses the ampere (A) for current, the volt (V) for voltage and the watt (W) for power.
- Like poles repel, unlike poles attract. A magnet attracts magnetic materials such as iron, steel, nickel and cobalt. Double · Physics
- Hard magnetic materials (steel) keep their magnetism; soft ones (iron) magnetise easily and lose it easily. Double · Physics
- A field line shows the direction of the force on a north pole. Outside a magnet the lines go from N to S; closer lines mean a stronger field.
- A piece of iron in a magnetic field becomes an induced magnet, and is attracted. Double · Physics
- Between a flat N pole and a flat S pole facing each other, the field is uniform: straight, parallel, equally spaced lines.
Which units do I need for Topic 6? Single · Double · Physics
Topic 6 links magnetism with electric circuits, so the units are the electrical ones you met in Topic 2.
| Quantity | Unit | Symbol | Route |
|---|---|---|---|
| current | ampere | A | All |
| voltage (potential difference) | volt | V | All |
| power | watt | W | All |
- Convert prefixes before you substitute: 1 mA = 0.001 A, 1 kV = 1000 V, 1 kW = 1000 W, 1 MW = 1 000 000 W.
- The three units are linked by power = current × voltage (\(P=I\times V\)), which is on the Equation List and is taught in 2(b). One watt is one ampere multiplied by one volt.
(a) A small motor takes a current of 450 mA from a 12 V battery. Calculate the power supplied to the motor.
(b) A 1.5 kW heater is connected to a 230 V supply. Calculate the current in the heater.
(a) Convert first: 450 mA = 0.45 A. \(P=I\times V=0.45\times12=\) 5.4 W
(b) 1.5 kW = 1500 W. \(I=\dfrac{P}{V}=\dfrac{1500}{230}=6.52=\) 6.5 A
Common error in (a): substituting 450 and getting 5400 W — a power-of-ten error from not converting mA to A.
How do magnets attract and repel? Double · Physics
Not required for Single Award (4SS0).
Every magnet has two poles, a north (N) pole and a south (S) pole. The magnetic forces are strongest at the poles.
- Like poles repel (N and N, or S and S).
- Unlike poles attract (N and S).
- A magnet attracts magnetic materials — iron, steel, nickel and cobalt — whichever pole is brought near. It never repels them.
- Most metals (copper, aluminium, brass, gold) and all non-metals are not magnetic materials, so a magnet has no effect on them.
What are magnetically hard and soft materials? Double · Physics
Not required for Single Award (4SS0).
| Magnetically hard | Magnetically soft | |
|---|---|---|
| Example | steel | iron |
| Becoming magnetised | hard to magnetise | easy to magnetise |
| When the magnetising field is removed | keeps its magnetism — becomes a permanent magnet | loses its magnetism quickly — a temporary magnet |
| Typical use | permanent magnets, such as compass needles and fridge-door magnets | things that must switch their magnetism on and off, such as the core of an electromagnet |
“Hard” and “soft” here describe magnetic behaviour, not how hard the metal feels to the touch.
A student has two rods that look identical. One is iron and the other is steel. Describe how she could use a strong magnet and a box of iron paper clips to find out which rod is steel.
- Hold each rod against (or stroke it with) the strong magnet in the same way for the same time, so both become magnetised.
- Move the magnet well away.
- Dip the end of each rod into the paper clips and count how many clips each one lifts.
- The rod that still lifts clips (or lifts more) has kept its magnetism, so it is the magnetically hard material: steel. The iron rod lifts few or none because it loses its magnetism.
A fair test needs the same magnet, the same time in contact and the same test for both rods.
What is a magnetic field line? Single · Double · Physics
- Direction: arrows on field lines point from N to S outside the magnet, because a north pole is pushed away from N and pulled towards S.
- Strength: the closer together the lines, the stronger the field. The field of a bar magnet is strongest at the poles and gets weaker further away.
- Field lines never cross. At any point the force on a north pole has only one direction.
- A plotting compass is a tiny magnet; its needle lines up along the field line, and its N end points along the direction of the field.
Fields between two magnets
- Unlike poles facing (N–S): lines join from the N pole of one magnet to the S pole of the other, crowded together in the gap.
- Like poles facing (N–N or S–S): lines from the two magnets push away from each other. Midway there is a neutral point, where the two fields cancel and a compass would not point in any particular direction.
- For two S poles facing, the pattern has the same shape as the N–N one, but every arrow points the other way (towards the S poles).
A plotting compass is placed at point X, the neutral point between two bar magnets with their north poles facing. Explain why the needle does not settle in a particular direction.
- At X the field from the left-hand magnet points to the right (away from its N pole).
- The field from the right-hand magnet points to the left with the same strength.
- The two fields cancel, so the resultant field at X is zero and there is no magnetic force to turn the needle.
What is induced magnetism? Double · Physics
Not required for Single Award (4SS0).
When a piece of magnetic material, such as iron, is placed in a magnetic field it becomes a magnet itself. This is induced magnetism.
- The end of the material nearest the magnet always becomes the opposite pole, so the induced magnet is always attracted. This is why a magnet attracts magnetic materials with either pole.
- Induced magnets can magnetise other pieces in turn — this is how a chain of paper clips hangs from a magnet.
- A soft material (iron) loses most of its induced magnetism as soon as it is removed from the field: the chain of clips falls apart. A hard material (steel) keeps some and becomes a permanent magnet.
A student brings the N pole of a bar magnet towards each end of an unlabelled bar, X. Both ends of X are attracted. She concludes that X is a magnet. Explain why her conclusion is wrong and what X must be.
- If X were a magnet, one of its ends would be an N pole, and that end would be repelled by the N pole of the bar magnet.
- Both ends are attracted because the magnet induces an S pole in whichever end is nearest, so attraction happens every time.
- X is therefore an unmagnetised piece of magnetic material (for example iron or steel), not a magnet.
Practical: how do you investigate magnetic field patterns? Single · Double · Physics
You need to be able to describe how to show the shape and direction of the field around one bar magnet, and between two bar magnets. There are two methods: plotting compasses (shape and direction) and iron filings (shape only, quickly).
Mapping the field pattern of a bar magnet and of two bar magnets
- Aim: find the shape and direction of the magnetic field around a bar magnet, and between two bar magnets with like and with unlike poles facing.
- Apparatus: two bar magnets, sheet of plain paper, pencil, several small plotting compasses, iron filings in a shaker, a clear plastic sheet or thin card.
- Independent variable: the arrangement of the magnets (one magnet; N facing S; N facing N).
- Dependent variable: the pattern of field lines (shape, direction, where lines are closest together).
- Control variables: the same magnets, the same gap between the magnets, nothing else magnetic (steel rulers, other magnets) nearby.
Method A — plotting compasses (shows direction)
- Place the magnet in the middle of the paper and draw round it. Label the N and S ends.
- Put a plotting compass next to the N pole. Mark a dot on the paper just beyond the N end of the needle.
- Move the compass so the S end (tail) of its needle is over that dot, and mark a new dot beyond the N end.
- Repeat until the line reaches the S pole or the edge of the paper. Join the dots with a smooth curve and add an arrow pointing the way the compass needle pointed (N to S).
- Start again from a different point near the N pole to draw more lines. For two magnets, place them on the paper with the gap marked and repeat.
Method B — iron filings (shows shape)
- Lay the plastic sheet or card over the magnet (or magnets).
- Sprinkle iron filings thinly and evenly from a height of about 20 cm, then tap the sheet gently.
- The filings become induced magnets and line up along the field lines. Sketch or photograph the pattern.
Processing and evaluation
- Look for where the lines are closest together (strongest field: at the poles) and for any neutral point between like poles.
- Iron filings show the shape but not the direction; add the direction with one plotting compass.
- Use small compasses and mark the dots close to the needle — a large compass averages the field over a wide area and gives a less accurate line.
- Keep other magnets, steel objects and current-carrying wires away; they distort the pattern.
- Safety: keep iron filings away from eyes (wear eye protection and do not rub your eyes); keep the filings off the magnet itself by using the plastic sheet; wash hands afterwards.
How do you make a uniform magnetic field? Single · Double · Physics
- Place two permanent magnets so that a north pole faces a south pole (unlike poles — like poles would give a neutral point instead).
- Use magnets with wide, flat pole faces, held parallel to each other.
- Keep the gap small compared with the width of the faces. The field is uniform in the middle of the gap; near the edges the lines bulge outwards.
- Draw the uniform region as straight, parallel lines with equal spacing and arrows from N to S.
Uniform fields are used whenever a steady, predictable force is wanted, for example between the poles of the magnet in an electric motor (see 6(c) Electromagnetism).
Exam tips for 6(b)
When you draw a field pattern, put at least one arrow on each line, pointing from N to S. Lines without arrows, lines that cross, or lines that start in mid-air usually lose marks.
Field lines should start and finish on the magnet, mostly at or near the poles. Draw them as smooth curves, symmetrical about the magnet, and closer together near the poles.
To describe a uniform field, say that the lines are straight, parallel and equally spaced. To describe how to make one, say unlike poles facing, flat and parallel faces, and a small gap.
A magnet attracts iron, steel, nickel and cobalt, not “metals”. Copper and aluminium are metals but are not attracted. Answers that just say “metal” are not credited. Double · Physics
For hard and soft materials, make the comparison explicit: “iron is easier to magnetise and loses its magnetism when the field is removed; steel is harder to magnetise but keeps it”. Naming the materials alone earns little. Double · Physics
Common misconceptions
| Misconception | Why it is wrong | Correct idea |
|---|---|---|
| Magnets attract all metals. (Double · Physics) | Copper, aluminium and most other metals are not magnetic materials. | Only magnetic materials (iron, steel, nickel, cobalt) are attracted. |
| If an object is attracted to a magnet, it must be a magnet. (Double · Physics) | Induced magnetism means any magnetic material is attracted. | Only repulsion proves that an object is a magnet. |
| Field lines go from S to N outside the magnet. | The direction is the way a north pole would be pushed. | Outside a magnet, field lines go from N to S. |
| Field lines are real strings of force; between them there is no field. | Lines are a model; we draw a few of them. | The field exists everywhere around the magnet; line spacing shows its strength. |
| A “soft” magnetic material is physically soft. (Double · Physics) | The words describe magnetic behaviour. | Soft = easily magnetised and demagnetised (iron); hard = keeps its magnetism (steel). |
Key facts
| Fact | Route |
|---|---|
| Current in A, voltage in V, power in W; \(P=I\times V\) links them (on the Equation List, all papers). | All |
| Like poles repel, unlike poles attract; magnets attract iron, steel, nickel and cobalt. | Double · Physics |
| Steel is magnetically hard (permanent magnets); iron is magnetically soft (temporary magnets, electromagnet cores). | Double · Physics |
| A field line shows the direction of the force on a north pole: N to S outside a magnet; closer lines mean a stronger field; lines never cross. | All |
| A magnetic material in a magnetic field becomes an induced magnet, with the opposite pole nearest the magnet. | Double · Physics |
| Field patterns are mapped with plotting compasses (direction) or iron filings (shape). | All |
| Uniform field: unlike flat poles facing, parallel, with a small gap — straight, parallel, equally spaced lines. | All |
Check your understanding
1. State the unit of power and the unit of voltage, with their symbols. All
Power: watt (W). Voltage: volt (V).
2. Write 0.75 kW in watts and 20 mA in amperes. All
0.75 kW = 750 W; 20 mA = 0.020 A.
3. State two things the field lines around a bar magnet tell you about the field. All
The direction of the field (the direction of the force on a north pole, shown by the arrows, N to S) and the strength of the field (stronger where the lines are closer together, at the poles).
4. Describe how to use a plotting compass to draw one field line around a bar magnet. All
- Place the compass near the N pole and mark a dot at the N end of the needle.
- Move the compass so the tail of the needle is over the dot; mark a new dot.
- Repeat until the S pole is reached; join the dots with a smooth line and add an arrow from N to S.
5. Describe how to arrange two magnets to produce a uniform field, and describe the field lines you would draw. All
Flat N pole facing flat S pole, faces parallel, with a small gap. Lines in the gap: straight, parallel, equally spaced, with arrows from N to S.
6. A cooking pan is not attracted to a magnet. A spoon is attracted to both poles. Suggest what each is made of. Double · Physics
Pan: a non-magnetic material, for example aluminium or copper. Spoon: a magnetic material that is not magnetised, for example steel — the magnet induces magnetism in it, so it is attracted by either pole.
7. Explain why the core of an electromagnet that lifts and drops scrap is made of iron, not steel. Double · Physics
Iron is magnetically soft: it is magnetised easily when the current is on and loses its magnetism when the current is switched off, so the scrap is released. Steel would stay magnetised and keep holding the scrap.
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