OxfordAQA International GCSE Physics: Magnets, magnetic fields and electromagnets (9203) – Revision Notes

These notes condense section 3.5.2 (Magnetism and electromagnetism) of the OxfordAQA International GCSE Physics (9203) specification, for exams May/June 2018 onwards. For full explanations, start with the study guide. When you are ready, try the practice questions.

More links: course hub, printable checklist, exam preparation and the 10-minute diagnostics.

Key definitions

Term	Meaning
Magnetic pole	The place on a magnet where the force is strongest
Non-contact force	A force acting across a gap, such as magnet on magnet
Magnetic field	The space around a magnet where a second magnet or a piece of magnetic material experiences a force
Magnetic materials	Iron, steel, cobalt, nickel
Uniform field	Field lines parallel, so the same strength and direction everywhere
Induced magnet	A material that becomes a magnet in a magnetic field and loses most or all of it quickly when removed
Solenoid	A wire shaped into a coil
Electromagnet	A solenoid with an iron core

Rules for forces between poles

• Like poles repel. Unlike poles attract.
• Induced magnetism always attracts.
• Only repulsion proves that an object is a magnet.

Method: predict an interaction

1. Name the pole nearest the gap on each magnet.
2. Compare them. Same name means repel. Different means attract.
3. State the direction each magnet is pushed or pulled.

Field line rules

Rule	Detail
Direction	Out of north, into south
Spacing	Closer together means a stronger field
Crossing	Lines never cross
Uniform field	Parallel, equally spaced lines with arrows all one way

Patterns to sketch

Arrangement	What the lines look like
One bar magnet	Curves from north to south, crowded at the poles
Unlike poles facing	Straight, nearly parallel lines across the gap
Like poles facing	Lines curve apart, with a point of zero field between the poles
Flat opposite poles, close together	Uniform field in the gap

Induced magnetism: explain in four lines

1. The magnet’s field magnetises the iron.
2. The nearer end becomes the opposite pole to the magnet’s pole.
3. Opposite poles attract, so the force is attraction.
4. Away from the magnet, the iron loses most or all of its magnetism quickly.

Earth’s field and the compass

• The Earth has a magnetic field, most concentrated at the magnetic north and south poles.
• A plotting compass is a small magnet that turns freely. It lines up with the field and shows its direction.
• Its north-seeking end points towards the Earth’s north, so it helps navigation.
• Magnets, iron, steel and currents nearby will change the reading.

Method: plot a field line

1. Place the compass near one pole and mark both ends of the needle.
2. Move it so the tail sits on the last head dot.
3. Repeat until you reach the other pole.
4. Join the dots and add an arrow.

Field around a straight wire

Fact	Detail
Shape	Concentric circles in a plane perpendicular to the wire
Distance	Stronger closer to the wire
Current size	Larger current, stronger field
Current direction	Reversing the current reverses the field lines

Solenoid and electromagnet

Fact	Detail
Solenoid	Increases the field strength compared with a straight wire
Field inside	Strong and uniform
Field outside	Similar shape to a bar magnet
Iron core	Increases field strength
Switching	The magnet can be switched off, because the core loses its magnetism quickly

Ways to strengthen an electromagnet: a larger current, more turns, an iron core.

Uses: scrapyard crane (lifts then releases scrap), electric bell (the coil pulls the armature, which breaks the circuit, then a spring resets it), relay (a small current in the coil closes a switch in a second circuit).

Required practical in steps

1. Wind insulated wire on an iron nail and connect to a low-voltage dc supply, ammeter, variable resistor and switch.
2. Change one factor only: turns, current or core.
3. Count the paper clips lifted. Do at least three trials.
4. Calculate a mean for each setting.
5. Plot a graph or describe the pattern.
6. Switch off between readings so the coil does not overheat.

Worked reminder: a core comparison

Without an iron core, three trials lift 2, 3 and 2 clips. With the core, they lift 9, 10 and 9.

• Mean without core = (2 + 3 + 2) / 3 = 2.33 clips
• Mean with core = (9 + 10 + 9) / 3 = 9.33 clips
• Ratio = 9.33 / 2.33 = 4.0
• Percentage increase = (9.33 − 2.33) / 2.33 × 100 = 300%

The core makes the electromagnet four times as strong by this measure.

Worked reminder: an anomaly

Four trials give 6, 6, 11 and 7 clips. The 11 is much further from the others, so it is an anomaly. Check the trial for an error, or repeat it. Without it, the mean is (6 + 6 + 7) / 3 = 6.33 clips.

Worked reminder: turns and field strength

At 1.0 A, the mean clips lifted are 5.33 for 20 turns and 10.33 for 40 turns.

• Ratio = 10.33 / 5.33 = 1.94
• Doubling the turns almost doubles the clips, so the strength is close to proportional to the number of turns.

Write “close to” and not “exactly”, because the clip count is a whole number and each reading carries some scatter.

Show that a wire has a field

1. Pass a vertical wire through a card and place plotting compasses on the card around it.
2. With no current, all the needles point the same way, along the Earth’s field.
3. Switch the current on. The needles turn to follow circles round the wire.
4. Reverse the current. The needles turn the opposite way.
5. Increase the current. The turns get bigger. Move a compass further away and its turn gets smaller.

The electric bell in order

1. Press the switch, so current flows through the coil.
2. The coil and iron core become an electromagnet.
3. The iron armature is pulled towards the core and the hammer hits the gong.
4. The armature movement opens the contact, so the current stops.
5. The core loses its magnetism and a spring pulls the armature back.
6. The contact closes again and the cycle repeats while the switch is held down.

Must-know distinctions

Compare	Point
Permanent and induced magnet	A permanent magnet keeps its magnetism and can repel. An induced magnet loses it quickly and only attracts
Magnet and magnetic material	Both are attracted by a magnet, but only a magnet can be repelled
Field of a magnet and of a wire	A magnet’s lines run between poles. A straight wire’s lines are circles round it
Field lines and field strength	Direction comes from the arrows. Strength comes from how close the lines are

Quick self-test

1. What are the four magnetic materials?
2. A south pole is brought near the south pole of another magnet. What happens?
3. How do you know a bar of metal is a magnet and not just iron?
4. Describe the field lines in a uniform field.
5. A nail is attracted to a magnet. Which pole forms at the end nearest the magnet’s north pole?
6. What is the shape of the field around a straight current-carrying wire?
7. State two ways of making the field around a straight wire stronger.
8. What happens to the field direction if the current is reversed?
9. Name the three changes that make an electromagnet stronger.
10. A relay uses a 0.5 A coil current to switch a second circuit. Is this the same circuit? Give the reason.
11. Two trials lifted 12 and 14 clips. Calculate the mean.
12. A compass reading falls from 36 degrees to 27 degrees as the compass moves away from a wire. Calculate the percentage decrease.

Answers

1. Iron, steel, cobalt and nickel.
2. They repel, because like poles repel.
3. Test for repulsion. A magnet will repel a known pole. Iron is only attracted.
4. Parallel lines, equally spaced and all pointing the same way.
5. A south pole.
6. Concentric circles in a plane perpendicular to the wire.
7. Increase the current, or move closer to the wire.
8. The direction reverses.
9. A larger current, more turns and an iron core.
10. No. A relay uses a small current in the coil circuit to close a separate circuit that can carry a larger current.
11. (12 + 14) / 2 = 13 clips.
12. (36 − 27) / 36 × 100 = 25%.

Where marks are usually lost

• Saying “magnets attract metal” instead of naming iron, steel, cobalt or nickel.
• Drawing field lines without arrows, or with arrows pointing into the north pole.
• Drawing lines that cross, or lines that stop in the air.
• Leaving out the word “unlike” when explaining attraction.
• Explaining induced magnetism with no mention of the nearer end taking the opposite pole.
• Writing that the wire’s field is “around” the wire without saying circles and perpendicular.
• Giving “more power” instead of “larger current” or “more turns”.
• Counting clips once and not repeating the trial.
• Forgetting to control the other variables.

Official syllabus

OxfordAQA International GCSE Physics (9203) specification, for exams May/June 2018 onwards, published by OxfordAQA. This page covers section 3.5.2.