This note covers magnets and magnetic fields: which materials are magnetic, how poles attract and repel, how magnets are made and unmade, and how to show the shape and direction of a field with field lines and a plotting compass. Magnets are used in motors, speakers, fridge door seals, compasses and many other devices.
Key ideas
Poles and forces. A magnet has two poles: a north-seeking pole (N) and a south-seeking pole (S). The poles are where the magnetic force is strongest.
- Like poles repel (N and N, or S and S).
- Unlike poles attract (N and S).
The only sure test for a magnet is repulsion. If one end of an object is repelled by the north pole of a known magnet, then the object is a magnet. Attraction alone is not enough, because an unmagnetised piece of iron is also attracted to either pole.
Magnetic materials. Iron, steel, nickel and cobalt are magnetic materials. They are attracted by magnets and can be magnetised. Copper, aluminium, plastic, wood and glass are non-magnetic. Brass is non-magnetic because it is made from copper and zinc.
Induced magnetism. A piece of magnetic material (such as iron) placed near or touching a magnet becomes a magnet itself. It is a magnet only while it is close. The end nearest the magnet gets the opposite pole, so the force is attractive. This is why an iron nail attracted by a magnet can in turn pick up a paper clip.
Soft and hard magnetic materials.
- Soft magnetic materials (such as iron) are easy to magnetise and lose their magnetism easily. They are used for the cores of electromagnets and transformers.
- Hard magnetic materials (such as steel) are harder to magnetise but keep their magnetism. They are used for permanent magnets.
Making and removing magnetism.
- Magnetising by stroking: stroke a piece of steel many times in the same direction with one pole of a magnet. The steel becomes a magnet. The end where the stroke finishes gets the opposite pole from the stroking pole.
- Magnetising with a coil: place the steel inside a coil of wire (a solenoid) and pass a direct current through it.
- Demagnetising: hammer or heat the magnet, or place it in a coil carrying alternating current and slowly pull it out of the coil far away. Each method shakes the magnetic pattern into disorder.
Magnetic fields. A magnetic field is a region where a magnetic material or a magnet experiences a force. The field is represented by field lines.
- Field lines point from north to south outside the magnet. The direction of the field at a point is the direction of the force on the north pole of a small compass placed there.
- The lines are closer together where the field is stronger. The field is strongest at the poles.
- Field lines never cross.
Typical patterns:
- A bar magnet: lines leave the N pole, curve round and enter the S pole.
- Two unlike poles facing each other: lines run straight from N to S across the gap, and the field in the gap is nearly uniform (parallel, evenly spaced lines).
- Two like poles facing each other: the lines bend away from each other, and there is a point in between where the field is zero.
Plotting a field. Place a plotting compass near the magnet and mark the position of both ends of the needle with dots. Move the compass so that its tail sits on the last dot, mark the new position of the head, and repeat. Join the dots to draw one field line. Start at different points to draw more lines. Alternatively, sprinkle iron filings on a card over the magnet and tap it gently. The filings line up along the field and show its shape, but not its direction.
A compass needle is a small magnet. It lines up with any magnetic field it is in. In the absence of other magnets, its north pole points towards the Earth's geographic north, because the Earth has its own magnetic field, like a giant bar magnet.
Definitions and formulae
| Term or formula | Meaning / when to use |
|---|---|
| Magnetic pole | The place on a magnet where the force is strongest. Every magnet has a N and a S pole. |
| Law of magnetic poles | Like poles repel. Unlike poles attract. |
| Magnetic material | Material attracted by a magnet and that can be magnetised: iron, steel, nickel, cobalt. |
| Induced magnetism | A magnetic material becomes a temporary magnet when placed in a magnetic field. |
| Soft / hard magnetic material | Easily magnetised and demagnetised (iron) / keeps its magnetism (steel). |
| Magnetic field | Region in which a magnet or magnetic material experiences a force. |
| Field line | Line showing field direction: from N to S, closer together where the field is stronger. |
Worked examples
Example 1 A student has a steel bar X and a known bar magnet. The north pole of the magnet is brought near end A of X. End A is repelled. Then the north pole is brought near end B of X, and B is attracted. What can the student conclude about X and its ends?
Repulsion only occurs between like poles. End A is repelled by a north pole, so end A is a north pole and X is a magnet.
A magnet has two opposite poles, so end B is a south pole. It is attracted to the north pole of the known magnet, which agrees.
Answer: X is a magnet. End A is a north pole and end B is a south pole.
Example 2 An iron nail is held with its head touching the north pole of a bar magnet. Its point is brought near some iron filings, which are picked up. Explain what happens and what happens when the magnet is pulled away.
The iron is a magnetic material, so it becomes magnetised by induction while it touches the magnet.
The head (nearest the north pole of the magnet) becomes a south pole. The point therefore becomes a north pole.
The north pole at the point induces opposite poles in the filings, so they are attracted to it.
Iron is a soft magnetic material. When the magnet is pulled away, the nail loses almost all of its magnetism and the filings fall off.
Answer: The nail is a temporary magnet by induction. Its point is a north pole, and it loses its magnetism when the magnet is removed.
Example 3 Describe the magnetic field between the north pole of one bar magnet and the south pole of another, facing each other with a small gap, and state where the field is strongest.
Unlike poles face each other, so the field lines run from the N pole straight across the gap to the S pole.
Inside the gap the lines are parallel and evenly spaced, so the field is uniform.
The lines are most crowded near the poles, so the field is strongest at the poles.
Answer: Parallel lines from N to S across the gap, a uniform field; strongest at the poles.
Common errors
- Common error: saying attraction proves that an object is a magnet. Only repulsion does, because magnetic materials are attracted by either pole.
- Common error: drawing field lines that cross, that have no arrows, or that point from south to north outside the magnet.
- Common error: saying all metals are magnetic. Copper, aluminium, zinc and brass are not.
- Common error: saying that the end of the nail touching a pole of the magnet has the same pole. Induced poles are always opposite to the pole causing them, which is why the force is attractive.
- Common error: mixing up soft and hard materials. Soft magnetic materials are easy to magnetise and demagnetise, so iron suits electromagnets, and steel suits permanent magnets.
Exam tip: When drawing field lines, draw smooth curves, add an arrow on every line, space the lines more closely where the field is strongest and make sure that none of them touch or cross.
Check yourself
- Two bar magnets are placed with their north poles facing each other. Do they attract or repel?
Answer
They repel, because like poles repel. - Name two magnetic materials and two non-magnetic metals.
Answer
Magnetic: any two of iron, steel, nickel, cobalt. Non-magnetic metals: any two of copper, aluminium, zinc, brass. - Why is iron used for the core of an electromagnet but steel is used for a compass needle?
Answer
Iron is a soft magnetic material, so it magnetises quickly and loses its magnetism when the current is switched off, which is what an electromagnet needs. Steel is a hard magnetic material that stays magnetised, so the compass needle remains a permanent magnet. - Give two ways of demagnetising a steel magnet.
Answer
Hammer it, heat it strongly, or place it in a coil carrying alternating current and slowly withdraw it from the coil.