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Magnetism is one of the fundamental forces of nature, and understanding magnetic fields is essential for making sense of how compasses, electric motors, generators, and MRI scanners work. This lesson covers the properties of magnets, the concept of magnetic fields, and how to represent them using field line diagrams.
A permanent magnet produces its own magnetic field without needing an electric current. The most common permanent magnets are made from iron, steel, cobalt, or nickel — these are called ferromagnetic materials. At GCSE level, you need to know that only ferromagnetic materials can be made into permanent magnets or are strongly attracted to magnets.
There is an important distinction between hard and soft magnetic materials:
| Property | Soft Magnetic Material (e.g. iron) | Hard Magnetic Material (e.g. steel) |
|---|---|---|
| Ease of magnetisation | Easy to magnetise | Harder to magnetise |
| Ease of demagnetisation | Easy to demagnetise | Hard to demagnetise |
| Use | Electromagnet cores | Permanent magnets |
| Retains magnetism? | No — loses it quickly | Yes — retains it |
Iron is used in electromagnets because it can be magnetised and demagnetised rapidly. Steel is used for permanent magnets because it retains its magnetism once magnetised.
Every magnet has a north pole and a south pole. These poles are where the magnetic field is strongest. The fundamental rules of magnetic poles are:
If you break a magnet in half, you do not get a separate north pole and south pole. Instead, each piece becomes a complete magnet with its own north and south pole. This is because magnetism arises from the alignment of atomic magnetic domains throughout the material.
Common GCSE exam mistake: Students sometimes write that breaking a magnet gives you an isolated north pole and an isolated south pole. This is wrong — you cannot create a magnetic monopole by breaking a magnet. Every piece always has both poles.
A magnetic field is the region around a magnet where a magnetic force acts on another magnetic material or moving charge. Magnetic fields are invisible, but we can represent them using magnetic field lines.
Key rules for drawing and interpreting magnetic field lines:
graph LR
subgraph "Bar Magnet Field"
N["N pole"] -->|"Field lines curve outward"| S["S pole"]
end
A["Key facts"] --> B["Lines: N → S outside magnet"]
A --> C["Closer lines = stronger field"]
A --> D["Lines never cross"]
A --> E["Strongest at the poles"]
The magnetic field of a bar magnet has a characteristic pattern:
The shape of this field is identical to the field produced by a solenoid (a coil of wire carrying a current), which you will meet in the next lesson.
A plotting compass is a small compass that can be used to trace field lines around a magnet. The method is:
This method works because the compass needle always aligns with the local magnetic field direction. The north pole of the compass needle points in the direction of the field (from north to south).
A uniform magnetic field has field lines that are parallel and equally spaced. The field has the same strength and direction at every point. This can be produced between two flat, parallel magnets with opposite poles facing each other (a north pole facing a south pole).
A non-uniform field has field lines that vary in spacing and direction. The field around a single bar magnet is non-uniform because it is stronger near the poles and weaker further away.
| Field type | Field line pattern | Strength | Example |
|---|---|---|---|
| Uniform | Parallel, equally spaced | Same everywhere | Between two flat magnets |
| Non-uniform | Curved, varying spacing | Varies with position | Around a bar magnet |
When an unmagnetised magnetic material is placed near a magnet, it becomes a magnet itself. This is called induced magnetism.
For example, if you hold a paper clip near a magnet, the paper clip becomes magnetised and can attract other paper clips. The end of the paper clip nearest to the magnet's north pole becomes an induced south pole (because unlike poles attract). This is always the case — the induced pole nearest to the permanent magnet is always the opposite pole.
When the permanent magnet is removed, a soft magnetic material (like iron) quickly loses its induced magnetism, while a hard magnetic material (like steel) retains it.
Common GCSE exam mistake: Some students say the paper clip is attracted because it is "magnetic". The better answer is that the paper clip becomes an induced magnet — the magnet induces magnetism in it, creating an attractive force between the induced south pole and the permanent north pole.
Question: Two bar magnets are placed end to end and repel each other. The left magnet has its north pole on the right side. What is the pole arrangement of the right magnet?
Answer: Since like poles repel, the left side of the right magnet must also be a north pole. So the right magnet has north on the left and south on the right.
Question: A student draws the magnetic field around a bar magnet. She notices that at point A near the north pole, the field lines are 2 mm apart, while at point B further from the magnet, the field lines are 8 mm apart. Compare the magnetic field strength at A and B.
Answer: The field lines are much closer together at point A than at point B. This means the magnetic field is stronger at point A and weaker at point B. The field strength decreases with distance from the magnet, and the spacing of field lines is a visual indicator of this.
Question: A steel nail is placed near the north pole of a bar magnet. Explain why the nail is attracted to the magnet and what happens when the magnet is removed.
Answer: The magnetic field of the bar magnet causes the magnetic domains in the steel nail to align. The end of the nail nearest the north pole becomes an induced south pole. Since unlike poles attract (north attracts south), the nail is pulled towards the magnet. When the magnet is removed, the nail retains its magnetism because steel is a hard magnetic material — it does not easily lose its induced magnetism.
The Earth behaves as though it has a giant bar magnet inside it. The geographic North Pole is actually near a magnetic south pole (which is why the north pole of a compass needle points towards geographic north — it is attracted to the magnetic south pole located there).
The Earth's magnetic field is important for navigation (compasses) and for protecting life on Earth by deflecting charged particles from the Sun (the solar wind). Without the magnetic field, the solar wind would strip away the atmosphere.