Magnets and Magnetism
Magnets and Magnetism
Magnetism gives 1 question in the Part-III paper, and it is one of the easiest marks in the science half. The properties of magnets, the electromagnet and the difference between the two kinds of magnet are the whole of it.
Properties of a Magnet
- Every magnet has two poles, north and south, and the poles cannot be separated.
- Breaking a magnet in half gives two complete magnets, each with both poles.
- Like poles repel and unlike poles attract.
- A freely suspended magnet always comes to rest pointing north and south.
- Magnets attract magnetic materials: iron, cobalt, nickel and steel.
The most reliable test of magnetism is repulsion, not attraction, because a magnet attracts an ordinary iron piece as well but only another magnet is repelled. That is a genuine reasoning question and a good discriminator.
✗ Attraction is the surest test of whether an object is a magnet | ✓ Repulsion is the surest test; attraction happens with plain iron too
Making and Losing Magnetism
| Method of magnetising | Method of demagnetising |
|---|---|
| Stroking with a bar magnet | Heating strongly |
| Passing current through a coil | Hammering repeatedly |
| Placing in a strong magnetic field | Dropping from a height |
| — | Storing carelessly without keepers |
Magnets should be stored in pairs with unlike poles facing and with soft iron keepers across the ends, and kept away from heat, mobile phones, televisions and other magnets. This storage question appears in the class 6 textbook and is asked directly.
Electromagnets
A current flowing through a coil produces a magnetic field. Winding the coil round a soft iron core makes an electromagnet.
| Point | Permanent magnet | Electromagnet |
|---|---|---|
| Magnetism | Always present | Only while current flows |
| Strength | Fixed | Can be varied |
| Polarity | Fixed | Can be reversed |
| Core | — | Soft iron |
An electromagnet's strength increases with more turns in the coil and with a larger current. Soft iron is used for the core, not steel, because soft iron loses its magnetism as soon as the current stops, which is exactly what an electromagnet needs. Steel is used for permanent magnets for the opposite reason.
Electromagnets are used in electric bells, cranes lifting scrap iron, loudspeakers, and in separating iron from mixed waste. The crane example is the one most often set.
The Earth as a Magnet
The Earth behaves like a huge magnet, which is why a compass needle aligns north to south. The geographic north attracts the north-seeking pole of a magnet, which means the Earth's magnetic south pole lies near the geographic north. This inversion is a favourite question, because it sounds like a contradiction until the reasoning is clear.
A compass is simply a small pivoted magnet free to rotate, and it was the earliest practical use of magnetism in navigation.
चुंबकत्व की सबसे पक्की पहचान आकर्षण नहीं, प्रतिकर्षण है।
TRE pointer: Two reasoning questions carry this topic: repulsion is the sure test of a magnet, and the Earth's magnetic south lying near the geographic north. Both sound counterintuitive, which is exactly why they are set. The soft-iron-versus-steel choice for an electromagnet core is the third. Since these are one-line facts, they are among the cheapest marks in the Part-III science paper and should never be left to option E.
60-Second Recap
- Poles always come in pairs; breaking a magnet gives two complete magnets.
- Like poles repel, unlike attract; repulsion is the sure test of magnetism.
- Iron, cobalt, nickel and steel are magnetic materials.
- Heating, hammering and dropping destroy magnetism; store with keepers.
- An electromagnet uses a soft iron core and works only while current flows.
- The Earth's magnetic south pole lies near the geographic north pole.
Frequently Asked Questions
What is the surest test that an object is a magnet?
Repulsion. A magnet will attract an ordinary piece of iron just as it attracts another magnet, so attraction proves nothing. Only another magnet can be repelled, which makes repulsion the definitive test. This reasoning is asked regularly in teaching examinations.
Why is soft iron used in an electromagnet?
Because soft iron becomes strongly magnetic while current flows and loses that magnetism the moment the current stops, which is exactly what an electromagnet needs. Steel retains magnetism, which is why steel is used for permanent magnets instead.
Where is the Earth's magnetic south pole?
Near the geographic north pole. That is why the north-seeking pole of a compass needle points north, since unlike poles attract. The apparent contradiction is deliberate in questions, and understanding the attraction rule is what resolves it.
How can a magnet lose its magnetism?
By being heated strongly, hammered repeatedly, dropped from a height, or stored carelessly near other magnets. Correct storage means keeping magnets in pairs with unlike poles facing and soft iron keepers across the ends, away from heat and electronic devices.