Magnetism

ICSE Class 9 · Physics · Chapter 10

What this chapter covers

Magnets and magnetic materials
Induced magnetism
The magnetic field and magnetic field lines
The Earth's magnetic field
Neutral points

Key points

Like poles repel and unlike poles attract; poles always occur in pairs.
Iron, steel, nickel and cobalt are magnetic; soft iron is easily magnetised but loses it quickly, steel keeps it.
Magnetic induction makes a magnetic material a temporary magnet; the near end gets the opposite pole.
Induction precedes attraction; repulsion is the sure test of magnetism.
A magnetic field line is a closed curve whose tangent gives the field direction; outside a magnet it runs from N to S.
Field lines never intersect; they are crowded where the field is strong.
A uniform field has parallel, equally spaced lines (the Earth's field locally); a bar magnet's field is non-uniform.
The Earth behaves like a large magnet with its magnetic south pole near the geographic North Pole.
Evidence: freely suspended magnets point north–south, iron posts become magnetised, neutral points exist.
A magnetic compass is a pivoted magnetised needle that settles along the magnetic meridian.
At a neutral point the magnet's field cancels the Earth's horizontal field; a compass there does not settle.
N pole pointing north: neutral points on the equatorial line (east and west). N pole pointing south: neutral points on the axial line.

Questions answered

Name four magnetic materials and two non-magnetic metals.

Iron, steel, nickel and cobalt are magnetic. Copper and aluminium are non-magnetic.

What is magnetic induction?

It is the process by which a piece of magnetic material becomes a magnet when placed near or in contact with a magnet.

The north pole of a magnet is brought near one end of a soft iron bar. What polarity is induced at the near end and at the far end?

The near end becomes a south pole and the far end a north pole.

Explain the statement "induction precedes attraction".

A magnet first induces an opposite pole at the near end of the iron, and only then attracts it, since unlike poles attract.

Why is repulsion considered the sure test of magnetism?

An unmagnetised magnetic material is attracted by both poles because of induction, but only a magnet can be repelled by another magnet, since repulsion occurs only between like poles.

Define a magnetic field line.

It is a continuous curve in a magnetic field whose tangent at any point gives the direction of the field at that point.

Why do two magnetic field lines never intersect?

At the point of intersection, there would be two directions of the field, so a compass needle would have to point two ways at once, which is impossible.

How can you tell from a field-line diagram where a field is strong?

The lines are crowded together where the field is strong and spread out where it is weak.

Past board exam questions

ICSE 2019 sample paper · 2 marks

State two differences between an electromagnet and a permanent magnet.

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1. An electromagnet is a temporary magnet: it is magnetised only while current flows through its coil; a permanent magnet keeps its magnetism. 2. The strength of an electromagnet can be changed (by changing the current or number of turns); the strength of a permanent magnet cannot be changed easily. (Also: the polarity of an electromagnet can be reversed by reversing the current; an electromagnet has a soft iron core, a permanent magnet is made of steel/alloys.)

ICSE 2019 sample paper · 2 marks

Why does a magnet suspended freely from its CG, always come to rest along the north south direction of the earth?

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The earth behaves like a huge magnet whose magnetic south pole is near the geographic north and magnetic north pole is near the geographic south. The north pole of the freely suspended magnet is attracted by the earth's magnetic south pole (near geographic north) and its south pole by the earth's magnetic north pole, so the magnet aligns itself along the earth's magnetic field, i.e. in the north–south direction.

ICSE 2019 sample paper · 3 marks

Pins are suspended from the same magnet to their maximum limit in two different cases. In each case a bar magnet lies horizontally (S at the left end, N at the right end) and a chain of pins hangs vertically, one below the other, from its N end. The chain of pins A is longer (more pins hang) than the chain of pins B. State with a reason whether the set of pins A or the set of pins B are made out of soft iron. Also define the magnetic process which enables us to suspend the pins one below the other.

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The set of pins A is made of soft iron. Soft iron has a high magnetic permeability (susceptibility), so it gets magnetised easily and strongly by induction; therefore more pins can hang one below the other, forming the longer chain A. (Pins B, of steel, are magnetised less easily, so fewer pins hang.) The process is magnetic induction: the process in which a magnetic substance placed near (or in contact with) a magnet becomes a magnet itself. Each pin becomes a magnet and attracts the next pin.

ICSE 2019 sample paper · 4 marks

(i) Define a neutral point. (ii) AB is a magnet and CD is an iron bar. The magnet AB lies horizontally with end B on the left and end A on the right; a magnetic field line curves over the top of the magnet from end A to end B, its arrow pointing towards B. The iron bar CD lies in line with the magnet to its right, with end C nearer to A and end D at the far right. Determine the polarities at the ends A, B and D.

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(i) A neutral point is a point in the magnetic field where the resultant magnetic field is zero, i.e. the field of the magnet is exactly balanced by the earth's horizontal magnetic field; a compass needle placed there shows no particular direction. (ii) Magnetic field lines go from the north pole to the south pole outside a magnet. The field line goes from A to B, so A is a north pole and B is a south pole. The iron bar CD gets magnetised by induction: end C, nearest to A (N), becomes a south pole, so end D becomes a north pole. A – North pole, B – South pole, D – North pole.