Magnetism — ICSE Class 9 Physics Important Questions

Answer: (b) north-south direction.

This directive property is why a magnet was historically used as a compass; its north pole points towards the geographic north.

Answer: (c) iron.

Iron (like cobalt and nickel) is attracted by a magnet and can be magnetised, so it is a magnetic material; copper, aluminium and brass are non-magnetic.

Answer: (b) from north pole to south pole.

By convention, outside the magnet the field lines emerge from the north pole and enter the south pole (inside the magnet they run from south to north, forming closed loops).

Answer: (c) be a complete magnet with both poles.

Magnetic poles always exist in pairs; an isolated pole cannot exist. So each broken piece becomes a smaller magnet with its own north and south poles.

Answer: (a) Soft iron is easily magnetised and just as easily demagnetised, so it makes a strong temporary magnet that switches off with the current, which is exactly what an electromagnet needs. R correctly explains A.

1. A freely suspended magnet always rests in the north-south direction (directive property).
2. Like poles repel and unlike poles attract each other.
3. The attracting power is strongest at the poles and least at the middle.
4. Magnetic poles always exist in pairs; an isolated pole cannot exist. (It also attracts magnetic materials such as iron.)

Induced magnetism is the temporary magnetism developed in a piece of magnetic material (such as soft iron) when it is placed in or brought near the magnetic field of a magnet, without touching it. The nearer end acquires an opposite polarity, so it is attracted.

Example: An unmagnetised iron nail brought close to a magnet becomes a temporary magnet and can itself pick up small iron pins; it loses this magnetism when the magnet is removed.

The field lines emerge from the north pole (N), curve around, and enter the south pole (S), forming closed loops.

Properties: (1) Magnetic field lines never cross one another. (2) They are crowded (closer) where the field is strong, such as near the poles, and farther apart where the field is weak.

Magnetic substances are those that are attracted by a magnet and can be magnetised, for example iron and nickel (also cobalt, steel).

Non-magnetic substances are those that are not attracted by a magnet and cannot be magnetised, for example copper and wood (also aluminium, plastic, brass).

A natural magnet is an ore of iron (magnetite, $Fe_3O_4$Fe_3O_4) found in nature that has the property of attracting small pieces of iron; it is also called lodestone.

Single-touch method: Place the steel bar on a table. Take a strong bar magnet and rub one of its poles along the length of the steel bar from one end to the other, always in the same direction. Lift the magnet high at the end of each stroke and repeat many times. Repeated stroking aligns the tiny magnetic domains of the steel in one direction, so the bar becomes a permanent magnet, with the end where the stroking finishes acquiring a polarity opposite to the stroking pole.

Demagnetisation (any one): heating the magnet strongly and then letting it cool, or hammering it roughly, or placing it inside a solenoid carrying alternating current, disturbs the alignment of the domains and destroys the magnetism.

(a) A magnetic field is the region around a magnet within which its magnetic force of attraction or repulsion can be experienced by another magnet or a magnetic material.

(b) Place the bar magnet on a sheet of paper and mark its outline. Put a small plotting compass near the north pole and mark two dots showing the positions of the compass needle's ends. Move the compass so its tail is where the head dot was, and mark the new head position. Repeat, joining the dots, until you reach the south pole; this traced curve is one field line. Repeat starting from different points around the magnet to obtain the complete pattern, with arrows pointing from N to S.

(c) Temporary magnet: made of soft iron, magnetism is strong but lost easily (e.g. electromagnet). Permanent magnet: made of steel, harder to magnetise but retains its magnetism for a long time. (Also: temporary magnets can be switched off; permanent magnets cannot.)

(a) A neutral point is a point near a magnet where the magnetic field of the magnet is exactly equal and opposite to the horizontal component of the Earth's magnetic field, so the resultant field there is zero and a compass needle placed there shows no definite direction. There are two such points.

(b) When the north pole of the magnet points towards geographic north (a 'north-pole-north' setting), the magnet's field and the Earth's field oppose each other on the sides. The two neutral points lie on the equatorial line of the magnet, that is, symmetrically on either side (east and west) of the magnet.

(c) Uses of magnets: (1) in making compass needles for navigation; (2) in electrical devices such as electric bells, loudspeakers, electric motors and generators; (3) for separating magnetic materials (iron scrap) from non-magnetic waste, and in storing data on magnetic strips and hard disks.

(i) The phenomenon is induced magnetism (magnetic induction).

(ii) The end of the nail nearer to the magnet's north pole develops an opposite polarity, i.e. a south pole, which is why it is attracted.

(iii) Soft iron produces only temporary induced magnetism. When the magnet is removed, the magnetising field is gone, the domains of the soft iron lose their alignment, the nail loses its magnetism, and so the pins are no longer held and fall off.

(iv) No. Copper is a non-magnetic material; magnetism cannot be induced in it, so it would neither be attracted by the magnet nor pick up any pins.