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) If two field lines crossed, a compass needle at the crossing point would have to point in two directions at once, which is impossible because the field at a point has a single direction. 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, $Fe_3O_4$Fe₃O₄) found in nature that has the property of attracting small pieces of iron; it is also called lodestone.

Evidences: (1) A freely suspended magnet always rests in the north-south direction. (2) Neutral points are found in the field of a bar magnet, where its field cancels the Earth's field. (3) A soft-iron bar buried in the ground in the north-south direction for a long time becomes magnetised.

Compass: its magnetised needle lines up with the Earth's magnetic field, so it always shows north and south. It is used to find directions at sea, in forests and in aircraft.

(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) A magnet attracts a piece of iron only after it has first induced magnetism in it: the near end of the iron becomes an opposite pole, and then the unlike poles attract. So induction happens first, attraction follows.

(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) Now the neutral points lie on the axial line of the magnet, one beyond each pole. Beyond the north pole (to the south) the magnet's field points south, and beyond the south pole (to the north) it also points south, opposite to the Earth's northward horizontal field. At one point on each side the two fields are equal and cancel.

(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.