Question Bank

Spread iron filings on a sheet of paper and place a bar magnet over them, then tap the paper. The maximum iron filings stick near the two ends of the magnet and very few stick in the middle. These ends, where attraction is strongest, are the poles.

Because the Earth itself behaves like a giant magnet. A freely suspended magnet lines up with the Earth's magnetism and comes to rest along the north–south direction, with its North-seeking pole pointing north.

It is a small circular box with a transparent cover. A needle-shaped magnet balanced on a pin can rotate freely; its north end is usually painted red, and a dial marked with directions lies below. To use it, place it where you want to find directions, wait for the needle to rest north–south, then rotate the box until N and S on the dial line up with the needle.

A magnet attracts both another magnet (unlike pole) and a plain iron bar, so attraction cannot tell them apart. But only a magnet can be repelled — when like poles face each other. An iron bar is attracted at both ends by both poles and never repels.

Do not heat, drop or hammer magnets, and keep them away from mobile phones and remote controls. Store bar magnets in pairs with unlike poles on the same side, a piece of wood between them, and two pieces of soft iron placed across their ends.

1. Collect a cork, an iron sewing needle, a permanent bar magnet, a glass bowl and water.
2. Place the needle on a wooden table. Put one pole of the magnet at one end of the needle and move it along the length of the needle to the other end. Lift the magnet up.
3. Bring the same pole back to the same starting end and repeat. Do this 30 to 40 times.
4. Bring iron filings or steel pins near the needle. If they are attracted, the needle has become a magnet.
5. Push the needle horizontally through the cork and float the cork in a bowl of water so the needle stays above the water.

Checking it works: when the needle comes to rest, note the direction each end points. Rotate the cork gently and let it stop again; repeat a few times. If the needle always comes to rest pointing the same way (north–south), the compass works. This is very like the ancient Indian matsya-yantra, a magnetised fish-shaped iron piece floating in oil.

Activity 4.5: magnet A is placed on 5–6 round pencils so it can roll. One end of magnet B is brought near one end of A (without touching), then B's other end. A rolls towards B when unlike poles face each other and rolls away when like poles face each other.

Activity 4.6: when the North pole of a bar magnet is brought near the North end of a compass needle, the needle moves away; when the South pole is brought near, the needle moves closer.

What we learn: unlike poles attract, like poles repel — and a compass needle is itself a magnet.

Telling a magnet from iron: when an iron bar replaces one magnet, both its ends are attracted by both poles — it never repels. So if an object is repelled by some pole of a known magnet, it must be a magnet. Repulsion is the sure test.

Unlike poles attract. The compass needle's North pole is pulled towards the geographic north, so the Earth's magnetic pole there must be a South magnetic pole. In other words, the Earth's magnet has its South pole near the geographic north and its North pole near the geographic south. (This answers textbook question 8.)

Not necessarily. A magnet attracts any magnetic material — a plain steel pin is attracted even if it is not a magnet. Attraction alone proves nothing.

Test: bring the pin near the North pole of the magnet, then near the South pole, at each end of the pin. If any end is repelled, the pin is a magnet. If both ends are attracted by both poles, it is just magnetic material. (He could also hang the pin freely and see whether it always settles north–south.)

(i) Unlike poles of two magnets attract each other, whereas like poles repel each other. (ii) magnetic materials. (iii) north–south. (iv) two.

(i) F — poles always exist in pairs. (ii) T — like poles repel. (iii) F — filings stick mostly near the ends (poles). (iv) T.

N – N → Repulsion · N – S → Attraction · S – N → Attraction · S – S → Repulsion.

Option (i) 10, 2, 10. Positions A and C are the ends (poles), where attraction is strongest, so many clips stick; B is the middle, where very few stick.

Bring the bars near each other in pairs, trying both ends. The two bars that repel each other in some position are the magnets (like poles facing). The iron bar is always attracted, never repelled, by the other bars — so it is the bar that never shows repulsion.

Bring the North pole of the marked magnet near one end of the unmarked magnet. If that end is repelled, it is a North pole (like poles repel); if it is attracted, it is a South pole. The other end is the opposite pole.

Suspend the magnet freely with a thread tied at its middle so it balances horizontally. When it comes to rest, it lies north–south. The end pointing towards the north (find north from where the Sun rises in the east) is the North pole.

The compass needle's North pole points to geographic north. Since unlike poles attract, the Earth's magnetic South pole must be near the geographic north, and its magnetic North pole near the geographic south.

Magnetise the tip of the steel screwdriver by stroking it with one pole of a magnet many times in the same direction (as in Activity 4.4), or use a screwdriver with a magnet at its tip. The steel screws will then be attracted and stay on the screwdriver.

The faces of X and Y that point towards each other are like poles, so they repel and X stays floating above Y. To bring them together, take X off, turn it upside down and put it back on — now unlike poles face each other, so X is attracted down onto Y.

1 – N, 2 – S, 3 – N, 4 – S, 6 – S (5 – N given).

The three magnets form a Z shape: ends 1–2 (top magnet), 3–4 (upright magnet), 5–6 (bottom magnet), with end 2 touching 3 and end 4 touching 5.

Reasoning: touching ends stay together, so they must be unlike poles. 5 = N → 4 = S → the other end of that magnet, 3 = N → 2 = S → 1 = N. And 6, the other end of the magnet with 5, = S.