Chapter 4 — Exploring Magnets
Complete, detailed solutions to every in-text question and every “Let us enhance our learning” exercise question, with the original textbook figures.
💭In-text Questions
Yes. A magnet attracts only certain materials, called magnetic materials — mainly iron, nickel and cobalt (and some of their combinations with other metals).
Objects made of wood, plastic, rubber, glass, paper or cloth are non-magnetic materials and are not attracted. So a magnet sticks only to objects that are made of (or contain) a magnetic material.
The objects that are not attracted by the magnet are non-magnetic. In Table 4.1 these are the objects made of materials other than iron/nickel/cobalt:
- Pencil — wood
- Eraser — rubber
- Plastic scale / plastic pen — plastic
- Glass tumbler — glass
- Candle, cloth (duster), paper — non-magnetic
Only the iron/steel objects (iron nail, screw, key, paper clip, safety pin, blade, steel part of scissors and stapler) are magnetic — they get attracted.
No. All parts do not attract equally. The pull is strongest near the two ends of the magnet and weakest in the middle.
When iron filings are sprinkled near a bar magnet, the maximum filings cling at the two ends and very few stick in the middle — as seen in Fig. 4.4. These strong ends are called the poles of the magnet.
Yes. Every magnet — whether bar, U-shaped, ring or disc — has two poles where the attraction is strongest.
So with any shape, the iron filings again gather mostly at the poles and very little in between. The position of the poles changes with shape, but the property (strongest attraction at the poles) stays the same.
No. It is not possible to obtain a magnet with a single pole. Poles always exist in pairs.
Even if a magnet is broken into the smallest possible pieces, every single piece will still have both a North pole and a South pole. An isolated North pole or South pole cannot exist.
A freely suspended magnet always comes to rest along the north–south direction. So the line drawn along the resting magnet indicates the north–south line.
How to find it out: The Sun rises in the east and sets in the west. Using the direction of sunrise or sunset we get an approximate idea of east and west, and from these we can locate north and south — the directions along which the magnet has come to rest.
We can build a simple compass (Activity 4.4):
- Rub an iron sewing needle with one pole of a bar magnet, always moving in the same direction, and repeat 30–40 times to magnetise the needle.
- Check it is magnetised — it should now attract iron filings or steel pins.
- Push the needle horizontally through a small cork, and float the cork in a bowl of water so the needle stays above the water.
- When it stops moving, the needle settles along the north–south direction. Your compass is ready!
It depends on which poles face each other:
- Unlike poles (North–South) attract each other.
- Like poles (North–North or South–South) repel each other.
Yes. Because the compass needle is itself a tiny magnet, it behaves exactly like one (Activity 4.6):
- Bring the North pole of a bar magnet near the North pole of the needle → the needle moves away (like poles repel) — Fig. 4.9(a).
- Bring the South pole of the bar magnet near the North pole of the needle → the needle moves closer (unlike poles attract) — Fig. 4.9(b).
No. There is no appreciable change in the deflection of the needle.
The magnetic effect can pass through non-magnetic materials such as wood, cardboard, plastic and glass. So even with a sheet placed in between, the needle still gets deflected (Activity 4.7 & Table 4.2).
Yes, it is! A magnet exerts its force at a distance, so it can attract or move magnetic objects — and repel other magnets — without any physical contact.
This is why we can guide steel balls through a maze by moving a magnet under the tray, lift a steel paper clip out of water without wetting our fingers, or make a hanging garland stick to a magnet.
📘Let us enhance our learning
- (i) Unlike poles of two magnets attract each other, whereas like poles repel each other.
- (ii) The materials that are attracted towards a magnet are called magnetic materials.
- (iii) The needle of a magnetic compass rests along the north–south direction.
- (iv) A magnet always has two poles.
- (i) A magnet can be broken into pieces to obtain a single pole. ✗ False
Poles always exist in pairs — every piece keeps both a N and a S pole.
- (ii) Similar poles of a magnet repel each other. ✓ True
- (iii) Iron filings mostly stick in the middle of a bar magnet when brought near them. ✗ False
They stick mostly at the two ends (poles), not the middle.
- (iv) A freely suspended bar magnet always aligns with the north–south direction. ✓ True
| Column I (poles placed near) | Column II (interaction) |
|---|---|
| N – N | Repulsion |
| N – S | Attraction |
| S – N | Attraction |
| S – S | Repulsion |
Rule: like poles (N–N, S–S) → repulsion; unlike poles (N–S, S–N) → attraction.
The attraction is strongest at the poles (ends) and weakest at the middle. Positions A and C are near the ends, so they pick up many clips; B is the middle, so it picks up very few.
Correct option: (i) — most clips at the two ends (A and C), fewest in the middle (B).
Use the property of repulsion. This is the sure test of a magnet, because:
- A magnet attracts both a magnet and a piece of iron.
- But a magnet repels only another magnet — iron is never repelled.
Procedure: Bring an end of one bar near an end of another (try flipping the bars so different pole combinations meet). The pair that shows repulsion in some position — both of those are magnets. The bar that only ever attracts the others (and never repels any) is the plain iron piece.
Take the marked magnet whose North (N) and South (S) poles are known, and bring its known North pole near one end of the unmarked magnet:
- If that end is repelled → it is a N North pole (like poles repel).
- If that end is attracted → it is a S South pole (unlike poles attract).
The other end is automatically the opposite pole. Repulsion is the decisive test, so mark the pole that is pushed away as North.
Suspend the magnet freely by a thread tied at its middle so that it can rotate horizontally, and let it come to rest (as in Fig. 4.5).
- A freely suspended magnet always settles along the north–south direction.
- The end pointing towards the geographic North is the N North pole; the other end is the South pole.
Directions of north and south can be judged from where the Sun rises (east) and sets (west).
Yes. The North pole of a compass needle always points towards the geographic North.
Since unlike poles attract, the pole hidden near the geographic North must be the Earth’s magnetic South pole, and the pole near the geographic South must be the Earth’s magnetic North pole.
- Geographic North ↔ Earth’s magnetic S South pole
- Geographic South ↔ Earth’s magnetic N North pole
Magnetise the tip of the screwdriver (or use a magnetic screwdriver). Rub the metal tip several times with one pole of a magnet, always in the same direction, so the tip becomes a magnet.
Since steel screws are made of a magnetic material (iron), they will now cling to the magnetised tip and will not fall down while he works.
Reason: The two ring magnets are placed with their like poles facing each other. Like poles repel, so this repulsion pushes X upward and keeps it floating — it cannot move down to touch Y.
Solution (without pushing either magnet): Lift magnet X off the rod, turn it upside down (invert it), and slide it back on. Now unlike poles face each other, so they attract — magnet X comes down and rests in contact with magnet Y.
Rule used: (a) The two ends of a single bar magnet are always opposite poles. (b) Where two magnets meet at a corner, the touching ends must be unlike poles so that they attract and hold the arrangement together.
Step-by-step (starting from end 5 = N):
- End 5 = N (given) → its own magnet’s other end 6 = S.
- End 4 touches end 5 (N), so to attract 4 = S → same magnet’s other end 3 = N.
- End 2 touches end 3 (N), so to attract 2 = S → same magnet’s other end 1 = N.
| End | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|
| Pole | N | S | N | S | N | S |