Fill in the blanks
(i) Unlike poles of two magnets ............... each other, whereas like poles ............... each other.
(ii) The materials that are attracted towards a magnet are called ................
(iii) The needle of a magnetic compass rests along the ............... direction.
(iv) A magnet always has ............... poles.
Answer
(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.
State whether the following statements are True (T) or False (F).
(i) A magnet can be broken into pieces to obtain a single pole.
(ii) Similar poles of a magnet repel each other.
(iii) Iron filings mostly stick in the middle of a bar magnet when it is brought near them.
(iv) A freely suspended bar magnet always aligns with the north-south direction.
Answer
(i) False
Corrected Statement — A magnet cannot be broken to obtain a single pole. The north and south poles always exist in pairs, even in the smallest piece of the magnet.
(ii) True
(iii) False
Corrected Statement — Iron filings mostly stick near the ends (poles) of a bar magnet, and very few stick at its middle.
(iv) True
Column I shows different positions in which one pole of a magnet is placed near that of the other. Column II indicates the resulting interaction between them for different situations. Fill in the blanks.
| Column I | Column II |
|---|---|
| N – N | ________ |
| N – ________ | Attraction |
| S – N | ________ |
| ________ – S | Repulsion |
Answer
| Column I | Column II |
|---|---|
| N – N | Repulsion |
| N – S | Attraction |
| S – N | Attraction |
| S – S | Repulsion |
Atharv performed an experiment in which he took a bar magnet and rolled it over a heap of steel U-clips (Fig. 4.15). According to you, which of the options given in Table 4.3 is likely to be his observation?
Table 4.3: Number of pins attracted by the magnet at its various positions
| Position A | Position B | Position C | |
|---|---|---|---|
| (i) | 10 | 2 | 10 |
| (ii) | 10 | 10 | 2 |
| (iii) | 2 | 10 | 10 |
| (iv) | 10 | 10 | 10 |
Answer
Position A — 10, Position B — 2, Position C — 10
Reason — The magnetic strength of a magnet is maximum at its poles (the two ends) and least at the middle. In Fig. 4.15, positions A and C are near the two poles of the bar magnet, so they attract the maximum number of pins (10 each), while position B lies at the middle, where the attraction is least, so it attracts very few pins (2).
Reshma bought three identical metal bars from the market. Out of these bars, two were magnets and one was just a piece of iron. How will she identify which two amongst the three could be magnets (without using any other material)?
Answer
Reshma can identify the magnets using the property of repulsion. She should bring the ends of the bars close to one another, two at a time, and reverse the ends when necessary. Repulsion occurs only between like poles of two magnets, whereas a magnet attracts an unmagnetised iron bar. Therefore, the pair of bars that repel each other in one arrangement must be the two magnets. The remaining bar is the piece of iron.
You are given a magnet which does not have the poles marked. How can you find its poles with the help of another magnet which has its poles marked?
Answer
Bring the marked North pole of the known magnet close to one end of the unmarked magnet and observe what happens.
- If that end is pushed away (repelled), it is the North pole, because like poles repel each other.
- If that end is pulled towards the known magnet (attracted), it is the South pole.
The other end of the magnet will be the opposite pole. Since repulsion takes place only between like poles, it gives a sure way of marking the poles.
A bar magnet has no markings to indicate its poles. How would you find out near which end its North pole is located without using another magnet?
Answer
Tie a thread to the middle of the bar magnet and suspend it so that it can rotate freely in the horizontal direction. Let it come to rest. A freely suspended magnet always rests along the north-south direction. The end of the magnet that points towards the geographical north is its North pole. That end can be marked as the North pole.
If the earth is itself a magnet, can you guess the poles of earth's magnet by looking at the direction of the magnetic compass?
Answer
Yes. The North pole of the magnetic compass needle always points towards the geographical north. Since unlike poles attract each other, the Earth's magnet must have its south pole near the geographical north and its north pole near the geographical south.
While a mechanic was repairing a gadget using a screw driver, the steel screws kept falling down. Suggest a way to solve the problem of the mechanic on the basis of what you have learnt in this chapter.
Answer
The mechanic should magnetise the tip of his steel screwdriver. He can do this by stroking the tip several times with one pole of a magnet, each time in the same direction. As steel is a magnetic material, the screws are attracted to the magnetised screwdriver and stick to its tip. This will hold the steel screws firmly, so they will not fall down while he works.
Two ring magnets X and Y are arranged as shown in Fig. 4.16. It is observed that the magnet X does not move down further. What could be the possible reason? Suggest a way to bring the magnet X in contact with magnet Y, without pushing either of the magnets.
Answer
Possible reason: The two ring magnets are placed with their like poles facing each other. Like poles repel each other, so magnet X is pushed up by magnet Y. Due to this force of repulsion, magnet X stays floating above magnet Y and does not move down further.
Way to bring them in contact: Take out magnet X, turn it upside down (invert it) and place it back on the rod. Now unlike poles of the two magnets face each other. Since unlike poles attract, magnet X will move down and come in contact with magnet Y, without pushing either of the magnets.
Three magnets are arranged on a table in the form of the shape shown in Fig. 4.17. What is the polarity, N or S, at the ends 1, 2, 3, 4 and 6 of the magnets? Polarity of one end (5) is given for you.
Answer
At every point where two magnets meet, the poles facing each other must be unlike poles, because unlike poles attract and hold the arrangement together. Also, the two ends of the same magnet are always opposite poles. Starting from the given end 5, which is N:
- Ends 5 and 6 are the two ends of the same magnet, so end 6 is S.
- End 4 faces end 5 (N), so by attraction end 4 is S.
- Ends 3 and 4 are the two ends of the same magnet, so end 3 is N.
- End 2 faces end 3 (N), so by attraction end 2 is S.
- Ends 1 and 2 are the two ends of the same magnet, so end 1 is N.
Therefore, the polarities are:
| End | Polarity |
|---|---|
| 1 | N |
| 2 | S |
| 3 | N |
| 4 | S |
| 5 | N (given) |
| 6 | S |