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Do magnets stick to objects made of certain materials only?
Yes, magnets stick only to objects made of magnetic materials like iron, nickel, and cobalt. They do not attract non-magnetic materials like wood, plastic, glass, or rubber.
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Activity 4.1: Let us explore
Table 4.1: Identifying the materials attracted by a magnet
| Name of the object | Material which the object is made of (plastic/wood/glass/iron/any other) | Attracted by the magnet (Yes/No) | |
|---|---|---|---|
| Prediction | Observation | ||
| Pencil | Wood | No | No |
| Eraser | Rubber | No | No |
| Iron nail / screw | Iron | Yes | Yes |
| Paper clip | Steel / Iron | Yes | Yes |
| Plastic scale | Plastic | No | No |
| Glass tumbler | Glass | No | No |
| Steel key | Metal (Iron/Steel) | Yes | Yes |
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Was your prediction correct for all objects? Which materials stuck to the magnet? What conclusion can you draw?
Yes, most predictions were correct. Objects made of iron and steel stuck to the magnet. We conclude that magnets attract only magnetic materials (like iron, nickel, cobalt) and do not attract non-magnetic materials.
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Which materials listed in Table 4.1 were found to be non-magnetic?
Wood (pencil), rubber (eraser), plastic (scale), and glass (tumbler) were found to be non-magnetic materials.
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Do all parts of a magnet attract magnetic materials equally?
No, magnetic attraction is strongest at the two ends (poles) of the magnet and weakest near the middle.
Activity 4.2: Let us investigate
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Do you observe anything special about the way they stick to the magnet? Do the iron filings stick all over the magnet uniformly? Or do the iron filings stick more at some places?
The iron filings do not stick uniformly. Maximum iron filings stick near the two ends (poles) of the bar magnet, and very few stick near the middle.
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If we repeat this activity with magnets of other shapes, do we get the same result?
Yes, regardless of the shape of the magnet (horseshoe, U-shaped, ring, cylindrical), iron filings always attract the most at its poles.
Page 114
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Can we find a magnet with a single pole?
No, an isolated single magnetic pole (monopole) cannot exist. Magnetic poles always exist in pairs (North pole and South pole), even if a magnet is broken into smaller pieces.
Activity 4.3: Let us experiment
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Now again rotate the magnet by giving a gentle push at its one end and wait till it comes to rest. Does the magnet rest along the same line?
Yes, every time the magnet comes to rest, it aligns itself along the exact same North-South line.
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What direction does this line indicate along which the magnet rests? How can we find it out?
The line indicates the geographic North-South direction. We can confirm this by knowing that the Sun rises in the East and sets in the West; facing the rising sun (East), North is to our left, which aligns with the North-seeking pole of the magnet.
Page 116
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Repeat this activity with a small iron bar in place of the bar magnet. What do you observe this time? Does it always rest along north-south direction?
No, an ordinary iron bar does not always rest in the north-south direction; it can come to rest in any random direction. Only a magnetized object always aligns along the North-South direction.
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How can we make our own magnetic compass?
We can make a magnetic compass by magnetizing an iron sewing needle by stroking it with a bar magnet 30-40 times in one direction, inserting it through a small cork disc, and floating it in a bowl of water.
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Activity 4.4: Let us construct
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Rotate the cork gently and wait till it stops rotating. Repeat this a few more times. Do the ends of the needle always point in the same direction?
Yes, the magnetized needle floating on the cork will always point along the North-South direction whenever it comes to rest.
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What happens when we bring two magnets closer to each other?
When two magnets are brought close, unlike poles (North-South) attract each other, whereas like poles (North-North or South-South) repel each other.
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Activity 4.5: Let us experiment
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Next, bring the other end of magnet B near the same end of magnet A (Fig. 4.8(b)). Does the magnet A on the pencils begin to move? Does it always move in the direction of the approaching magnet? What do these observations suggest?
Yes, Magnet A moves. It moves towards Magnet B when opposite poles face each other (attraction) and moves away when similar poles face each other (repulsion). This suggests that like poles repel each other and unlike poles attract each other.
Page 122
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Repeat the activity by using an iron bar in place of one of the magnets. What do you observe this time?
Both ends of the plain iron bar get attracted to both the North and South poles of the magnet. The iron bar never shows repulsion. Therefore, repulsion is the sure test of magnetism.
Activity 4.6: Let us experiment
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The compass needle is also a magnet. Will it show the same behaviour if a magnet is brought closer to it?
Yes, bringing the North pole of a bar magnet near the North pole of the compass needle repels it (pushes it away), while bringing the South pole attracts the North pole of the needle.
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Now repeat the above step with the South pole of the bar magnet. Do you observe a difference this time?
Yes, when the South pole of the bar magnet is brought near the North pole of the compass needle, the needle is attracted towards the magnet.
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Suppose we place a piece of wood between the compass needle and the magnet. Will this affect the deflection of the compass needle?
No, the deflection of the compass needle is not stopped because the magnetic force can easily pass through non-magnetic materials like wood.
Activity 4.7: Let us investigate
Table 4.2: Observing the effect of magnet through non-magnetic materials
| S. no. | Material placed between the magnet and the compass needle | Observations |
|---|---|---|
| 1. | Wood | Compass needle continues to deflect normally |
| 2. | Cardboard | Compass needle continues to deflect normally |
| 3. | Plastic | Compass needle continues to deflect normally |
| 4. | Glass | Compass needle continues to deflect normally |
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Can we make a garland using pins and a magnet?
Yes, each steel pin in contact with a magnet gets temporarily magnetised and attracts the next pin, forming a chain or garland of pins hanging from the magnet.
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Can we take the steel balls out of the maze by moving a magnet below the cardboard tray?
Yes, because magnetic force acts through the cardboard tray (a non-magnetic material), allowing us to guide and roll the steel balls along the paths of the maze.
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Can we pick out a steel paper clip fallen in water using a magnet, without getting our fingers or the magnet wet?
Yes, by bringing a strong magnet near the outer surface of the glass tumbler and sliding it upwards, the steel paper clip is attracted through the glass and water and follows the magnet to the top.
Page 128
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Will the two cars speed towards each other or run away from each other when brought closer? (Fig. 4.14)
Since the two matchbox cars have like poles facing each other, they will repel and run away from each other. If unlike poles were facing each other, they would speed towards each other.
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Let us enhance our learning
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1. Fill in the blanks:
(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. -
2. State whether the following statements are True (T) or False (F).
Statement Ans (i) A magnet can be broken into pieces to obtain a single pole. False (ii) Similar poles of a magnet repel each other. True (iii) Iron filings mostly stick in the middle of a bar magnet when it is brought near them. False (iv) A freely suspended bar magnet always aligns with the north-south direction. True -
3. 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 Repulsion S - N Attraction S - S Repulsion -
4. Deepak 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?
Answer: Option (i) — Position A: 10, Position B: 2, Position C: 10
Reason: Positions A and C are the poles (ends) of the bar magnet where magnetic attraction is strongest, while position B is in the middle where magnetic force is weakest.
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5. Hasini 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)?
She can test the ends of the bars against one another:
1. A plain iron bar will only show attraction with every end of both magnets.
2. The two magnets will show repulsion when their like poles are brought together.
Since repulsion occurs only between two magnets, the two bars that repel each other at certain ends are the magnets, and the remaining bar is the iron piece. -
6. 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?
Bring the known North (N) pole of the marked magnet near one end of the unmarked magnet:
- If it repels, that end of the unmarked magnet is the North pole.
- If it attracts, that end is the South pole. -
7. 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?
Tie a thread at the centre of the bar magnet and suspend it freely from a stand so that it can rotate horizontally. Once it comes to rest, the end pointing towards the geographic north is its North pole, and the other end is its South pole.
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8. 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?
Since opposite poles attract, the North pole of the compass needle points towards geographic North. This means the Earth's magnetic pole located near the geographic North Pole behaves like a magnetic South pole, and the one near the geographic South Pole behaves like a magnetic North pole.
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9. 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.
The mechanic can magnetize the tip of the steel screwdriver by rubbing a strong bar magnet along its length several times in one direction. The magnetic tip will firmly hold the steel screws in place and prevent them from falling.
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10. 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.
- Reason: The bottom face of magnet X and the top face of magnet Y have like poles facing each other, causing magnetic repulsion that levitates magnet X.
- Way to bring them in contact: Remove magnet X, flip it upside down (invert its poles), and slide it back onto the wooden rod. Unlike poles will now face each other, causing them to attract and stick together. -
11. 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.
Given: End 5 = N
- For the bottom horizontal magnet: If 5 = N, then end 6 = S.
- For the middle vertical magnet: End 4 is joined with end 5 (N), so end 4 = S (attraction), and its opposite end 3 = N.
- For the top horizontal magnet: End 2 is joined with end 3 (N), so end 2 = S (attraction), and its opposite end 1 = N.
Final Polarities:
- 1 = N
- 2 = S
- 3 = N
- 4 = S
- 6 = S
