Exploring Magnets

Reshma lives in a coastal town of Kerala and loves writing stories. She was writing a story for her grandmother's 60th birthday about a ship carrying spices from Kerala for trade in the olden days. Sailors used stars to find directions at night — but in her story a storm brought an overcast sky and the stars were hidden. Searching the internet and her school library, she learnt that travellers used a magnetic compass to find directions.

She had seen magnets in pencil boxes and purses (to keep them closed) and in a whiteboard duster, but had never looked at them carefully. She became curious about magnets.

• Sailors in the olden days used lodestones — naturally occurring magnets discovered in ancient times.
• Later, people found that magnets could be made from pieces of iron. Today magnets are made of different materials.
• Magnets in the school laboratory, pencil boxes, stickers and toys are artificial magnets.
• Shapes (Fig. 4.2 and p.78): bar, U-shaped, ring, disc, cylindrical, spherical. "My poles always occur in pairs, no matter my shape."

Activity 4.1: predict, then test, which objects stick to a magnet.

• Magnetic materials are attracted towards a magnet. Iron is magnetic; nickel and cobalt are other magnetic metals; some of their combinations with other metals are also attracted.
• Non-magnetic materials are not attracted — e.g. a wooden pencil, a rubber eraser, plastic, glass.

Activity 4.2: spread iron filings on paper and place a bar magnet over them. Maximum filings stick near the ends; very few stick in the middle.

• These ends are the two poles — the North pole and the South pole.
• Most filings stick to the poles of a magnet of any shape.
• A magnet with a single pole is not possible. If a magnet is broken into pieces, North and South poles exist in pairs even in the smallest piece.

Activity 4.3: hang a bar magnet from a thread tied at its middle so it balances horizontally; rotate it gently and let it rest. Mark the line. Push it again — it rests along the same line.

• A freely suspended magnet comes to rest along the north–south direction.
• The end pointing north = North-seeking (North) pole; the other = South-seeking (South) pole.
• It does this because our Earth itself behaves like a giant magnet.
• An iron bar suspended the same way can rest in any direction — so this activity is a way to test whether a piece of metal is a magnet.
• Hint for finding which way is north: the Sun rises roughly in the East and sets in the West.

• A small circular box with a transparent cover.
• A needle-shaped magnet is balanced on a pin so it can rotate freely.
• The end of the needle pointing North is usually painted red. Below the needle is a dial marked with directions.
• How to use it: keep it where you want to know directions; wait for the needle to rest north–south; gently rotate the box until N and S on the dial line up with the needle. Now all directions are as shown on the dial.

1. Place an iron sewing needle on a wooden table.
2. Put one pole of a bar magnet at one end of the needle and move it along the length to the other end; lift it up.
3. Bring the same pole back to the same starting end and repeat — 30 to 40 times.
4. Test: if iron filings or steel pins are attracted, the needle has become a magnet.
5. Push the needle horizontally through a cork and float it in a glass bowl of water, needle above the water. When it rests, your compass is ready — it always points the same way.

Activity 4.5: rest magnet A on 5–6 round pencils; bring each end of magnet B near one end of A without touching.

• Unlike poles attract (North of one + South of the other).
• Like poles repel (North + North, or South + South).
• With an iron bar instead of one magnet: both ends of the bar are attracted by both poles of the magnet — no repulsion.

Activity 4.6: a compass needle is also a magnet. Bring the North pole of a bar magnet near the North end of the needle → the needle moves away. Bring the South pole near it → the needle moves closer.

Place a piece of wood between the bar magnet and the compass, then try cardboard, thin plastic and thin glass. There is no appreciable change in the needle's deflection.

Conclusion: the magnetic effect can act through non-magnetic materials. "Magnets can move some objects without touching them!"

• Magnetic garland — pins/clips hanging in a chain from a magnet.
• Maze game — guide steel balls out of a cardboard maze by moving a magnet below the tray (works through cardboard).
• Paper clip in water — pull a steel clip out of a glass of water with a magnet outside, without wetting fingers or magnet.
• Matchbox-magnet cars — with like poles facing, the cars run away from each other.

How poles are marked: N and S letters; or a white dot for North; or North painted red and South blue.

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

Collect objects of different materials; predict which will stick to a magnet (Table 4.1), then test. Pencil (wood) and eraser (rubber) do not; iron objects do.

Bar magnet over iron filings; tap the paper. Filings cluster at the ends → the poles.

Suspend a bar magnet by a thread at its middle; it always comes to rest along the same line — north–south. Repeat with an iron bar: it rests in any direction.

Magnetise a sewing needle by stroking it with one pole 30–40 times, push it through a cork, float it in water — a home-made compass.

Magnet A on round pencils; bring each end of magnet B near it. Unlike poles attract, like poles repel. With an iron bar, both ends are always attracted.

N pole of bar magnet near N of compass needle → needle moves away; S pole → needle moves closer.

Place wood, cardboard, plastic and glass sheets between magnet and compass (Table 4.2) — no appreciable change in deflection.

Everyday items with magnets attached — pencil boxes, purses, a whiteboard duster, stickers, toys.

A bar magnet, a U-shaped magnet and a ring magnet.

A hand holding a magnet near various objects one by one to see which stick.

Thick tufts of iron filings at both ends of the bar magnet, very few in the middle.

A bar magnet hanging horizontally from a thread tied at its middle.

A round box with a transparent lid, a needle with a red north end balanced on a pin, and a direction dial (N, E, S, W) beneath.

(a) A bar magnet stroked along an iron needle; (b) the magnetised needle pushed through a cork, floating in a bowl of water.

Magnet A resting on round pencils; magnet B brought near (a) one way and (b) the other way — showing attraction and repulsion.

(a) N pole of the magnet near the needle's N end — the needle turns away; (b) S pole near it — the needle turns towards.

A piece of wood standing upright between the bar magnet and the compass.

A magnetic garland; steel balls in a cardboard maze; a steel paper clip in water; two matchbox cars with magnets, like poles facing.

Two bar magnets stored with unlike poles on the same side, a wooden piece between and soft iron pieces across the ends. Bar, disc, cylindrical, ring and spherical magnets, each with N and S marked.

1. 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.
2. True or False: (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. (iv) A freely suspended bar magnet always aligns with the north-south direction.
3. Column I (pole positions) / Column II (interaction) — fill the blanks: N–N, N–__ (Attraction), S–N, __–S (Repulsion).
4. Atharv rolled a bar magnet over a heap of steel U-clips (Fig. 4.15). Which option in Table 4.3 (pins at positions A, B, C) is likely? (i) 10, 2, 10 (ii) 10, 10, 2 (iii) 2, 10, 10 (iv) 10, 10, 10
5. Reshma has three identical metal bars — two are magnets, one is iron. How can she identify the magnets without using any other material?
6. You have a magnet with unmarked poles. How can you find its poles using another magnet with marked poles?
7. A bar magnet has no markings. How can you find its North pole without using another magnet?
8. If the Earth is itself a magnet, can you guess the poles of Earth's magnet from the direction of the magnetic compass?
9. Steel screws keep falling off a mechanic's screwdriver. Suggest a solution based on this chapter.
10. Two ring magnets X and Y (Fig. 4.16): X does not move down any further. Why? How can X be brought into contact with Y without pushing?
11. Three bar magnets are arranged as in Fig. 4.17. What is the polarity (N or S) at ends 1, 2, 3, 4 and 6? End 5 is N.

Worked answers to the textbook questions are in the Question Bank (section G).

1. Do magnets stick to objects made of certain materials only?
2. Do all parts of a magnet attract magnetic materials equally?
3. Can we find a magnet with a single pole?
4. What direction does the line along which the magnet rests indicate? How can we find it out?
5. How can we make our own magnetic compass?
6. What happens when we bring two magnets closer to each other?
7. The compass needle is also a magnet. Will it behave the same way when a magnet is brought near it?
8. If we place a piece of wood between the compass needle and the magnet, will it affect the deflection?

• Using 3–4 different magnets, see which lifts the most steel pins or U-clips; discuss why.
• Make a "Hopping Frog" toy with ring magnets fixed N–S alternately along a scale.
• Find out about the Maglev Train and try to make a model.
• Find out why magnets are made in different shapes.
• Collect information on the use of magnets in medicine.