19. Properties of a Magnetic Field
Complete solutions to the Exercises. Maharashtra State Board (Balbharati), Grade Seven General Science.
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Exercises
(a) The alloys called alnico and nipermag are used for making industrial magnets.
(b) A magnetic field can pass through cardboard and water.
(c) The intensity of a magnetic field is indicated by the lines of force.
(d) The real test of a magnet is repulsion.
| Group ‘A’ | Group ‘B’ |
|---|---|
| (a) Compass | 4. A magnetic needle |
| (b) Door of a cupboard | 3. A magnet |
| (c) Repulsion | 2. Like poles |
| (d) Magnetic pole | 1. The highest magnetic force |
(a) Distinguish between the two methods of making artificial magnets.
| Single touch method | Double touch method |
|---|---|
| Only one bar magnet is used. | Two bar magnets are used. |
| One pole of the magnet is rubbed from one end of the steel bar to the other end. | Opposite poles of two magnets are rubbed from the centre of the bar towards its two ends. |
| The magnet produced is weak. | The magnet produced is strong. |
| Magnetism lasts for a short time. | Magnetism lasts longer. |
(b) Which substances are used for making electromagnets?
An electromagnet is made by winding an insulated copper wire around a soft iron bar. When electric current flows through the wire, the soft iron becomes a magnet. When the current stops, it loses its magnetism.
(c) Write a note on ‘magnetic field’.
The space around a magnet in which its magnetic force acts is called its magnetic field. It is shown by magnetic lines of force.
- The lines of force go from the north pole to the south pole outside the magnet.
- They never cross each other.
- They are close together near the poles. So the field is strong there.
- A magnetic field can pass through cardboard, glass and water.
The earth also has a magnetic field.
(d) Why is a magnetic needle used in a compass?
A freely suspended magnet always rests in the north-south direction. A magnetic needle is light and thin. It can turn freely. It quickly rests in the north-south direction. Once we know north, we can find other directions. So a magnetic needle is used in a compass.
(e) Explain with the help of a diagram how the intensity and direction of the magnetic field of a bar magnet can be determined.
Magnetic lines of force around a bar magnet. The lines are crowded near the poles, where the field is strong, and spread out farther away, where it is weak.
Direction:
- Keep a bar magnet on a paper and draw its outline.
- Keep a small compass near the north pole. Mark dots at both ends of its needle.
- Move the compass ahead and mark dots again. Repeat until it reaches the south pole.
- Join the dots. This is a line of force. Its arrow shows the direction from the north pole to the south pole.
Intensity: The lines of force are close together near the poles. So the field is strong there. They spread apart away from the magnet. So the field is weak there.
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In olden times, merchants travelled far by land and sea for trade. In deserts and seas, it was difficult to find directions. So they used magnets.
- A piece of natural magnet (lodestone) was hung by a thread. It always rested in the north-south direction.
- The Chinese used a spoon-shaped lodestone placed on a plate.
- Indian sailors used a fish-shaped piece of magnetised iron floating in oil. It was called matsya yantra.
Thus, merchants found directions and kept to their route.
- A metal detector works on the principle of electromagnetism.
- A coil in it produces a magnetic field.
- When a metal object comes near, the detector gives a signal like a beep.
Uses:
- At airports, railway stations and malls to check people for metal objects.
- To find metals and ores under the ground.
- To find landmines.
- In factories, to find pieces of metal mixed in food.