Chapter 15 - Fun with Magnets
1. How will you do this ?
(a) Determine whether a material is magnetic or non-magnetic.
Ans: If a material sticks to a magnet, then it is a magnetic material; otherwise, it is a non-magnetic material.
(b) Explain that a magnet has a certain magnetic field.
Ans: Every magnet has magnetic lines that originate at the North Pole and enter the South Pole. Inside the magnet field, lines are directed from south to north.
(i) Take some iron fillings, a bar magnet, and a cardboard box.
(ii) Sprinkle the iron fillings on the cardboard.
(iii) Notice the position of the iron fillings.
(iv) Now bring the bar magnet near the fillings.
(v) You will find that the fillings get attracted to the bar magnet, and a pattern will form near the magnet.
The pattern will be somewhat similar to the one shown in the figure.
This pattern follows the pattern of the magnetic lines of the magnet, i.e., it is closer near the poles and farther apart near the centre of the magnet. The iron fillings tend to experience force along the direction of magnetic lines.
(c) Find the north pole of a magnet.
Ans: Tie a thread to the centre of a magnet and hang it from a stand as shown. Wait till the magnet settles itself.
The pole that points towards the geographic north is the north pole of the magnet.
2. Which magnet will you use ?
(a) Iron is to be separated from trash.
Ans: Electromagnets can be used to separate iron from trash.
(b) You are lost in a forest.
Ans: A bar magnet or loadstone can be used to find the direction in the forest. This is because a bar magnet, when suspended freely, always rests in the north-south direction.
(c) A window shutter opens and shuts continuously in the wind.
Ans: A permanent magnet can be used to stop the continuous shuttering of the window in the wind.
3. Fill in the blanks with the appropriate word.
(a) If a bar magnet is hung by a thread tied at its centre, its north pole becomes steady in the direction of the ……………. Pole of the earth.
(South, north, east, west)
Ans: north
(b) If a bar magnet is cut into equal pieces by cutting it at right angles to its axis at two places, ……….. bar magnets are formed, and a total of ……………. poles are formed.
(6,3,2)
Ans: 6
(c) There is repulsion between the ………. poles of a magnet, and attraction between its ………… poles.
(opposite, like.)
Ans: like, opposite
(d) When magnetic material is taken close to a magnet, the material acquires …………… .
(permanent magnetism, induced magnetism)
Ans: induced magnetism.
(e) If a magnet attracts a piece of metal, that piece must be made of …………. .
(any other metal but iron, magnetic material or iron, non-magnetic material)
Ans: magnetic material or iron.
(f) A magnet remains steady in a ………….. direction.
(east-west, north-south)
Ans: north-south.
4. Write the answers in your words.
(a) How is an electromagnet made ?
Ans: When an iron article is wound around a copper wire and an electric current is sent through the wire, magnetism is produced in the iron article. If the electric current is shut off, the magnetism is lost. This is how temporarily produced magnetism can be created by creating electromagnetism.
(b) Write the properties of a magnet.
Ans:
(i) A magnet has two poles, which are referred to as the North Pole and the South Pole.
(ii) A magnet, when suspended freely, always rests in the north-south direction.
(iii) A magnet has attractive properties, i.e., it always attracts magnetic materials towards it.
(iv) Like poles of magnets repel each other, whereas unlike poles attract each other.
(v) The two poles of a magnet are inseparable. It means that when a magnet is cut into two parts, two independent magnets are formed.
(vi) The magnetic force of a magnet is concentrated at its two poles.
(c) What are the practical uses of a magnet ?
Ans:
(i) Pin holders, doorbells, doors of a fridge, car doors, and magnets attracted to a fridge for decorations are some of the articles in which magnets are used.
(ii) The crane uses magnets, which are very powerful. They are usually electromagnets.
(iii) Hard discs for computers, audio and video CDs, etc. also use magnetic materials.
(iv) Modern medical techniques such as MRI (Magnetic Resonance Imaging) also use magnetism. ATM cards, credit cards, and debit cards also have a magnetic strip attached to them.