Class 7 Science Chapter 14 |
Electric Current and its Effect NCERT Solutions
Question 1. Draw in your notebook the symbols to represent the following components of electrical circuits: connecting wires, switch in the ‘OFF’ position, bulb, cell, switch in the ‘ON’ position, and battery.
Question 2. Draw the circuit diagram to represent the circuit shown in Fig.14.21.
Question 3. Fig.14.22 shows four cells fixed on a board. Draw lines to indicate how you will connect their terminals with wires to make a battery of four cells. Fig. 14.22
Answer 3 To make a battery of a cell, the positive terminal (+) of one cell always connected to negative terminal (-) of another cell.
Question 4. The bulb in the circuit shown in Fig.14.23 does not glow. Can you identify the problem? Make necessary changes in the circuit to make the bulb glow.
Answer 4 The positive terminal (+) of one cell always connected to negative terminal (-) of another cell.Here the two positive terminal are connected to each other.The correct arrangement is :
Question 5. Name any two effects of electric current.
Answer 5 The two effects of electric current are:
1) Heating effect of current: When an electric current passes through a wire, the wire gets heated. This effect is used in many heating appliances like electric heater, electric iron, light bulb etc.
2) Magnetic effect of current: When an electric current flows through a wire, it generates magnetic field around it. This effect has many applications like power life, electric bell, electric fan.
Question 6. When the current is switched on through a wire, a compass needle kept nearby gets deflected from its north-south position. Explain.
Answer 6 When an electric current flows through a wire, it generates magnetic field around it.When a compass is kept nearby this wire, the two magnetic field interact each other causing deflection in magnetic needle.When the current is switched off, there is no magnetic field produced by the wire, magnetic needle does not deflect from its north-south direction.
Question 7. Will the compass needle show deflection when the switch in the circuit shown by Fig.14.24 is closed? Fig. 14.24
Answer 7 No, the compass needle will not show deflection even though the switch is closed, because there is no source electric current.In the absence of electric current, there is no magnetic field produced by wires.
Question 8. Fill in the blanks:
(a) Longer line in the symbol for a cell represents its_________ terminal.
(b) The combination of two or more cells is called a______.
(c) When current is switched ‘on’ in a room heater, it________.
(d) The safety device based on the heating effect of electric current is called a______.
(a) Longer line in the symbol for a cell represents its positive terminal.
(b) The combination of two or more cells is called a battery
(c) When current is switched ‘on’ in a room heater, it becomes red hot.
(d) The safety device based on the heating effect of electric current is called a fuse
Question 9. Mark ‘T’ if the statement is true and ‘F’ if it is false:
(a) To make a battery of two cells, the negative terminal of one cell is connected to the negative terminal of the other cell. (T/F)
(b) When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks. (T/F)
(c) An electromagnet does not attract a piece of iron. (T/F)
(d) An electric bell has an electromagnet. (T/F)
(a) To make a battery of two cells, the negative terminal of one cell is connected to the negative terminal of the other cell:(F)
(b) When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks:(T)
(c) An electromagnet does not attract a piece of iron:(F)
(d) An electric bell has an electromagnet:(T)
Question 10. Do you think an electromagnet can be used for separating plastic bags from a garbage heap? Explain.
Answer 10 An electromagnet can attract iron piece only. Plastic does not have any magnetic property , so an electromagnet can not be used for separating plastic bags from a garbage heap.
Question 11. An electrician is carrying out some repairs in your house. He wants to replace a fuse by a piece of wire. Would you agree? Give reasons for your response.
Answer 11 No, we would not agree to allow to replace the fuse by a wire. Fuse is a device that is a safeguard to all electrical appliances. They are made up of special material which melt quickly and break when a large electric current passed through it. If an electrician uses ordinary wire as fuse, it increases the risk of overheating of wires due to flow of excessive current. It may lead to short circuit in electric equipment and appliances may catch fire.
Question 12. Zubeda made an electric circuit using a cell holder shown in Fig. 14.4, a switch and a bulb. When she put the switch in the ‘ON’ position, the bulb did not glow. Help Zubeda in identifying the possible defects in the circuit.
Answer 12 1) The bulb may be fused or defective.
2) There may be loose connections.
3) Cells are not connected properly.
4) The switch is not functioning well.
5) The cells are dried up.
Question 13. In the circuit shown in Fig. 14.25
(i) Would any of the bulb glow when the switch is in the ‘OFF’ position?
(ii) What will be the order in which the bulbs A, B and C will glow when the switch is moved to the ‘ON’ position?
Answer 13 1) None of the bulb will glow when the switch is in OFF position, since the electric circuit is not closed.
2) When the switch is in ON position, circuit is complete and electric current will flow immediately. All of the bulbs will glow instantly.