NCERT Solutions for Class 7th: Ch 14 Electric Current and its Effects Science
Page No: 170
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.
Answer
2. Draw the circuit diagram to represent the circuit shown in Fig.14.21.
Answer
In the fig., one end of the bulb is connected with the battery while other end is connected with safety pin which is a conductor. However, the other end battery is not connected with the safety pin which was required for the current to flow. Thus, the switch is in off position or circuit is not completed.
Answer
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
In the given circuit, the positive terminals of both the batteries are joined together so there is no flow of current and hence the bulb didn't glow.
Suggested changes: The current flows from positive terminals therefore the positive terminals of cell will be connected to negative terminal of the cell and vice versa.
5. Name any two effects of electric current.
Answer
Two effects of electric current:
(i) Heating effect
(ii) Magnetic effect
Page No: 171
6. When the current is switched on through a wire, a compass needle kept nearby gets deflected from its north-south position. Explain.
Answer
The current flowing through the wire produces magnetic effect around it which deflect the needle of the compass kept nearby as it is also a piece of magnet.
7. Will the compass needle show deflection when the switch in the circuit shown by Fig.14.24 is closed?
Answer
No, the circuit is not having any source of electricity. Therefore, there will be no flow of electric current through and thus there will be no magnetic effect and neither the deflection of the needle.
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 ____________ .
Answer
(a) Longer line in the symbol for 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 heats up.
(d) The safety device based on the heating effect of electric current is called a electric fuse.
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)
Answer
(a) F
(b) T
(c) F
(d) T
10. Do you think an electromagnet can be used for separating plastic bags from a garbage heap? Explain.
Answer
No, an electromagnet can be used for separating plastic bags from a garbage heap. Since plastic bags are non-magnetic material and electromagnet is used to seperate only the magnetic materials.
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
No, I will not agree with the electrician.
The wire use for making fuse have low melting point and therefore any wire cannot be used to replace the fuse. In most cases, the ordinary wires have high melting point.
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.
Possible defect:
(i) Bulb may have fused.
(ii) Rubber band of cell holder may not be tight and connections are loose.
(ii) Terminals of the cells may not be in the correct positions.
Page No: 172
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?
(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
(i) No, none of the bulb will glow when the switch is in the ‘OFF’ position.
(ii) All the bulbs will glow at once because connections are ok.
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