NCERT CLASS 7 SCIENCE CHAPTER 14 ELECTRIC CURRENT AND ITS EFFECTS
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NCERT CLASS 7 SCIENCE CHAPTER 14 ELECTRIC CURRENT AND ITS EFFECTS
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NCERT CLASS 7 SCIENCE CHAPTER 14 ELECTRIC CURRENT AND ITS EFFECTS
Question Answers
NCERT IN – TEXT QUESTIONS OF BOOJHO AND PAHELI
Q.1 Paheli and Boojho wonder whether the batteries used in tractors, trucks and inverters are also made from cells. Then why is it called a battery? Can you help them to find the answer to this question. (page 162)
Fig. Truck battery and its cutout
Ans. The battery used in tractors, trucks and inverters is a combination of several cells. These cells are not dry cells. These are the several sets of plates and each. Set of plates acts like a cell.
Q.2 Boojho could not see element in an electric iron. Paheli told him that electrical appliances, such as immersion heaters, hotplates, irons, geysers, electric kettles, hair dryers, have elements inside them. Have you ever seen the element in any appliance? (page 164)
Fig. Element of electric iron.
Ans. Yes, I have seen the element of an electric iron at the electric repair shop near my house.
Q.3 Paheli and Boojho saw a magic trick sometimes back. The magician placed an iron box on a stand. He then called Boojho and asked him to lift the box. Boojho could easily lift the box. Now the magician made a show of moving his stick around the box while muttering something. He again asked Boojho to lift the box. This time Boojho could not even move it. The magician again muttered something and now Boojho could lift the box. (page 172)
The audience, including Paheli and Boojho, were very impressed with the show and felt that the medicine had some Supernatural powers. However, after reading this chapter Paheli is wondering if the trick was indeed some magic or some science was involved? Can you guess what science might be involved?
Ans. The magician arranged an electromagnet below the box. The electromagnet could be turned ‘ON’ and ‘OFF’ as when the magician signals his assistance. When the electromagnet is turned ‘ON’ it attracts the iron box and hence it could not be lifted
NCERT TEXTBOOK EXERCISE (pages 170 – 172)
Q.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.
Ans. Table
Q.2 Draw the circuit diagram to represent the circuit shown in Fig.
Fig. Circuit.
Ans.
Q.3 Figure 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. A cell.
Ans. Fig. Battery.
Q.4 The bulb in the circuit shown in Fig. does not glow. Can you identify the problem? Make necessary changes in the circuit to make the bulb glow.
Ans. The problem with the circuit is that both the negative terminals are connected to the bulb. The corrected diagram is shown below:
Q.5 Name any two effects of electric current.
Ans. The two effects of current are heating effect and magnetic effect.
Q.6 When the current is switched on through a wire, a compass needle kept nearby gets deflected from its north – South position. Explain.
Ans. When electric current passes through a wire, it behaves like a magnet. This is the magnetic effect of the electric current due to attraction of the wire. So, the compass needle which is a magnet, gets deflected.
Q.7 Will the compass needle show deflection when the switch in the circuit shown by figure is closed?
Ans. No, because there is no cell, so no current will flow.
Q.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………
Ans. (a) Positive; (b) battery; (iii) get heated; (iv) fuse.
Q.9 Mark ‘T’ if the statement is true and ‘F’ if it is false:
(i) 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)
(ii) When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks. (T/F)
(iii) An electromagnet does not attract a piece of iron. (T/F)
(iv) An electric bell has an electromagnet. (T/F)
Ans. (i) False (F); (ii) True (T); (iii) False (F); (iv) True (T).
Q.10 Do you think an electromagnet can be used for separating plastic bags from a garbage heap? Explain.
Ans. No, the plastic bags do not get attracted by the magnet, so they cannot be separated by an electromagnet. The plastic bags are not magnetic materials, only magnetic materials like iron can be attracted by magnet.
Q.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.
Ans. No, because this piece of wire will not melt even if high current flows through it. So, it will not prevent the damage done by high current.
Q.12 Zubeda made an electric circuit using a cell holder shown in figure 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.
Fig. A cell holder.
Ans. It is important to put the cells in right series. The positive terminal of the cell should be connected with negative terminal of the second cell. The switch should be closed properly the bulb should not be fused. If Zubeda will check these things, then the bulb will certainly glow.
Q.13 In the circuit shown in figure 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?
Ans. (i) No, bulb will not glow.
(ii) The bulb A will glow first, followed by bulb B and then bulb C because bulb A comes first in the path of electric current flowing from positive terminal towards the negative terminal of the battery.
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