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Electric Current And Its Effects

Class 7th Science CBSE Solution
Exercise
  1. Draw in your notebook the symbols to represent the following components of electrical…
  2. Set up the circuit shown in Fig. 14.17 again. Move the key to ON position and watch…
  3. Draw the circuit diagram to represent the circuit shown in Fig.14.21.…
  4. Make four electromagnets with 20, 40, 60 and 80 turns. Connect them one by one to a…
  5. Fig.14.22 shows four cells fixed on a board. Draw lines to indicate how you will connect…
  6. Paheli and Boojho saw a magic trick sometime back. The magician placed an iron box on a…
  7. Using an electromagnet, you can make a working model of a railway signal as shown in…
  8. The bulb in the circuit shown in Fig.14.23 does not glow. Can you identify the problem?…
  9. Visit an electric shop. Request a mechanic to show you the various types of fuses and MCB…
  10. Name any two effects of electric current.
  11. When the current is switched on through a wire, a compass needle kept nearby gets…
  12. Will the compass needle show deflection when the switch in the circuit shown in Fig. 14.69…
  13. Fill in the blanks: a. Longer line in the symbol for a cell represents its…
  14. Mark T if the statement is true and F if it is false: a. To make a battery of two cells,…
  15. Do you think an electromagnet can be used for separating plastic bags from a garbage heap?…
  16. An electrician is carrying out some repairs in your house. He wants to replace a fuse by a…
  17. Zubeda made an electric circuit using a cell holder shown in Fig. 14.4, a switch and a…
  18. In the circuit shown in Fig. 14.25 (i) Would any of the bulb glow when the switch is in…
Exercise
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
Answer:

The various symbols which represents the following components of electrical circuits can be drawn as follows:


1. Connecting Wires:



2. Switch in the ‘OFF’ Position



3. Bulb



4. Cell



5. Switch in the ‘ON’ position



6. Battery
Question 2.

Set up the circuit shown in Fig. 14.17 again. Move the key to ‘ON’ position and watch careful in which direction the compass needle gets deflected. Switch ‘OFF’ the current. Now keeping rest of the circuit intact, reverse the connections at the terminal of the cell. Again, switch ‘on’ the current. Note the direction in which the needle gets deflected. Think of an explanation.
Answer:

On reversing the connections of cell, the direction of deflection of needle will also gets reversed because the polarity of magnet is dependable on the current direction in which it is flowing.
Question 3.

Draw the circuit diagram to represent the circuit shown in Fig.14.21.
Answer:

The components in above diagram are :


1. Connecting wires


2. Bulb


3. Key


4. Cell


So, the circuit diagram using the symbols can be drawn as follows:
Question 4.

Make four electromagnets with 20, 40, 60 and 80 turns. Connect them one by one to a battery of 2 cells. Bring the electromagnet near a box of pins. Count the number of pins attracted by it. Compare the strengths of the electromagnets.
Answer:

On increasing the number of turns in the coil, the strength of electromagnets will be increased. The electromagnet having maximum turns i.e. 80 will have highest strength and electromagnet with least no. of turns will have least strength.
Question 5.

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.
Answer:

To make a battery of four cells, the positive end of the first cell is connected to negative end of second cell. The negative end of the cell is connected to the switch. The positive end of the second cell is connected to negative of third cell and positive of third cell is connected to negative of fourth cell. The positive of fourth cell is connected to the bulb. Thus, making a battery of four cells.
Question 6.

Paheli and Boojho saw a magic trick sometime 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 not Boojho could lift the box.

The audience, including Paheli and Boojho, were very impressed with the show and felt that the magician had some supernatural powers. However, after reading this chapter Paheli wondering if the trick was indeed some magic or some science was involved? Can you guess what science might be involved?
Answer:

There was no magic involved in it instead there is role of science in it because the magician might have put an electromagnet below the box which is turned ‘ON’ and ‘OF’ when the magician moves his stick.


When it is ‘ON’ , it attracts the box of iron and therefore, it was not lifted by Boojho. When it was ‘OFF’, it was easily picked up by Boojho because there was no attractive force then.
Question 7.

Using an electromagnet, you can make a working model of a railway

signal as shown in Fig.14.26.
Answer:

1. Take a cardboard and make a hold into it.


2. Attach the carbon hole to a stand.


3. Tie a thread to one end of the cardboard and tie an iron nail to the thread.


4. Make an electromagnet and rotate the coil of wire on it. Connect the wire to a battery and a key.


In this way, a railway signal can be made.
Question 8.

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:

The bulb didn’t glow because the negative terminals of both the battery have been connected to the bulb. The positive terminal of first cell should be connected to the negative terminal of another cell. The other ends should be connected to bulb and key.


The Correct Diagram will be as follows:
Question 9.

Visit an electric shop. Request a mechanic to show you the various

types of fuses and MCB and to explain how they work.
Answer:

The various types of MCB are:


1. MCB


2. MCCB


3. ACB


4. VCB


5. RCCB


The various types of fuses and MCB are:


1. AC fuses:


AC fuses are little bit smaller than the DC fuses because in AC the voltage with 60 Hz changes its amplitude from 0 to 60 times every second.


2. DC fuses:


When the metallic wire melts, heat generated due to large current, an arc is produced. DC fuses are bigger than AC fuses to increase the distance between the electrodes. So that the arc is reduced.


3. One time use only fuses:


These types of fuses are used once only. When the high current flows, they melt and gets burnt out. These fuses are replaced manually.


4. Cartridge fuses:


These types of fuses have plastic body and two metal caps for fitting in the socket. These are used generally in automobiles.


5. Resettable fuses:


These fuses can work multiple times. They open the circuit when high current flows through it and after some time they again connect the circuit.
Question 10.

Name any two effects of electric current.
Answer:

Electric current produces following two effects:


(i) Heating effect : When the energy from the source dissipates as heat.


(ii) Magnetic effect: When electric current flowing in the wire produces a magnetic field around it.
Question 11.

When the current is switched on through a wire, a compass needle kept nearby gets deflected from its north-south position. Explain.
Answer:

The compass consists of a tiny magnet and a needle which gives the direction of the poles.

When electric current is allowed to pass through a wire, due to this a magnetic force is generated which will led to compass needle deflection from its north to south position.
Question 12.

Will the compass needle show deflection when the switch in the circuit shown in Fig. 14.69 is closed?
Answer:

When the switch in the circuit shown in the question is closed, there will be no current flowing in the circuit and hence there will be no deflection in the compass needle.
Question 13.

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 a cell represents its positive terminal. A battery has two ends. The longer line represents positive end and the shorter line represents negative end.


b. The combination of two or more cells is called a battery. When two or more cells are combined together, they are known as battery.


c. When current is switched ‘on’ in a room heater, it gets heated. This happens because of heating effects of current.


d. The safety device based on the heating effects of electric current is called a fuse. Using an electric fuse prevents the flow of unduly high electric current and protects the appliances from the damage.
Question 14.

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 electro-magnet. (T/F)
Answer:

a. F


To make a battery of two cells, the negative terminal of one cell is connected to the positive terminal of the other cell and not to the negative terminal. So this statement is false.


b. T


When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks. This statement is true. Using an electric fuse prevents the flow of unduly high electric current and protects the appliances from the damage.


c. F


This statement is false. An electromagnet attracts a piece of iron.


d. T


An electric bell has an electromagnet and it shown an oscillatory motion. As the hammer hits the bell and it vibrates rapidly.
Question 15.

Do you think an electromagnet can be used for separating plastic bags from a garbage heap? Explain.
Answer:

No, an electromagnet can’t be used for separating plastic bags from a garbage heap because plastic bags are not attracted by the electromagnets and therefore it can’t separate them from garbage heap.
Question 16.

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 won’t agree because it is possible that the wire might not melt when there is flow of high current and can damage appliances also.


Using an electric fuse prevents the flow of unduly high electric current and protects the appliances from the damage.
Question 17.

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:

If the bulb didn’t glow, there are chances that the bulb taken by Sumedha is fused already, or the cells used in the circuit is exhausted or she hasn’t made correct connections in the circuit. It is possible that she have connected same terminals of cells.
Question 18.

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:

(i) No, bulb will not glow when the switch is in the ‘OFF position’ because in ‘OFF’ position, there will be no current flowing through the circuit.


(ii) When the switch is ‘ON’, the bulbs will glow at the same time as they are connected in series and same amount of current is flowing through all of them.