NCERT Exemplar Solutions Class 7 Science Electric Current and Its Effects

Multiple Choice Questions............

Question 1:  When an electric current flows through a copper wire AB as shown in Figure14.1, the wire

(a) deflects a magnetic needle placed near it.

(b) becomes red hot.

(c) gives an electric shock.

(d) behaves like a fuse. 

Solution 1:  (a) deflects a magnetic needle placed near it.

With a magnetic field around it, a current-carrying wire acts as a magnet, the strength of which depends on the amount of current passing through it.

Question 2:   Choose the statement which is not correct in the case of an electric fuse.

(a) Fuses are inserted in electric circuits of all buildings.

(b) There is a maximum limit on the current which can safely flow through the electric circuits.

(c) There is a minimum limit on the current which can safely flow in the electric circuits.

(d) If a proper fuse is inserted in a circuit it will blow off if current exceeds the safe limit. 

Solution 2:  (c) There is a minimum limit on the current which can safely flow in the electric circuits.

In the case of an electrical fuse, there is only a maximum current limit that can flow safely through the electrical circuits.

Question 3:   Three bulbs A, B, C are connected in a circuit as shown in Figure 14.2. When the switch is ‘ON’

(a) bulb C will glow first.

(b) bulb B and C will glow simultaneously and bulb A will glow after some time.

(c) all the bulbs A, B and C will glow at the same time.

(d) the bulbs will glow in the order A, B and C.  

Solution 3:  (c) all the bulbs A, B and C will glow at the same time. 

As there is no lag time in the flow of current through the circuit, all the bulbs will glow simultaneously.

Question 4:  When a switch is in OFF position,

(i) circuit starting from the positive terminal of the cell stops at the switch.

(ii) the circuit is open.

(iii) no current flows through it.

(iv) current flows after some time. Choose the combination of the correct answer from the following.

(a) all are correct

(b) (ii) and (iii) are correct

(c) only (iv) is correct

(d) only (i) and (ii) are correct 

Solution 4:  (b) (ii) and (iii) are correct 

The circuit becomes incomplete when the turn is off, so it is open and no current passes into it.

Question 5:  Which of the following precautions need not be taken while using electric gadgets/appliances/circuit?

(a) We should never touch a lighted electric bulb connected to the mains.

(b) We should never experiment with the electric supply from the mains or a generator or an inverter.

(c) We should never use just any wire or strip of metal in place of a fuse.

(d) We should never turn the switch in ON position. 

Solution 5:  (d) We should never turn the switch in ON position. 

It is important to turn on electric gadgets/appliances/circuit switch to use, so statement d) is incorrect.

Very Short Answer Questions.............

Question 6:  Which property of a conducting wire is utilised in making electric fuse?

Solution 6:   In making electric fuses, the low melting point of the wire is used.

Question 7:   Name the device used these days in place of electric fuses in electrical circuits. 

Solution 7:    Miniature Circuit Breaker (MCB) is the device used these days in place of electric fuses in electrical circuits.

Question 8:   Fill in the blanks:

(i) Our body is a ________________ of electricity.

(ii) An electric cell produces electricity from the __________ ___________ in it.

(iii) In an electric circuit a fuse is a _________ _______ to prevent possible fire.

(iv) A combination of two or more cells is called a _________. 

Solution 8:

(i) Our body is a conductor of electricity.

(ii) An electric cell produces electricity from the Chemicals stored in it.

(iii) In an electric circuit, a fuse is a Safety device to prevent a possible fire.

(iv) A combination of two or more cells is called a switch.

Question 9:  Unscramble the following words:

(i) TBTAYER

(ii) SFEU

(iii) HTRCO

(iv) HICWTS 

Solution 9:

(i) Battery

(ii) Fuse

(iii) Torch

(iv) Switch

Question 10:  Paheli does not have a night lamp in her room. She covered the bulb of her room with a towel in the night to get dim light. Has she taken the right step? Give one reason to justify your answer. 

Solution 10:   No, Paheli's measures are incorrect because the towel may be burnt due to the heat generated by the bulb and the energy would be unnecessarily wasted by the glowing bulb as well.

Question 11:  Why are compact fluorescent lamps (CFLs) preferred over electric bulbs? 

Solution 11:   Compact fluorescent lamps (CFLs) are favoured to electric bulbs because CFL's absorb less electricity and, in the form of heat, do not lose energy.

Question 12:  Why is an electric fuse required in all electrical appliances? 

Solution 12:   For it works as a safety mechanism, an electric fuse is required and it tests excessive current flow.

Short Answer Questions........................

Question 13:  Can we use the same fuse in a geyser and a television set? Explain. 

Solution 13:   No, since a geyser and television set need a particular volume of current, we should not use the same fuse in a geyser and television. The fuse used in this would then have multiple scores.

Question 14:  Name two electric devices for each were (i) heating effect of current is used and (ii) magnetic effect of current is used. 

Solution 14:

(i) Heating effect of current is used

Geyser, room heater

(ii) Magnetic effect of current is used

Electric bell, Cranes to lift magnetic material

Question 15:  Why do we cover plug pinholes which are within the reach of children with cellotape or a plastic cover when not in use? 

Solution 15:  We cover plug pinholes which are within the reach of children with cellotape or a plastic cover when not in use so that the infant will not be able to insert his/her fingertips into the outlet which may result in a catastrophic electrical shock.

Question 16:  Boojho made an electromagnet by winding 50 turns of wire over an iron screw. Paheli also made an electromagnet by winding 100 turns over a similar iron screw. Which electromagnet will attract more pins? Give reason.

Solution 16:   Paheli’s electromagnet will attract more pins as it has more number of turns of wire on it and thus a stronger electromagnet. 

Long Answer Questions....................

Question 17:  Your teacher has shown you the following activity.

Activity: Teacher has wound a long insulated piece of wire around an iron nail in the form of a coil. Free ends of the wire are connected to a cell through a switch as shown in Figure 14.3. The current is switched on and some pins are placed near the ends of the nail. 

Write down any three questions that come to your mind about this activity.  

Solution 17:  

(i) Why would the nail attract the pins? 

(ii) What happens if we use some other substance in place of the nail, like a piece of wood? 

(iii) What will happen if we switch off the current?  

Question 18:   Paheli took a wire of length 10 cm. Boojho took a wire of 5 cm of the same material and thickness. Both of them connected the wires as shown in the circuit given in Figure 14.4. The current flowing in both circuits is the same. 

1. Will the heat produced in both the cases be equal? Explain.
2. Will, the heat produced be the same if the wires taken by them are of equal lengths but of different thickness? Explain.