Class 7 Science · 20 questions · 20 with answers
When an electric current flows through a copper wire AB as shown in Figure14.1, the wire Fig. 14.1
This question refers to a figure in the original PDF.
(a) deflects a magnetic needle placed near it.
Choose the statement which is not correct in the case of an electric fuse.
(c) There is a minimum limit on the current which can safely flow in the electric circuits.
Three bulbs A, B, C are connected in a circuit as shown in Figure 14.2. When the switch is ‘ON’
This question refers to a figure in the original PDF.
(c) all the bulbs A,B and C will glow at the same time.
When a switch is in OFF position, (i) circuit starting from the positive terminal of the cell stops at the switch. (ii) circuit is open. (iii) no current flows through it. (iv) current flows after some time. Choose the combination of correct answer from the following.
(b) (ii) and (iii) are correct
Which of the following precautions need not be taken while using electric gadgets/appliances/circuit?
(d) We should never turn the switch in ON position.
Which property of a conducting wire is utilised in making electric fuse?
Low melting point.
Name the device used these days in place of electric fuses in electrical circuits.
Miniature Circuit Breaker (MCB).
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 _________.
Unscramble the following words:
(i) TBTAYER
(ii) SFEU
(iii) HTRCO
(iv) HICWTS
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.
No. The towel may burn due to the heat produced by the bulb. Besides, it will also result in wastage of electric energy.
Why are compact fluorescent lamps (CFLs) preferred over electric bulbs?
CFLs do not waste electricity as heat.
Why is an electric fuse required in all electrical appliances?
To check the excessive flow of electric current. SHORT ANSWER QUESTIONS
Can we use the same fuse in a geyser and a television set? Explain.
No, a geyser and a television set require different amount of current. Therefore the fuse used in these will be of different ratings.
Name two electric devices for each where
Heating effect – Geyser, room heater. Magnetic effect – Electric bell, Cranes to lift magnetic material.
Why do we cover plug pin holes which are within the reach of children with cellotape or a plastic cover when not in use?
The child may put his/her fingers into the socket and he/she may get an electric shock which could be fatal.
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 electro magnet will attract more pins? Give reason.
Paheli’s electomagnet will attract more pins as it has more number of turns of wire on it and thus a stronger electromagnet. ANSWERS 129 LONG ANSWER QUESTIONS
Your teacher has shown you the following activity. Fig. 14.3 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 the 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.
This question refers to a figure in the original PDF.
Some of the questions can be (i) Why does the nail attract the pins? (ii) What will happen if we connect more cells in the circuit? (iii) What will happen if we use some other material like a straw in place of the nail? (iv) What will happen if we wrap the wire on the nail more tightly? Any other appropriate question.
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 the circuits is the same.
This question refers to a figure in the original PDF.
(i) Will the heat produced in both the cases be equal? Explain. ELECTRIC CURRENT AND ITS EFFECTS 79
(ii) Will the heat produced be the same if the wires taken by them are of equal lengths but of different thickness? Explain. Fig. 14.4
How does the magnetic effect of electric current help in the working of an electric bell? Explain with the help of a diagram.
Hint: Working of an electric bell.
Draw the symbols of the following circuit components.
(i) electric cell
(ii) switch in off position
(iii) electric bulb
(iv) battery