NCERT Solutions for Class 7 science Chapter 3: Electricity Circuits and Their Components Class 7

Why does the torch lamp glow in one position of its switch? (Page 24)
Answer:
The torch lamp glows when the switch is in the ‘ON’ position because it completes the electrical circuit, allowing current to flow through the lamp. When the switch is in the ‘OFF’position, the circuit is open, and current cannot flow, so the lamp does not glow

In a torch, we generally use more than one cell. Are those placed in any particular order? (Page 25)
Answer:
Yes, in a torch, cells are usually placed in series. The positive terminal of one cell is connected to the negative terminal of the next. This arrangement increases the total voltage, which powers the lamp.

How does a switch turn ‘ON’ or ‘OFF’ the torchlight? (Page 32)
Answer:
The switch controls the flow of electricity in the circuit. When the switch is ‘ON’, it closes the circuit, allowing electricity to flow through the lamp and make it glow. When the switch is ‘OFF’, it opens the circuit, stopping the flow of electricity, and the lamp goes off.

Can we represent the circuit in a simpler manner? (Page 33)
Answer:
Yes, the circuit (often representing components like cells, bulbs, or switches in a circuit diagram) can be represented by symbols. For example, a battery is represented by a pair of short and long parallel lines, and a lamp is represented by a circuit with an ‘X’ inside it.

Why did we use metal wire for making the electric circuit? Can we not use some other material for the wires? (Page 34)
Answer:
Metal wires (usually copper) are used because metals are good conductors of electricity. They allow electric current to flow easily. While other materials could be used but they would not conduct electricity as efficiently. For example, rubber and plastic are insulators, so they can’t carry current.

Why are electric wires covered with plastic or rubber? (Page 34)
Answer:
Electric wires are covered with plastic or rubber because these materials are insulators. They prevent electric current from escaping the wires and also protect people from electric shocks. Additionally, the insulation keeps the wires from touching each other and causing short circuits.

Choose the incorrect,statement.
(i) A switch is the source of electric current in a circuit.
(ii) A switch helps to complete or break the circuit.
(iii) A switch helps us to use electricity as per our requirement.
(iv) When the switch is in ‘OFF’ position, there is an air gap between its terminals.
Answer:
(i) A switch is the source of electric current in a circuit.

Observe Fig. 3.16. With which material connected between the ends A and B, the lamp will not glow?

Answer:
To prevent the lamp from lighting in Fig. 3.16, connect an insulator (like rubber or plastic) between A and B. This will break the circuit and stop the flow of electricity.

In Fig. 3.17, if the filament of one of the lamps is broken, will the other glow? Justify your answer.

Answer:
In Fig. 3.17, the lamps are connected end to end (as indicated by the circuit diagram). If the filament of one of the lamps is broken, the other lamp will not glow. This is because in a line circuit, the current flows through all components one after another. If one component (like a lamp) is broken, the entire circuit is interrupted, and the current cannot flow through the rest of the circuit.

A student forgot to remove the insulator covering from the connecting wires while making a circuit. If the lamp and the cell are working properly, will the lamp glow?
Answer:
No, the lamp will not glow in this case. The insulator covering on the connecting wires prevents the flow of electric current. Since the current cannot pass through the insulated parts of the wires, the circuit remains incomplete, and the lamp will not light up.

Draw a circuit diagram for a simple torch using symbols for electric components.
Answer:

(i) If S2 is in ‘ON’ position, S1 is in ‘OFF’ position, which lamp(s) will glow?
(ii) If S2, is in ‘OFF’ position, S1, is in ‘ON’ position, which lamp(s) will glow?
(iii) If Si; and S2, both are in ‘ON’ position, which lamp(s) will glow?
(iv) If both S1, and S2, are in ‘OFF’ position, which lamp(s) will glow?
Answer:
(i) S2 ‘ON’, St ‘OFF’: Lamp L2 will glow.
(ii) S2 ‘OFF’, St ‘ON’: Lamp L1 will glow.
(iii) S1 ‘ON’, S2 ‘ON’: Both lamps (L1 and L2) will glow.
(iv) S1 ‘OFF’, S2 ‘OFF’: Neither lamp will glow.

Vidyut has made the circuit as shown in Fig. 3.19. Even after closing the circuit, the lamp does not glow. What can be the possible reasons? List as many possible reasons as you can for this faulty operation. What will you do to find out why the lamp did not glow?

In Fig. 3.20, in which case(s) the lamp will not glow when the switch is closed?

Answer:
Case (a): The Lamp will glow.
Case (b): Lamp will not glow.
Case (c): Lamp may not glow if the battery is reversed.
Case (d): Lamps will glow.

Suppose the ‘+’ and symbols cannot be read on a battery. Suggest a method to identify the two terminals of this battery. Answer:
Use a Circuit: Connect the battery to a light bulb. The correct connection will make the bulb glow brightly.
Check the Shape: The flat side is usually the negative (-) terminal, and the smaller protruding side is positive (+) terminal.

You are given six cells marked A, B, C, D, E, and F. Some of these are working and some are not. Design an activity to identify which of them are working.
(i) List the items that you require.
(ii) Write the procedure that you will follow.
(iii) With the items, carry out the activity to identify the cells that are working.
Answer:
(i) Items Required:

An LED requires two cells in series to glow. Tanya made the circuit as shown in Fig. 3.21. Will the lamp glow? If not, draw the wires for correct connections.

Answer:
The LED bulb will glow when the longer wire i.e., the positive terminal of LED is connected with positive terminal of battery and shorter wire i.e., the negative terminal of LED is connected with negative terminal of battery.