Class 7 Science · Chapter 3

3. Electricity: Circuits and their Components

Complete solutions to Let Us Enhance Our Learning. CBSE / NCERT Class 7 Science (Curiosity).

Let Us Enhance Our Learning

1Choose 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.

Incorrect statement: (i) A switch is the source of electric current in a circuit.

The electric cell (or battery) is the source of current. A switch only completes or breaks the circuit.

2Observe Fig. 3.16. With which material connected between the ends A and B, the lamp will not glow?

The lamp will not glow if an insulator is connected between A and B.

Examples: plastic scale, rubber eraser, wooden stick, paper, glass, cotton thread.

These materials do not allow electric current to pass through them. So the circuit remains incomplete.

3In Fig. 3.17, if the filament of one of the lamps is broken, will the other glow? Justify your answer.

No, the other lamp will not glow.

Both lamps are joined one after the other in a single path. A broken filament breaks this path. So the circuit becomes open and no current flows through the other lamp.

4A 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?

No, the lamp will not glow.

The plastic covering on the wire is an insulator. It stops the metal of the wire from touching the terminals. So the circuit is not complete and current cannot flow.

5Draw a circuit diagram for a simple torch using symbols for electric components.

A torch has a cell, a switch and a lamp joined in one closed path.

Circuit diagram of a simple torch (switch shown in OFF position).

In a real torch, two or more cells are usually joined together. Draw the longer line for the positive terminal and the shorter, thicker line for the negative terminal of the cell.

6In Fig. 3.18, L1, S2 and L2 are on the upper wire and S1 is on the lower wire with the battery. Which lamp(s) will glow when: (i) S2 ON, S1 OFF; (ii) S2 OFF, S1 ON; (iii) S1 and S2 both ON; (iv) S1 and S2 both OFF?

All the components are in one single loop. Current flows only when both switches are ON.

(i) S2 ON, S1 OFF: No lamp glows. The circuit is broken at S1.

(ii) S2 OFF, S1 ON: No lamp glows. The circuit is broken at S2.

(iii) Both ON: Both L1 and L2 glow. The circuit is complete.

(iv) Both OFF: No lamp glows.

7Vidyut 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? What will you do to find out why the lamp did not glow?

Possible reasons:

The cell is used up (dead).
The filament of the lamp is broken (fused bulb).
The lamp is loose in its holder.
A wire is loose or not properly joined at a terminal.
The insulation is not removed from the ends of a wire.
The safety pin of the switch is not touching the drawing pin properly.
A wire is broken inside its covering.

How to find out:

Check all the joints and tighten loose wires.
Replace the cell with a new one.
Test the lamp in another circuit that is working.
Check each wire one by one with a working tester.

8In Fig. 3.20, in which case(s) the lamp/LED will not glow when the switch is closed?

The lamp/LED will not glow in case (c).

In (a) and (b), a lamp glows whichever way the cell is connected.
In (c), the positive side of the LED is joined to the negative terminal of the battery. Current cannot pass through an LED in this direction. So it does not glow.
In (d), the LED is joined correctly. So it glows.

9Suppose the ‘+’ and ‘–’ symbols cannot be read on a battery. Suggest a method to identify the two terminals of this battery.

We can use an LED.

Connect the two legs of an LED to the two terminals of the battery.
If the LED glows, the terminal joined to the longer leg of the LED is positive, and the other is negative.
If it does not glow, reverse the connections and check again.

In a common cell, the terminal with the metal cap is positive and the flat metal disc is negative.

10You 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) Items required: the six cells, a torch bulb (or LED) with a holder, two connecting wires with ends cleaned, a switch and a cell holder.

(ii) Procedure:

Make a simple circuit with the bulb, switch, wires and the cell holder.
Place cell A in the holder and close the switch.
If the bulb glows, cell A is working. If it does not glow, cell A is not working.
Repeat the same steps with cells B, C, D, E and F, one by one.

(iii) Record your result in a table with two columns — ‘Cell’ and ‘Bulb glows (Yes/No)’.

Make sure the bulb itself is working. Test it first with a new cell.

11Using an LED that 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.

No, the LED will not glow.

In Tanya’s circuit, the negative terminals of the two cells are joined together. The cells are not in series. Also, both positive terminals go to the LED.

Correct connections:

Join the negative terminal of the first cell to the positive terminal of the second cell.
Join the free positive terminal to the longer leg (+) of the LED.
Join the free negative terminal to the shorter leg (–) of the LED.

Correct circuit: two cells in series with the LED.

Exploratory Projects

Exploratory Projects are hands-on tasks. Do them with your classmates under the guidance of your teacher, and write your own observations.

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