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NCERT Solutions · Class 7 Science · Chapter 3

Chapter 3: Electricity: Circuits and their Components (Physics)

Answers to all "Let Us Enhance Our Learning" questions of Chapter 3, Electricity: Circuits and their Components (NCERT Class 7 Science, Curiosity, 2026-27): cells and batteries, lamps and LEDs, switches, conductors and insulators, circuit diagrams, and finding faults in circuits. All 11 questions are answered, with the key answer highlighted.

A lamp glows only in a closed (complete) circuit. An LED glows only when its longer leg (+) is connected towards the + terminal of the battery. In a circuit symbol of a cell, the longer line is + and the shorter thicker line is −.

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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.

Solution

(i) is incorrect. The cell or battery is the source of electric current; a switch only completes or breaks the circuit.

(i)

Observe Fig. 3.16 (a cell and a lamp with a gap between ends A and B). With which material connected between the ends A and B will the lamp not glow?

Solution

The lamp will not glow if A and B are joined by an insulator: a rubber band or eraser, a plastic scale or pen, a wooden stick, a piece of paper, glass, cotton thread or a dry leaf. It will glow with a conductor: an iron nail, a safety pin, a key, aluminium foil, a copper wire, a steel spoon.

With an insulator (rubber, plastic, wood, paper, glass) the lamp will not glow; with a conductor (iron, copper, aluminium) it will.

In Fig. 3.17 (two lamps connected one after the other with a battery), if the filament of one of the lamps is broken, will the other glow? Justify your answer.

Solution

No. The two lamps are connected in a single loop, one after the other. Current flows through a lamp only through its filament. If one filament breaks, there is a gap in the loop, the circuit becomes open, and no current flows through the other lamp either.

No: the lamps are in one loop, so a broken filament opens the circuit and current stops in both.

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?

Solution

No. The plastic covering of the wire is an insulator. If it is not removed at the ends, the metal of the wire does not touch the terminals of the cell and the lamp, so the circuit is not complete and no current flows.

No: the plastic covering is an insulator, so the metal wire does not make contact and the circuit stays open.

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

Solution

A torch has two cells (joined + to −), a switch and a lamp, all in one loop:

Circuit diagram of a torch: two cells, a switch and a lamp (bulb) joined in one loop

Two cells in series, a switch and a lamp joined in a single closed loop.

In Fig. 3.18 (lamps L1 and L2 and switch S2 in the top wire; switch S1 and the battery in the bottom wire): (i) S2 ON, S1 OFF (ii) S2 OFF, S1 ON (iii) both ON (iv) both OFF. Which lamp(s) will glow in each case?

Solution

Everything in Fig. 3.18 is in one single loop, so current flows only when both switches are ON.

(i) None (ii) None (iii) Both L1 and L2 (iv) None

(i) none (ii) none (iii) both L1 and L2 (iv) none

Vidyut has made the circuit in Fig. 3.19 (a cell, a lamp and a switch made with a safety pin on a board). 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?

Solution

Possible reasons:

The cell is used up (dead) or is not touching the wires properly.
The lamp's filament is broken (fused), or the lamp is loose in its holder.
A wire is broken inside its covering, or the plastic covering was not removed at the ends.
The connections are loose at the cell, the lamp holder or the drawing pins.
The safety-pin switch does not touch the other drawing pin properly (rust or paint on it).
The wires are connected to the wrong points, so the circuit is not one complete loop.

How to find out: check each component one at a time: tighten all connections; replace the cell with a new one; replace the lamp with one that works in another circuit; test each wire with a working cell and lamp; clean the switch contacts. The part whose replacement makes the lamp glow was faulty.

A dead cell, fused lamp, broken or uncovered wire, loose connections, or a switch not making contact; test by checking and replacing each component one at a time.

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

Solution

(a) and (b): lamps glow whichever way the cell is connected. Both glow.
(c): the battery's + terminal (longer line) is on the right, so current enters the LED from its negative side. An LED lets current pass only one way, so it does not glow.
(d): the + terminal is on the left, so current enters the LED from its positive side. It glows.

Answer: only (c) will not glow.

Only (c): the LED is connected the wrong way round (its + side is not towards the battery's + terminal).

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

Solution

Use an LED: connect its longer leg to one terminal and its shorter leg to the other terminal of the battery. If the LED glows, the terminal joined to the longer leg is +. If it does not glow, reverse the connections; now it should glow, and again the terminal at the longer leg is +.

(Also, in a common torch cell the terminal with the small metal cap (bump) is + and the flat end is −.)

Connect an LED: when it glows, the terminal joined to its longer leg is positive.

You are given six cells marked A to F, some working and some not. Design an activity to identify which of them are working: (i) list the items you require (ii) write the procedure (iii) carry out the activity.

Solution

(i) Items: a lamp (bulb) with a holder that you know works (or an LED), two connecting wires with ends uncovered, a cell holder or tape, and a notebook.

(ii) Procedure:

Make a simple circuit with one cell, the lamp and the wires.
Connect cell A, close the circuit and see whether the lamp glows. Record "working" or "not working".
Replace A with B, then C, D, E and F one by one, keeping everything else the same, and record each result.
To be sure, retest any cell that did not light the lamp after checking the connections.

(iii) Record the results in a table (Cell – Lamp glows? – Working?). The cells that make the lamp glow are working.

Make a circuit with a working lamp and wires, connect each cell in turn, and note which cells make the lamp glow; those cells are working.

Using an LED that requires two cells in series to glow, Tanya made the circuit in Fig. 3.21 (the two cells' negative ends joined together, one + terminal to the LED's − leg and the other + terminal to the LED's + leg). Will the LED glow? If not, draw the wires for correct connections.

Solution

No. The two cells are joined negative to negative, so they push current in opposite directions and cancel each other out; there is no push through the LED.

Correct connections: join the + terminal of one cell to the − terminal of the other (cells in series). Then connect the remaining free + terminal to the longer (+) leg of the LED and the free − terminal to the shorter (−) leg.

No: the cells are joined − to −, so they oppose each other. Join + of one cell to − of the other, then the free + to the LED's long leg and the free − to its short leg.

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