EN हिं

NCERT Solutions Curiosity Chapter 3 –39End-of-chapter questions — Let Us Enhance Our Learning

Book page 37 Updated on2026-09-05

Q1.
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

The incorrect statement is (i) — 'A switch is the source of electric current in a circuit.'

A switch is not a source of current at all. The source in these circuits is the electric cell or battery; the switch only controls whether the current is allowed to flow.

StatementCorrect?Reason
(i) A switch is the source of electric currentIncorrectThe cell or battery is the source; the switch supplies no energy
(ii) A switch helps to complete or break the circuitCorrectThis is exactly what a switch does
(iii) A switch helps us use electricity as per our requirementCorrectWe can switch a fan or lamp on only when it is needed
(iv) In 'OFF' position there is an air gap between its terminalsCorrectAir is an insulator, so the gap stops the current
Q2.
Observe Fig. 3.16. With which material connected between the ends A and B, the lamp will not glow?
Answer

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

Fig. 3.16 is a conduction tester — a cell and a lamp joined by wires whose two ends, A and B, are left free. Whatever is placed between A and B becomes part of the circuit.

Material joined between A and BTypeLamp
Rubber, plastic, wood, glass, paper, wax, dry cloth, thermocolInsulatorDoes not glow
Copper wire, iron nail, key, coin, aluminium foil, pencil leadConductorGlows
Why it happens: Current cannot pass through an insulator, so the loop stays broken at the gap between A and B even though the cell and lamp are perfectly good. Only a conductor bridges the gap and completes the circuit.
Q3.
In Fig. 3.17, if the filament of one of the lamps is broken, will the other glow? Justify your answer.
Answer

No, the other lamp will not glow either.

In Fig. 3.17 the two lamps and the battery are joined one after another in a single loop — the current has only one path to follow, and that path passes through both filaments.

lamp 1 lamp 2 — filament broken battery no current anywhere
The two lamps lie in the same single loop. A break in one filament opens the whole loop, so no current flows and neither lamp glows.

Justification: A broken filament leaves a gap inside that bulb. The circuit is therefore open, so no current flows through any part of the loop — including the second lamp. The second lamp is perfectly good, but there is no current for it to use.

Did you know? Old strings of decorative lights were wired like this, which is why one fused bulb used to switch off the whole string.
Q4.
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.

The plastic covering on a connecting wire is an insulator. If it is not stripped off at the ends, the copper inside never actually touches the terminals of the cell or of the lamp.

Cell terminal → plastic → copper → plastic → lamp terminal
Current cannot cross the plastic → circuit is open → lamp stays dark
Why it happens: A circuit needs an unbroken path of conductors from one terminal of the cell to the other. Insulating plastic at the two joints breaks that path just as surely as a missing wire would. This is why the activities tell you to remove about 1 cm of the covering from both ends of every wire.
Tip: After stripping, the exposed copper should be shiny. If it looks dull or blackened, rub it gently with sandpaper — a coating of dirt or oxide can also stop the current.
Q5.
Draw a circuit diagram for a simple torch using symbols for electric components.
Answer

A simple torch has just three components in a single loop — a battery of two cells, an electric lamp, and a switch, joined by wires.

electric lamp + – battery (two cells) switch (ON) current
Circuit diagram of a simple torch: a battery of two cells, a switch shown in the ON position, and an electric lamp, all in one loop. Current is taken to flow from the + terminal, through the lamp, back to the – terminal.

Points to check in your own drawing:

  • The long line of each cell is its + terminal, the short line its – terminal, and the cells are drawn long–short, long–short.
  • The lamp is a circle with a cross inside it.
  • The switch is two small circles; join them with a straight line for 'ON', lift the line for 'OFF'.
  • All the wires are plain straight lines and the loop must be closed for the lamp to glow.
Tip: If your torch uses an LED instead of a bulb, replace the lamp symbol with the LED symbol and make sure the triangle points away from the + terminal of the battery.
Q6.
In Fig. 3.18: (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 S1 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

Look at Fig. 3.18 carefully: the battery, S1, L1, S2 and L2 all lie on one single loop. There is no branch anywhere, so current must pass through every component or through none.

CaseS1S2Is the loop complete?Lamp(s) that glow
(i)OFFONNo — broken at S1None
(ii)ONOFFNo — broken at S2None
(iii)ONONYes — the loop is closedBoth L1 and L2
(iv)OFFOFFNo — broken at two placesNone

Answers in one line each:

  1. No lamp glows — S1 is open, so the circuit is broken.
  2. No lamp glows — S2 is open, so the circuit is broken.
  3. Both L1 and L2 glow — the circuit is complete and the same current passes through both lamps.
  4. No lamp glows — the circuit is broken at both switches.
Why it happens: In a single loop every component shares one path. Opening a switch anywhere in that path stops the current everywhere in it. That is also why a switch can be placed anywhere in a circuit and still control the whole circuit.
Q7.
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?
Answer

The circuit in Fig. 3.19 has only four kinds of parts — the cell, the lamp, the wires and the switch — so the fault must lie in one of them.

Possible reasons:

PartWhat could be wrong
CellThe cell is used up (dead), or has been placed in the holder the wrong way round, or is not pressed firmly against the spring and the contact
LampThe lamp is fused — its filament is broken; or it is not screwed tightly into its holder, so a terminal is not touching
WiresThe plastic covering has not been removed from a wire end; a wire is broken inside its covering; a wire has come loose from a screw or a drawing pin; the exposed copper is dirty or rusted
SwitchThe safety pin is not really touching the second drawing pin, so the gap is still open; or the drawing pins have become loose in the cardboard
HoldersThe screws of the lamp holder or the cell holder are loose, so the joints are not making proper contact

How to find out — test one part at a time:

  1. Check the switch first. Bypass it — touch the two wires that go to the drawing pins directly to each other. If the lamp now glows, the fault was in the switch.
  2. Check the cell. Replace it with a cell that is known to be working. If the lamp glows, the old cell was dead.
  3. Check the lamp. Put the doubtful lamp into a torch that works, or replace it with a lamp known to be good. Also hold the bulb up to the light and look for a broken filament.
  4. Check the wires and joints. Look at every end for unstripped plastic; tighten every screw and drawing pin; gently tug each wire to see whether it is broken inside.
  5. Make a tester. Use a cell and a lamp that surely work, leave two ends free, and touch them across each wire in turn — the lamp glows only through a wire that is whole.
Tip: Change only one thing at a time and test after each change. If you change two things together and the lamp lights up, you will never know which one was the fault.
Q8.
In Fig. 3.20, in which case(s) the lamp/LED will not glow when the switch is closed?
Answer

Only in case (c) the LED will not glow. In (a), (b) and (d) the lamp or LED lights up when the switch is closed.

CaseWhat the circuit containsWill it glow?Reason
(a)One cell and an electric lampGlowsComplete circuit; an incandescent lamp works whichever way the cell is connected
(b)One cell and an electric lamp, cell reversedGlowsSame as (a) — reversing the cell only reverses the direction of current, and the filament still heats up
(c)Battery of two cells and an LEDDoes not glowThe LED is connected the wrong way round — its triangle points towards the + terminal of the battery, so current is blocked
(d)Battery of two cells and an LED, battery reversedGlowsHere the triangle points away from the + terminal, so current can pass through the LED
Why it happens: An incandescent lamp does not care about direction — current heats its filament either way. An LED conducts in one direction only. In the LED symbol the triangle shows that direction, so trace the current from the long line (+) of the battery: if it meets the triangle point-first it passes, if it meets the bar first it is blocked.
Check it yourself: In (c), simply interchanging the two wires at the LED — or turning the battery round — would make it glow, exactly as in Activity 3.7.
Q9.
Suppose the ‘+’ and ‘–’ symbols cannot be read on a battery. Suggest a method to identify the two terminals of this battery.
Answer

Use an LED, because an LED conducts in one direction only and so acts as a direction-detector.

Method:

  1. Take an LED and note its two wires — the longer wire is its + terminal, the shorter one its – terminal.
  2. Connect the longer wire of the LED to one terminal of the battery and the shorter wire to the other terminal, using two connecting wires.
  3. If the LED glows, the terminal joined to the longer wire is the positive (+) terminal of the battery, and the other one is the negative.
  4. If the LED does not glow, interchange the two connections. When it lights up, read off the terminals in the same way.
ObservationConclusion
LED glowsThe terminal joined to the LED's longer wire is +
LED does not glow, but glows after interchangingThe terminal now joined to the longer wire is +

A second, simpler check — look at the cells themselves. In every cell the small protruding metal cap is the positive terminal and the flat metal disc is the negative terminal. In a cell holder the spring always presses on the flat disc, so the spring end of the holder is the negative terminal of the battery.

Caution: Never try to test a battery by touching it with your tongue or by joining its two terminals directly with a wire. Use an LED or a lamp — that is both safe and reliable.
Q10.
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

The idea is to build one test circuit and pass every cell through it in turn, changing nothing else. Then the only thing that can change the result is the cell itself.

(i) Items required

  • The six cells marked A, B, C, D, E and F
  • One cell holder (for a single cell)
  • One torch lamp (incandescent bulb) and a lamp holder
  • Four short lengths of connecting wire, with about 1 cm of covering removed from each end
  • One cell that is known to be working, to check the test circuit itself
  • A notebook to record the observations

(ii) Procedure

  1. Fix the lamp in the lamp holder and join two wires to the screws of the holder.
  2. Join two wires to the two ends of the cell holder.
  3. Connect one wire of the lamp holder to one wire of the cell holder. Keep the other two ends free — these two free ends will be joined for each test.
  4. First test the tester. Put the known good cell in the holder, with its negative terminal towards the spring, and join the two free ends. The lamp must glow. If it does not, tighten the joints and check the bulb before going further.
  5. Take out the good cell. Put cell A in the holder the same way, join the free ends and watch the lamp.
  6. Record 'glows' or 'does not glow'. Take A out and repeat exactly the same steps with B, C, D, E and F, one at a time.
  7. A cell for which the lamp glows brightly is working; a cell for which the lamp does not glow (or glows very faintly) is not working.

(iii) Carrying out the activity — record your results in a table like this. The entries below are only a sample; write down what your six cells actually show.

CellDoes the lamp glow?Conclusion
AYes, brightlyWorking
BNoNot working
CYes, brightlyWorking
DNoNot working
EYes, brightlyWorking
FVery faint glowAlmost used up — treat as not working
Tip: Keep the same lamp, the same wires and the same joints for all six tests. If you change the bulb halfway through, a dark lamp may be blaming the wrong thing. Also do not leave a cell connected for long — it drains the cell.
Q11.
Using 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.
Answer

No, the LED will not glow. Tanya has made two mistakes at once.

  • The cells are not in series. In Fig. 3.21 the black wire joins the negative terminal of one cell to the negative terminal of the other. Two cells joined – to – push current in opposite directions, so their effects cancel and the battery gives no useful push at all.
  • The LED is joined the wrong way round. The wire from a positive terminal of a cell reaches the negative (shorter) wire of the LED, and current can pass through an LED in one direction only.

Correct connections:

+ – – + link:+ to – – + LED cell 1 cell 2 green wire → shorter (–) wire of LED yellow wire → longer (+) wire of LED
Correct wiring: one cell is turned round so that the link wire joins the + terminal of one cell to the – terminal of the other — that makes a battery of two cells in series. The free + terminal then goes to the longer wire of the LED and the free – terminal to the shorter wire.

The two rules to follow while re-drawing the wires:

  1. Join the + terminal of one cell to the – terminal of the other cell. The two terminals left free are then the + and – terminals of the battery.
  2. Join the + terminal of the battery to the longer wire of the LED and the – terminal of the battery to the shorter wire of the LED.
Why it happens: Two cells help each other only when they are joined + to –; joined – to – they oppose each other. And even a correct battery cannot light an LED that is facing the wrong way, because an LED conducts in one direction only. Both conditions must be satisfied together.
Was this helpful? Report an error