Question Answer Chapter 3 Class 7 Science Curiosity NCERT

Electricity: Circuits and their Components

1. 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.

Explanation: A switch is not a source of electric current; it only completes or breaks the circuit to control the flow of current. The source of electric current is an electric cell or battery.

2. 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 an insulator like plastic, rubber, or glass is connected between ends A and B.

Explanation: Insulators do not allow electric current to flow through them, so the circuit will be incomplete, and the lamp will not glow.

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

Answer: Yes, the other lamp will still glow.

Explanation: In Fig. 3.17, the lamps are likely connected in parallel. In a parallel circuit, each lamp has its own direct path to the power source (electric cell). If one lamp’s filament is broken, the circuit for the other lamp remains complete, allowing current to flow through it, so it will glow.

4. 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.

Explanation: The insulator covering (like plastic or rubber) on the wires prevents the metal inside from making contact with the lamp and cell terminals. This breaks the circuit, so no current can flow, and the lamp will not glow.

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

Answer: A circuit diagram for a simple torch includes:

• An electric cell (symbol: long line for positive terminal, short line for negative terminal).
• A switch (symbol: a line with a gap for OFF or connected for ON).
• An incandescent lamp (symbol: a circle with a loop inside).
• Connecting wires (symbol: straight lines).

Description of diagram (since I cannot draw): Draw a single electric cell with its positive terminal connected by a wire to one terminal of the switch. From the other terminal of the switch, draw a wire to one terminal of the lamp. From the other terminal of the lamp, draw a wire back to the negative terminal of the cell, forming a closed loop.

6. 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:

(i) If S2 is ON and S1 is OFF, only lamp L2 will glow.

Explanation: S2 being ON completes the circuit for L2, allowing current to flow through it. S1 being OFF breaks the circuit for L1, so L1 will not glow.

(ii) If S2 is OFF and S1 is ON, only lamp L1 will glow.

Explanation: S1 being ON completes the circuit for L1, allowing current to flow through it. S2 being OFF breaks the circuit for L2, so L2 will not glow.

(iii) If both S1 and S2 are ON, both lamps L1 and L2 will glow.

Explanation: Both switches being ON complete the circuits for both L1 and L2, allowing current to flow through both lamps.

(iv) If both S1 and S2 are OFF, no lamps will glow.

Explanation: Both switches being OFF break the circuits for both L1 and L2, so no current flows, and neither lamp glows.

7. 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: Possible reasons the lamp does not glow:

1. The lamp’s filament is broken (fused).
2. The electric cell is dead or not providing enough power.
3. The connections are loose or not properly made (e.g., wires not touching the terminals).
4. The insulator covering on the wires was not removed, preventing current flow.
5. The switch is faulty or not properly closing the circuit.
6. The cell is placed incorrectly (e.g., positive and negative terminals not aligned properly).
7. The lamp or cell terminals are dirty or corroded, preventing good contact.

Steps to find out why the lamp did not glow:

1. Check if the lamp’s filament is intact by replacing it with a working lamp.
2. Test the cell with a different working circuit to confirm it is providing power.
3. Ensure all wires are stripped of insulation at the ends and securely connected to the terminals.
4. Test the switch by bypassing it (connect the wires directly) to see if the lamp glows.
5. Verify the cell’s orientation (positive and negative terminals) matches the circuit’s requirements.
6. Clean the terminals of the lamp and cell with a dry cloth to remove any dirt or corrosion.
7. Rebuild the circuit step-by-step, testing after each connection to identify the fault.

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

Answer: The lamp will not glow in cases (b) and (d).

Explanation:

• In case (a), the circuit is complete (positive to lamp to negative), so the lamp glows.
• In case (b), both wires are connected to the same terminal (positive), so the circuit is incomplete, and the lamp does not glow.
• In case (c), the circuit is complete (positive to lamp to negative), so the lamp glows.
• In case (d), both wires are connected to the same terminal (negative), so the circuit is incomplete, and the lamp does not glow.

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

Answer: To identify the positive and negative terminals of the battery:

1. Take a known working LED (which has a longer wire for the positive terminal and a shorter wire for the negative terminal).
2. Connect one terminal of the battery to the longer wire of the LED and the other terminal to the shorter wire using wires.
3. If the LED glows, the battery terminal connected to the longer wire is the positive terminal, and the other is the negative terminal.
4. If the LED does not glow, reverse the connections. When the LED glows, the terminal connected to the longer wire is positive.

Explanation: An LED only glows when its positive terminal (longer wire) is connected to the battery’s positive terminal and its negative terminal (shorter wire) to the battery’s negative terminal.

10. 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.

Answer: (i) List the items that you require:

• A working incandescent lamp or LED with known polarity (longer wire for positive, shorter for negative if using an LED).
• A cell holder (optional, for easy connection).
• Connecting wires with stripped ends.
• Electrical tape (if no cell holder is available).
• A switch (optional, to control the circuit).

(ii) Write the procedure that you will follow:

1. Set up a simple circuit with a lamp (or LED), a cell holder (or wires with tape), and connecting wires. If using an LED, ensure the longer wire connects to the positive side and the shorter to the negative side of the circuit.
2. Insert cell A into the cell holder (or connect it with wires), ensuring the positive terminal (metal cap) and negative terminal (flat disc) are correctly aligned with the circuit.
3. Close the circuit (or turn on the switch if used) and observe if the lamp or LED glows.
4. If the lamp/LED glows, cell A is working. If it does not glow, cell A is not working.
5. Repeat steps 2-4 for cells B, C, D, E, and F, testing each cell one at a time.
6. Record the results for each cell (working or not working).

(iii) With the items, carry out the activity to identify the cells that are working: Follow the procedure above in a practical setup. For each cell:

• Connect the cell to the circuit.
• Check if the lamp/LED glows.
• Note down which cells (e.g., A, C, E) make the lamp/LED glow (these are working) and which do not (e.g., B, D, F, these are not working).

Explanation: A working cell will provide enough electrical energy to complete the circuit and make the lamp/LED glow. A non-working cell will not, leaving the circuit incomplete or without enough power.

11. 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: Will the lamp glow? No, the LED will not glow.

Explanation: In Fig. 3.21, the LED’s positive terminal (longer wire) is connected to the negative terminal of the battery, and the negative terminal (shorter wire) is connected to the positive terminal of the battery. An LED only glows when its positive terminal is connected to the positive terminal of the battery and its negative terminal to the negative terminal. Additionally, the cells must be in series (positive of one cell to negative of the other) to provide enough voltage.

Correct connections (description, as I cannot draw):

• Take two cells in series: Connect the positive terminal (metal cap) of the first cell to the negative terminal (flat disc) of the second cell using a wire or cell holder.
• Connect the positive terminal of the battery (the free positive terminal of the second cell) to the longer wire (positive terminal) of the LED using a wire.
• Connect the negative terminal of the battery (the free negative terminal of the first cell) to the shorter wire (negative terminal) of the LED using another wire.
• Ensure all connections are secure and the circuit is closed (add a switch if needed).

Circuit diagram description:

• Draw two electric cells in series (positive of one to negative of the other).
• From the positive terminal of the second cell, draw a wire to the LED symbol (triangle with arrows, ensuring the triangle’s base is toward the positive side).
• From the LED’s negative side, draw a wire to the negative terminal of the first cell, completing the circuit.

This ensures the LED is correctly connected and will glow with sufficient voltage from the two cells in series.