Chapter 3 Electricity: Circuits and their Components Class 7 Science Curiosity NCERT Solutions
Chapter 3 Electricity: Circuits and their Components NCERT Solutions Class 7 Science
Page No. 24
Intext Questions
1. 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.
Page No. 25
2. In a torch, we generally use more than one cell. Are those placed in any particular order?
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.
Page No. 32
3. 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.
Page No. 33
4. Can we represent the circuit in a simpler manner?
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.
Page No. 34
5. Why did we use metal wire for making the electric circuit? Can we not use some other material for the wires?
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.
6. 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.
Page No. 37
Let us Enhance Our Learning
1. Choose the incorrect statement.
(i) A switch is the source of electric current in a circuit.
Answer
Incorrect.
A switch does not generate electric current; it only controls the flow of current by opening or closing the circuit.
(ii) A switch helps to complete or break the circuit.
Answer
Correct.
A switch controls the flow of electricity by either closing or opening a circuit, depending on whether it is in the "ON" or "OFF" position.
(iii) A switch helps us to use electricity as per our requirement.
Answer
Correct.
A switch allows us to control the use of electricity by turning devices "ON" or "OFF".
(iv) When the switch is in ‘OFF’ position, there is an air gap between its terminals.
Answer
Correct.
In the 'OFF' position, the switch opens the circuit, creating a gap that stops the current from flowing.
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 the material between the ends A and B is an insulator (like rubber, plastic, or wood). These materials do not allow electricity to flow through them.
3. 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 if the filament of one lamp is broken.
- If the filament of one lamp breaks, it creates a gap in the circuit.
- This gap means that the path for the electric current is interrupted. When there's a break in the circuit, the current cannot flow through it anymore.
- Even though the other lamp might be perfectly fine, it won't glow because no electricity is reaching it due to the broken connection.
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 because the insulator prevents the electric current from passing through the wire, which is necessary for the lamp to glow.
To make the circuit work and the lamp glow, you need to remove the insulation from the parts of the wire that connect to the cell and the lamp. This will allow the electricity to flow and make the lamp light up.
5. Draw a circuit diagram for a simple torch using symbols for electric components.
Answer
A simple torch circuit can be represented as:
This is a simple series circuit where the battery provides power, the switch controls the current, and the lamp glows when the current flows.
6. In Fig. 3.18:
(i) If S2 is in ‘ON’ position, S1 is in ‘OFF’ position, which lamp(s) will glow?
Answer
Neither lamp will glow because both switch 1 is open.
(ii) If S2 is in ‘OFF’ position, S1 is in ‘ON’ position, which lamp(s) will glow?
Answer
Neither lamp will glow because both switch 2 is open.
(iii) If S1 and S2 both are in ‘ON’ position, which lamp(s) will glow?
Answer
Both Lamp 1 and Lamp 2 will glow, as both switches are closed, allowing current to flow to both lamps.
(iv) If both S1 and S2 are in ‘OFF’ position, which lamp(s) will glow?
Answer
Neither lamp will glow because both switches are open, preventing current flow.
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:
- Broken filament in the lamp (if it's an incandescent lamp).
- Loose connections or poor contact in the circuit.
- Dead battery or improper placement of battery terminals.
- Wires not properly connected to the lamp or battery.
- Open circuit due to a faulty switch or a disconnected part of the circuit.
To find out why the lamp did not glow, one should:
- Check the lamp for a broken filament.
- Ensure all connections are secure.
- Test the battery using a simple tester to ensure it is working.
- Verify that the switch is in the ‘ON’ position.
- Check the wiring to ensure proper electrical contact.
8. In Fig. 3.20, in which case(s) the lamp will not glow when the switch is closed?
Answer
Case (a), (b), and (d) will glow when switch will be closed but Case (c) will not glow because the negative terminal of the battery is connected to the positive terminal of the LED, meaning the LED is reverse-connected. In this case, no current will flow through the LED, and it will remain off.
9. Suppose the ‘+’ and ‘–’ symbols cannot be read on a battery. Suggest a method to identify the two terminals of this battery.
Answer
To check which end of batter is positive and which is negative:
LED Identification: An LED has two leads:
- Longer lead: This is the positive terminal (anode).
- Shorter lead: This is the negative terminal (cathode).
Connect the Battery: Connect the battery's terminals to the LED’s leads:
- Connect the longer lead of the LED to one end of the battery and the shorter lead of the LED to the other end.
Observation:
- The LED will glow only when the longer lead is connected to the positive terminal of the battery and the shorter lead to the negative terminal.
- If the LED doesn't glow, it means the leads are connected in reverse. You can swap the connections to test.
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.
(i) Items required:
- Electric lamp.
- Wires.
- Cell holders.
- Cells A, B, C, D, E, F.
(ii) Procedure:
- Connect each cell one by one in the circuit with the lamp.
- Observe if the lamp glows.
- Record which cells cause the lamp to glow.
(iii) Activity to identify the working cells:
- Test each cell individually in the circuit.
- Mark the cells that cause the lamp to glow as working, and those that don't glow the lamp as non-working.
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
No, the LED will not glow if the cells are not connected in the correct polarity. To make the LED glow, the positive terminal of the battery should be connected to the positive terminal of the LED (longer wire), and the negative terminal of the battery should be connected to the negative terminal of the LED (shorter wire).
Corrected connections:
