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NCERT Class 7 Science Chapter 3 Electricity: Circuits and Their Components
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Electricity: Circuits and Their Components
Chapter: 3
Page No : 29
Table 3.1: Trying to make the lamp glow.
Note: The lamps are not shown glowing in any circuit.
Ans:
Page No : 35
Table 3.3: Identifying Conductors and Insulators.
Ans:
| S.No | Object | Material it is made up of | Lamp glows (yes/ No) | Conclusion (conductor/ Insulator) |
| 1 | Stick | Wood | No | Insulator |
| 2 | Scale | Plastic | No | Insulator |
| 3 | Bangle | Glass | No | Insulator |
| 4 | Paper Strip | Paper | No | Insulator |
| 5 | Candle | Wax | No | Insulator |
| 6 | Key | Metal | Yes | Conductor |
| 7 | Eraser | Rubber | No | Insulator |
| 8 | Copper wire | Copper | Yes | Conductor |
| 9 | Iron nail | Iron | Yes | Conductor |
| Let Us Enhance Our Learning |
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.
Ans: (i) A switch is the source of electric current in a circuit.
2. Observe Fig. 3.16. With which material connected between the ends A and B, the lamp will not glow?
Ans: The lamp will not glow if an insulator (like rubber, plastic, wood, glass) is connected between ends A and B, as insulators prevent the flow of electricity needed to complete the circuit and light the bulb.
3. In Fig. 3.17, if the filament of one of the lamps is broken, will the other glow? Justify your answer.
Ans: No, the other lamp will not glow if the filament of one of the lamps is broken in the circuit.The two lamps connected in series, meaning the electric current has only one path to flow through both lamps. If the filament of one lamp breaks, it creates an open circuit, interrupting this single path. Consequently, the electric current cannot flow through the broken filament to reach the second lamp, preventing it from lighting up. In a series circuit, all components must be intact for the current to flow and the circuit to function.
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?
Ans: No, the lamp will not glow.The plastic insulator covering the wires does not conduct electricity.
If the student does not remove the insulator, the wires cannot make a proper electrical connection between the cell and the lamp.
5. Draw a circuit diagram for a simple torch using symbols for electric components.
Ans:
6. In Fig. 3.18:
(i) If S2 is in ‘ON’ position, S1 is in ‘OFF’ position, which lamp(s) will glow?
Ans: If S₂ is ON and S₁ is OFF, The path through S₂ is complete. So, L₂ will glow, but L₁ will not glow.
(ii) If S2 is in ‘OFF’ position, S1 is in ‘ON’ position, which lamp(s) will glow?
Ans: If S₂ is OFF and S₁ is ON, The path through S₁ is complete.So, L₁ will glow, but L₂ will not glow.
(iii) If S1 and S2 both are in ‘ON’ position, which lamp(s) will glow?
Ans: If both S₁ and S₂ are ON, Both paths are complete. So, both L₁ and L₂ will glow.
(iv) If both S1 and S2 are in ‘OFF’ position, which lamp(s) will glow?
Ans: If both S₁ and S₂ are OFF, The circuit is open.No complete path for current.
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?
Ans: Vidyut’s lamp is not glowing even after closing the circuit because there may be several faults. The possible reasons are: the cell may be dead, the lamp may be fused, the wires might have loose connections, or the ends of the wires may not be properly stripped. Also, the paper clip used as a switch may not be making proper contact. To find out the cause, Vidyut should check if the cell is fresh, test the lamp separately, ensure all wire connections are tight and clean, and verify that the switch is properly closing the circuit.
8. In Fig. 3.20, in which case(s) the lamp will not glow when the switch is closed?
Ans: When the switch is closed:
(i) In (a): The lamp will glow because the circuit is complete and the battery is connected correctly.
(ii) In (b): The lamp will not glow because the battery is connected in reverse, but for a normal bulb, polarity does not matter. So, the lamp should glow here also.
(iii) In (c): The device is a LED (Light Emitting Diode). Here the polarity seems correct (positive to longer leg of LED), so the LED will glow.
(iv) In (d): The LED is connected in reverse polarity, so the LED will not glow.
Therefore, the lamp (or LED in cases (c) and (d)) will not glow only in case (d) because the LED is connected in reverse bias.
9. Suppose the ‘+’ and ‘–’ symbols cannot be read on a battery. Suggest a method to identify the two terminals of this battery.
Ans: If the ‘+’ and ‘–’ symbols on a battery cannot be read, we can identify the two terminals using a simple method. Connect a small bulb to the two terminals of the battery through a switch. Close the switch and observe the bulb. If the bulb glows, then the circuit is complete, but to find the positive and negative terminals, we can connect an LED instead of a bulb. LEDs allow current to flow only in one direction — from positive to negative. If the LED glows, the terminal connected to the longer leg of the LED is the positive (+) terminal, and the terminal connected to the shorter leg is the negative (–) terminal. If the LED does not glow, reverse the connections and check again.
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) List the items that you require.
Ans: The six cells: Marked A, B, C, D, E, and F.
A low-voltage light bulb with a bulb holder,Connecting wires,Optional: A multimeter (voltmeter setting).
(ii) Write the procedure that you will follow.
Ans: (a) Connect two wires to the terminals of the bulb (or bulb holder).
(b) Connect one wire to the positive terminal of a cell and the other to the negative terminal.
(c) Observe if the bulb lights up.
(d) If the bulb glows, the cell is working.
(e) If the bulb does not glow, the cell may be dead or weak.
(f) Repeat the same process for each cell (A, B, C, D, E, and F) one by one.
(g) Record the results for each cell.
(iii) With the items, carry out the activity to identify the cells that are working.
Ans: Test each of the six cells (A to F) using the above method.
Note the result:
(i) If the bulb glows – Cell is working.
(ii) If the bulb doesn’t glow – Cell is not working.
For example:
| Cell | Bulb Glow | Working |
| A | Yes | Working |
| B | No | Not Working |
| C | Yes | Working |
| D | No | Not Working |
| E | Yes | Working |
| F | No | Not 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.
Ans: No, the lamp (LED) will not glow in the circuit shown in Fig. 3.21 because the two cells are not connected correctly in series. For an LED to glow, the positive terminal of one cell must be connected to the negative terminal of the other cell so that their voltages add up.However, in Tanya’s circuit, both the positive terminals of the cells are connected in the same direction, which means they are not in series and cannot provide the required voltage to light the LED. To correct the circuit, the negative terminal of the first cell should be connected to the positive terminal of the second cell. Then, the free positive terminal of the first cell and the free negative terminal of the second cell should be connected to the two terminals of the LED. This proper series connection will allow enough voltage to pass through the LED, making it glow.
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