Electricity: Circuits and their ComponentsClass 7 Science NCERT Solutions
11 Solutions
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Solution 1 of 11
Q1Let Us Enhance Our Learning
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
The incorrect statement is (i): A switch is the source of electric current in a circuit. A switch only helps to complete or break the circuit, it is not the source of electric current. The electric cell or battery is the source of electric current.
Q2Let Us Enhance Our Learning
Observe Fig. 3.16. With which material connected between the ends A and B , the lamp will not glow?
Solution
The lamp will not glow when an insulator is connected between ends A and B. Insulators do not allow electric current to pass through them. Examples of insulators include rubber, plastic, and wood.
Q3Let Us Enhance Our Learning
In Fig. 3.17, if the filament of one of the lamps is broken, will the other glow? Justify your answer.
Solution
If the filament of one of the lamps is broken, the other lamp will not glow. This is because a broken filament creates an open circuit, interrupting the flow of electric current. For both lamps to glow, the circuit must be complete, allowing electric current to flow through both filaments.
Q4Let Us Enhance Our Learning
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
If the student forgot to remove the insulator covering from the connecting wires, the lamp will not glow. The insulator covering prevents the electric current from flowing through the wires, thus the circuit will not be completed.
Q5Let Us Enhance Our Learning
Draw a circuit diagram for a simple torch using symbols for electric components.
Solution
The circuit diagram for a simple torch includes an electric cell (represented by a long and short parallel line), a switch, a lamp (represented by a circle with a cross inside), and connecting wires. The positive terminal of the cell is connected to one end of the switch, and the other end of the switch is connected to one terminal of the lamp. The other terminal of the lamp is connected back to the negative terminal of the cell, completing the circuit.
Q6Let Us Enhance Our Learning
In Fig. 3.18:
(i)
If is in ' ON ' position, is in ' OFF ' position, which lamp(s) will glow?
(ii)
If is in 'OFF' position, is in 'ON' position, which lamp(s) will glow?
(iii)
If and both are in 'ON' position, which lamp(s) will glow?
(iv)
If both and are in 'OFF' position, which lamp(s) will glow?
Solution
(i)
If is in the 'ON' position and is in the 'OFF' position, only lamp will glow.
(ii)
If is in the 'OFF' position and is in the 'ON' position, only lamp will glow.
(iii)
If both and are in the 'ON' position, both lamps and will glow.
(iv)
If both and are in the 'OFF' position, neither lamp will glow.
Q7Let Us Enhance Our Learning
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?
Solution
Possible reasons why the lamp does not glow even after closing the circuit include:
- The electric cell may be exhausted.
- The lamp may be fused (filament broken).
- The connections may be loose.
- The switch may be faulty.
- The wiring may be incomplete or broken.
To find out why the lamp did not glow, I will:
- Replace the electric cell with a new one.
- Replace the lamp with a known working lamp.
- Check all connections to ensure they are tight and secure.
- Test the switch for continuity.
- Examine the wires for any breaks or disconnections.
Q8Let Us Enhance Our Learning
In Fig. 3.20, in which case(s) the lamp will not glow when the switch is closed?
Solution
The lamp will not glow in the following cases when the switch is closed:
(a) If the circuit is incomplete, meaning there is no continuous path for the electric current to flow.
(b) If the battery is connected incorrectly, such as positive to positive or negative to negative.
(c) If the conducting wires are not properly connected to the terminals of the bulb and battery.
Q9Let Us Enhance Our Learning
Suppose the '+' and '-' symbols cannot be read on a battery. Suggest a method to identify the two terminals of this battery.
Solution
To identify the positive and negative terminals of a battery without '+' and '-' markings, connect the battery to an LED. An LED has a longer wire (positive terminal) and a shorter wire (negative terminal). If the LED glows, the longer wire is connected to the positive terminal of the battery, and the shorter wire is connected to the negative terminal. If the LED does not glow, reverse the connections; the terminal that makes the LED glow when connected to the longer wire is the positive terminal.
Q10Let Us Enhance Our Learning
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.
Solution
(i)
Items required:
- Six electric cells (A, B, C, D, E, and F)
- A small incandescent lamp
- Connecting wires
- A switch (optional)
(ii)
Procedure:
- Connect cell A to the lamp using connecting wires and a switch (if available).
- Close the switch (if used). Observe if the lamp glows.
- If the lamp glows, cell A is working. If the lamp does not glow, cell A is not working.
- Repeat steps 1-3 for cells B, C, D, E, and F.
- Record which cells made the lamp glow and which did not.
(iii)
Activity:
Carry out the procedure described above. The cells that make the lamp glow are working, and the cells that do not make the lamp glow are not working.
Q11Let Us Enhance Our Learning
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.
Solution
The lamp will not glow in the circuit shown in Figure 3.21 because the cells are not connected in series correctly. The positive terminal of one cell must be connected to the negative terminal of the other cell. In the given circuit, it appears the positive terminal of one cell is connected to the positive terminal of the other cell. To correct the connections, the positive terminal of one cell should be connected to the negative terminal of the other cell, forming a battery. Then, connect the positive terminal of the battery to the longer wire (positive terminal) of the LED, and the negative terminal of the battery to the shorter wire (negative terminal) of the LED.