Electricity: Circuits and their Components

Correct Answer: (i) A switch is the source of electric current in a circuit.

Justification: A switch is NOT a source of electric current. The source of electric current in a circuit is the electric cell or battery. A switch is simply a device that either completes (closes) or breaks (opens) the circuit, thereby controlling the flow of current. Statements (ii), (iii), and (iv) are all correct descriptions of a switch.

Given: A circuit with a lamp, a cell, and two free ends A and B where different materials can be connected.

Concept Used: Materials that do not allow electric current to pass through them are called insulators. When an insulator is connected between A and B, the circuit remains open (broken) and the lamp does not glow.

Answer: The lamp will not glow when an insulating material is connected between ends A and B.

Examples of such materials include: rubber, plastic, glass, wood, paper, wax (candle), cork, and eraser.

Reason: These materials are poor conductors (insulators) of electricity. They do not allow electric current to flow through them, so the circuit remains incomplete and the lamp does not glow.

*(Note: The specific options in Fig. 3.16 are not fully visible in the OCR, but the answer is whichever option is an insulating material such as rubber, plastic, or wood.)*

Given: Two lamps are connected in a circuit (as shown in Fig. 3.17, they appear to be connected in series — one after the other in a single path).

Concept Used: In a series circuit, all components are connected in a single path. If any one component fails (breaks), the entire circuit becomes open (incomplete), and no current flows through any part of the circuit.

Answer: No, the other lamp will not glow.

Justification: When the filament of one lamp breaks, it creates a gap in the circuit. Since both lamps are connected in series (in a single path), the break in one lamp's filament makes the entire circuit open/incomplete. As a result, no current can flow through the circuit, and hence the other lamp also does not glow.

This is similar to how a fused lamp stops the flow of current — a broken filament acts like a gap in the wire.

Given: A circuit is made with connecting wires whose insulating (plastic/rubber) covering has NOT been removed at the ends. The lamp and cell are both working properly.

Concept Used: For electric current to flow through a circuit, there must be a complete conducting path. The metal part of the wire conducts electricity, but the plastic or rubber covering around it is an insulator.

Answer: No, the lamp will not glow.

Reason: The plastic or rubber covering on the wires is an insulator. If the insulating covering is not removed from the ends of the wires, the metal parts of the wires cannot make proper contact with the terminals of the cell and the lamp. As a result, the circuit remains incomplete (open), no current flows, and the lamp does not glow.

Conclusion: It is essential to strip (remove) about 1 cm of the insulating covering from both ends of each connecting wire before making a circuit, so that the bare metal can make good electrical contact.

Given: A simple torch consists of:
- A battery (two cells connected in series)
- A switch (S)
- A lamp (incandescent bulb or LED)
- Connecting wires

Concept Used: A circuit diagram uses standard symbols to represent electrical components.

Standard Symbols used:
- Electric cell: a long line (positive terminal) and a short thick line (negative terminal)
- Battery (two cells): two sets of long and short lines
- Switch (open): a gap shown by a line that does not connect
- Incandescent lamp: a circle with an X or a circle with a filament symbol inside
- Connecting wires: straight lines

Circuit Diagram Description:

The circuit diagram of a simple torch is drawn as follows:

$$\text{[Battery (+)]} \longrightarrow \text{[Switch S]} \longrightarrow \text{[Lamp]} \longrightarrow \text{[Battery (−)]}$$

All components are connected in series in a closed loop:
- The positive terminal of the battery connects via a wire to one terminal of the switch.
- The other terminal of the switch connects via a wire to one terminal of the lamp.
- The other terminal of the lamp connects via a wire back to the negative terminal of the battery.

Note: When the switch S is in the ON position, the circuit is complete and current flows from the positive terminal of the battery through the switch, through the lamp (making it glow), and back to the negative terminal of the battery.

Given: A circuit (Fig. 3.18) with two switches $S_1$ and $S_2$ and two lamps $L_1$ and $L_2$.

Assumption based on standard circuit diagrams of this type: $S_1$ controls $L_1$ (in series with $L_1$ on one branch) and $S_2$ controls $L_2$ (in series with $L_2$ on another branch), or $S_1$ is in the main line and $S_2$ is in a branch. Based on the typical circuit shown in NCERT Class 7 Fig. 3.18, $L_1$ is in series with $S_1$, and $L_2$ is in series with $S_2$, and both branches are connected in parallel to the battery.

Concept Used: A lamp glows only when the circuit through it is complete (closed). A switch in 'ON' position closes the circuit; 'OFF' position opens it.

(i) $S_2$ is ON, $S_1$ is OFF:
- The branch containing $L_2$ and $S_2$ is complete → $L_2$ glows.
- The branch containing $L_1$ and $S_1$ is open (broken) → $L_1$ does not glow.
- Answer: Only $L_2$ glows.

(ii) $S_2$ is OFF, $S_1$ is ON:
- The branch containing $L_1$ and $S_1$ is complete → $L_1$ glows.
- The branch containing $L_2$ and $S_2$ is open → $L_2$ does not glow.
- Answer: Only $L_1$ glows.

(iii) Both $S_1$ and $S_2$ are ON:
- Both branches are complete.
- Answer: Both $L_1$ and $L_2$ glow.

(iv) Both $S_1$ and $S_2$ are OFF:
- Both branches are open (broken).
- No current flows through either lamp.
- Answer: Neither lamp glows (no lamp glows).

Given: Vidyut has made a circuit with a cell, lamp, switch, and connecting wires. The switch is closed but the lamp does not glow.

Possible Reasons for the Lamp Not Glowing:

1. Dead/discharged cell: The electric cell may have run out of energy and can no longer provide current.
2. Fused lamp: The filament of the lamp may be broken (fused), creating a gap in the circuit.
3. Loose connections: The wires may not be properly connected to the terminals of the cell, lamp, or switch.
4. Insulating covering not removed: The plastic/rubber covering of the connecting wires may not have been stripped at the ends, preventing metal-to-metal contact.
5. Broken wire: One or more of the connecting wires may be broken internally (even if the outer covering looks fine).
6. Faulty switch: The switch may not be making proper contact even in the 'ON' position.
7. Incorrect connection: The wires may not be connected to the correct terminals.

Steps to Find Out the Reason:

Step 1 — Check the cell: Replace the existing cell with a new cell. If the lamp glows, the original cell was dead.

Step 2 — Check the lamp: Replace the lamp with a new lamp (or a known working lamp). If it glows, the original lamp was fused.

Step 3 — Check the connections: Inspect all wire connections to ensure they are tight and that the insulating covering has been removed from the wire ends.

Step 4 — Check the wires: Replace the connecting wires one by one with known good wires to identify any broken wire.

Step 5 — Check the switch: Bypass the switch by directly connecting the two wires that go to the switch. If the lamp glows, the switch is faulty.

By replacing components one at a time, we can identify the exact cause of the fault.

Given: Four circuit diagrams (a), (b), (c), (d) in Fig. 3.20 with a switch, cell, and lamp/LED.

Concept Used:
1. A lamp (incandescent) glows regardless of which terminal connects to positive or negative, as long as the circuit is complete.
2. An LED glows only when its positive terminal (longer wire/leg) is connected to the positive terminal of the battery and its negative terminal (shorter wire/leg) is connected to the negative terminal of the battery. Current flows through an LED in one direction only.
3. If the circuit is incomplete (open), no lamp or LED will glow.

Analysis of each case:

(a) — If this shows a complete circuit with an incandescent lamp correctly connected: Lamp will glow.

(b) — If this shows an LED connected with reversed polarity (positive terminal of LED connected to negative terminal of battery): LED will NOT glow, because current cannot flow through an LED in the reverse direction.

(c) — If this shows a complete circuit with correct connections: Lamp/LED will glow.

(d) — If this shows an open/incomplete circuit or an LED with reversed polarity: Lamp/LED will NOT glow.

Answer: Based on the standard version of this NCERT question, the lamp/LED will not glow in cases (b) and (d).
- In (b): The LED is connected with reversed polarity — its positive terminal is connected to the negative terminal of the battery, so current cannot flow through it.
- In (d): The circuit is incomplete/open or the LED polarity is reversed.

*(Note: Since the figures are partially visible in OCR, the answer is based on the standard NCERT Class 7 question where cases with reversed LED polarity or open circuits are identified.)*

Given: A battery whose '+' and '−' markings are not readable.

Method to Identify the Terminals:

Materials required: A known working LED (with clearly identifiable longer and shorter wires), two connecting wires.

Procedure:

Step 1: Take an LED. Identify its positive terminal (longer wire/leg) and negative terminal (shorter wire/leg).

Step 2: Connect the longer wire (positive terminal) of the LED to one terminal of the battery, and the shorter wire (negative terminal) of the LED to the other terminal of the battery.

Step 3: Observe whether the LED glows.

Observation and Conclusion:
- If the LED glows: The terminal of the battery connected to the longer wire of the LED is the positive terminal (+), and the terminal connected to the shorter wire is the negative terminal (−).
- If the LED does not glow: Interchange the connections (connect the longer wire to the other terminal). Now the LED will glow. The terminal now connected to the longer wire of the LED is the positive terminal (+).

Reason: An LED allows current to flow in only one direction — from its positive terminal to its negative terminal. It glows only when connected correctly. This property helps us identify the terminals of an unknown battery.

Given: Six cells marked A, B, C, D, E, and F. Some are working and some are not.

(i) Items Required:
1. Six cells — A, B, C, D, E, F
2. One small incandescent lamp (torch bulb) with a lamp holder
3. Connecting wires (with insulation removed at ends)
4. A cell holder

(ii) Procedure:

Step 1: Set up a simple circuit with the lamp holder, lamp, and two free wire ends (a conduction tester circuit — similar to Activity 3.11).

Step 2: Take cell A and place it in the cell holder. Connect the two free wire ends to the positive and negative terminals of cell A.

Step 3: Observe whether the lamp glows.
- If the lamp glows → Cell A is working.
- If the lamp does not glow → Cell A is not working (discharged/dead).

Step 4: Repeat Steps 2 and 3 for each of the remaining cells B, C, D, E, and F, one at a time.

Step 5: Record observations in a table.

| Cell | Lamp Glows (Yes/No) | Working / Not Working |
|------|--------------------|-----------------------|
| A | | |
| B | | |
| C | | |
| D | | |
| E | | |
| F | | |

(iii) Carrying out the Activity:

Test each cell one by one as described above. The cells for which the lamp glows are the working cells. The cells for which the lamp does not glow are dead/discharged cells.

Conclusion: This method uses the lamp as an indicator — it glows only when sufficient current flows through the circuit, which happens only when the cell is working and has enough energy.

Given: An LED that requires two cells in series to glow. Tanya has made a circuit as shown in Fig. 3.21.

Concept Used:
1. Two cells in series means the positive terminal of one cell is connected to the negative terminal of the next cell.
2. An LED glows only when its positive terminal (longer wire) is connected to the positive terminal of the battery and its negative terminal (shorter wire) is connected to the negative terminal of the battery.

Analysis of Tanya's Circuit (Fig. 3.21):

Based on the standard version of this NCERT question, Tanya's circuit has one or more of the following errors:
- The two cells are NOT connected in series correctly (e.g., positive terminal of one cell is connected to positive terminal of the other, instead of positive to negative).
- OR the LED is connected with reversed polarity.

Answer: The LED will not glow in Tanya's circuit because the connections are incorrect.

Correct Connection:

For the LED to glow, the correct circuit should be:

$$\text{Cell 1: (−) [spring side]} \longrightarrow \text{Cell 1: (+)]} \longrightarrow \text{Cell 2: (−)} \longrightarrow \text{Cell 2: (+)} \longrightarrow \text{LED positive (longer wire)} \longrightarrow \text{LED negative (shorter wire)} \longrightarrow \text{back to Cell 1 (−)}$$

Steps for correct connection:
1. Connect the two cells in series: the positive terminal of Cell 1 connects to the negative terminal of Cell 2.
2. The free positive terminal (of Cell 2) connects to the longer wire (positive terminal) of the LED.
3. The free negative terminal (of Cell 1) connects to the shorter wire (negative terminal) of the LED.

With this correct connection, the circuit is complete, current flows in the proper direction through the LED, and the LED will glow.