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).**