An electric bulb, marked `40 W` and `200 V`, is used in a circuit of supply voltage `100 V`. Now its power is

Options

A. `100 W`
B. `40 W`
C. `20 W`
D. `10 W`

Explanation

To solve the problem of finding the power consumed by a bulb marked `40 W` and `200 V` when used in a circuit with a supply voltage of `100 V`, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the given values**:
- Rated Power (P1) = 40 W
- Rated Voltage (V1) = 200 V
- Supply Voltage (V2) = 100 V

2. **Determine the resistance of the bulb**:
The power consumed by the bulb at its rated voltage can be expressed using the formula:
\[ P = \frac{V^2}{R} \]
Rearranging this gives us the resistance (R):
\[ R = \frac{V1^2}{P1} = \frac{200^2}{40} \]

3. **Calculate the resistance**:
\[ R = \frac{40000}{40} = 1000 \, \Omega \]

4. **Calculate the power consumed at the new voltage (100 V)**:
Using the power formula again with the new voltage:
\[ P2 = \frac{V2^2}{R} \]
Substituting the values we have:
\[ P2 = \frac{100^2}{1000} \]

5. **Calculate the new power**:
\[ P2 = \frac{10000}{1000} = 10 \, W \]

6. **Conclusion**:
The power consumed by the bulb when used at a supply voltage of `100 V` is **10 W**.

### Final Answer:
The power consumed by the bulb is **10 W**.