Question

The minimum number of identical bulbs of rating 4V, 6W, that can work safely with desired brightness, when connected in series with a 240V mains supply is:

• A. 20
• B. 40
• C. 60
• D. 80

Hint:

To calculate the total number of bulbs in series for a given voltage, you need to first find the resistance of each bulb and then use the total voltage.

Answer 1

The Correct Option is B

For a 4V, 6W bulb, resistance is calculated using: \[ P = \frac{V^2}{R} \] Therefore, \( R = \frac{V^2}{P} = \frac{4^2}{6} = \frac{16}{6} \approx 2.67 \, \Omega \). To operate safely on a 240V supply, the total series resistance must be: \[ R_{{total}} = \frac{V_{{total}}^2}{P_{{total}}} \] With a total power of 6W per bulb and a 240V supply, 40 bulbs are needed.
Thus, the correct answer is option (b).