A cell connected across a 9 Ω resistance produces heat at the rate of 400 J per second. What is the potential difference applied across the resistance?

1. Heat of 400 J per second means power $P = 400$ W. 2. Use $P = \dfrac{V^2}{R}$, so $400 = \dfrac{V^2}{9}$. 3. $V^2 = 400 \times 9 = 3600$. 4. $V = \sqrt{3600} = 60$ V. 5. The option 20 V would give $P = 400/9 \approx 44$ W, far below 400 W, so it is wrong. _Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 4 "Effects of Electric Current", p.59_