Question

A coil develops heat of 800 cal/sec. When 20 volts is applied across its ends. The resistance of the coil is (1 cal = 4.2 joule)

• A. $1.2\Omega$
• B. $1.4\Omega$
• C. $0.12\Omega$
• D. $0.14\Omega$

Answer 1

Answer: (C)

$0.12\Omega$

Answer 2

$H = \frac{V^{2}t}{4.2R}$ or $\frac{H}{t} = \frac{V^{2}}{4.2R}$

⇒ $800 = \frac{20 \times 20}{4.2 \times R}$⇒ $R = \frac{5}{42} = 0.119 \approx 0.12\Omega$