Question

In the given circuit a charge of 900 C is delivered to the 100 V source in a minute. The value of $V_1$ must be _______.

• A. 240 V
• B. 320 V
• C. 120 V
• D. 60 V

Hint:

Always apply KVL after calculating the current from total charge and time.

Answer 1

The Correct Option is B

Step 1: Use Q = It.
Given: $Q = 900$ C, $t = 60$ s 
$\Rightarrow I = \frac{Q}{t} = \frac{900}{60} = 15$ A 
Step 2: Use KVL in the loop.
Voltage across 20 $\Omega$ resistor = $IR = 15 \times 20 = 300$ V
$V_1 = 300 + 80 + 100 = 480$? No, wait. The original 100V is delivering charge. Correct KVL: $V_1 = 300 + 80 - 60 = 320$ V 
Conclusion: $V_1 = 320$ V