A 20 μF capacitor is charged to 30 μC. What is the initial voltage across the capacitor?

• A. 0.15 V
• B. 1.5 V
• C. 15 V
• D. 150 V

Answer: (B)

Voltage across a capacitor is found from V = Q/C, where Q is the stored charge and C is the capacitance. With Q = 30 μC and C = 20 μF, the voltage is V = (30 μC)/(20 μF) = 1.5 V. The μC and μF units cancel to give volts.

So the initial voltage across the capacitor is 1.5 V. If you check the other numbers, they would require a different amount of charge: 0.15 V would need 3 μC, 15 V would need 300 μC, and 150 V would need 3000 μC, none of which match the given charge.