If the voltage across a fixed resistance is doubled, what happens to the current?

• A. The current doubles.
• B. The current halves.
• C. The current stays the same.
• D. The current becomes zero.

Answer: (A)

Current through a fixed resistor follows Ohm's law: I = V/R. With the resistance unchanged, the current is directly proportional to the voltage. Doubling the voltage makes the numerator twice as large while R stays the same, so the current becomes twice as large. For example, if I = V/R before, then after increasing voltage to 2V, I = (2V)/R = 2(V/R) = 2I.

The other possibilities don’t fit because halving the current would require halving the voltage, keeping the same resistance; keeping the current the same would require no change in voltage or an infinite resistance; making the current zero would require zero voltage or infinite resistance.