Doubling the source voltage in the same series circuit with the same resistors, what happens to the current?

• A. It remains the same
• B. It halves
• C. It increases by a factor of four
• D. It doubles

Answer: (D)

In a series circuit, the current is set by Ohm's law: I = V / R_total, and the same current flows through every component. The total resistance is the sum of the individual resistances, so it stays the same if the resistors don’t change. If you double the source voltage while keeping the resistors the same, the current becomes I' = (2V) / R_total = 2 × (V / R_total) = 2I. So the current doubles. Since the current doubles and each resistor has its resistance unchanged, the power dissipated in each resistor increases by a factor of four (P = I^2R).