In the series circuit described earlier across 30 V with a 10-ohm resistor in series with a 20-ohm resistor, what is the current?

• A. 2 A
• B. 1 A
• C. 0.5 A
• D. 3 A

Answer: (B)

In a series circuit the same current flows through every component and the total resistance is the sum of all resistances. Here, the two resistors in series add up to 10 Ω + 20 Ω = 30 Ω. With a 30 V supply, the current is I = V / R = 30 V / 30 Ω = 1 A. This means the same 1 A flows through both resistors, with voltage drops of V = I × R, so 1 A × 10 Ω = 10 V across the first resistor and 1 A × 20 Ω = 20 V across the second, totaling 30 V.

If you see other currents listed, they’d come from treating only one resistor or a different total resistance, which isn’t correct for a series circuit where the current is determined by the combined resistance.