When three resistors of 10.0 ohms each are placed in series, the total resistance is

• A. 10.0 ohms
• B. 20.0 ohms
• C. 40.0 ohms
• D. 30.0 ohms

Answer: (D)

Resistors placed in series add their resistances because the same current flows through each component and the voltage drops across them add up. So the total resistance is simply the sum of the individual resistances. For three resistors of 10.0 ohms each in series, you get 10.0 + 10.0 + 10.0 = 30.0 ohms.

If you see 20.0 ohms, that would come from counting only two of the resistors in series. A value like 40.0 ohms would imply four tens were added, which isn’t the case here. In contrast, placing resistors in parallel would yield a total resistance smaller than any single resistor, not 30.0 ohms.