In a series circuit, the total resistance equals the sum of the individual resistances.

• A. The product of the resistances.
• B. The sum of the resistances.
• C. The reciprocal of the sum.
• D. The difference of the resistances.

Answer: (B)

When resistors are in series, the same current flows through each one and the voltage across the whole string is the sum of the individual voltages. Each voltage drop is V_i = I · R_i, so the total voltage is V_total = V1 + V2 + … = I · (R1 + R2 + …). The total resistance is V_total divided by the current, giving R_total = (I · (R1 + R2 + …)) / I = R1 + R2 + …. So the total resistance in a series circuit is the sum of the individual resistances. The other expressions don’t apply here: the product isn’t how series resistance adds, the reciprocal of the sum relates to parallel circuits (1/R_total = 1/R1 + 1/R2 + …), and taking a difference isn’t the standard way resistances combine in simple networks.