In a series circuit, the current Is is the same through all components. Which expression shows this?

• A. Is = I1 = I2 = I3
• B. Vs = V1 + V2 + V3
• C. Rs = R1 + R2 + R3
• D. V1 = V2 = V3

Answer: (A)

Current is the same through all parts of a series circuit because there is only one path for charge to flow; the same amount of current that leaves the source must pass through each component in sequence. That’s why Is = I1 = I2 = I3 shows the current staying constant throughout the loop.

As a contrast, voltage adds up in series (Vs = V1 + V2 + V3) and total resistance also adds (Rs = R1 + R2 + R3). The voltages across each component aren’t generally equal in a series circuit—V1, V2, and V3 can differ depending on each component’s resistance—so a statement like V1 = V2 = V3 isn’t the defining relationship here. The key idea for this question is the constant current, captured by Is = I1 = I2 = I3.