Resistors in series

In a series circuit there is only one path for the current. So the current is the same in every part. The battery's potential difference is shared between the resistors. The shares add up to the total.

The equivalent resistance is the sum: Rs = R1 + R2 + R3. It is larger than any single resistor. NCERT notes one drawback: if one part fails, the whole circuit stops. That is why home appliances are not joined in series.

A small example

A 4 Ω and an 8 Ω resistor are joined in series to a 6 V battery.

• Rs = 4 Ω + 8 Ω = 12 Ω.
• I = V/Rs = 6 V / 12 Ω = 0.5 A.
• Across the 4 Ω resistor: V = 0.5 A × 4 Ω = 2 V. Across the 8 Ω one: 4 V.
• Check: 2 V + 4 V = 6 V, the battery's voltage.

Where marks go

Question 37 of CBSE's 2026-27 Science sample paper has a 3-mark series version, for visually impaired students. A 15 V battery drives 2 Ω, 3 Ω and 5 Ω in series. The 2026-27 Science marking scheme gives 1 mark each for three lines: 10 Ω total, 1.5 A current, and 4.5 V across the 3 Ω resistor.

Each line uses the one before it. A slip in the total carries into the current and the voltage. See how step marking treats each line, and how resistors in parallel differ.

About the author

Deepti Sardana

Founder Member & Academic Lead

An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.

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