Ohm's law

In symbols, V = IR. V is the potential difference in volts (V). I is the current in amperes (A). R is the resistance in ohms (Ω). R stays the same for a given wire at a given temperature. So if V doubles, I doubles too.

The same law can be turned round: R = V/I and I = V/R. A graph of V against I for such a wire is a straight line through the origin. If the resistance doubles, the current halves.

A small example

• A wire carries 0.5 A when 12 V is applied across it.
• R = V/I = 12 V / 0.5 A = 24 Ω.
• At 6 V, the current is I = V/R = 6 V / 24 Ω = 0.25 A.

Half the voltage gives half the current, as the law says.

Where marks go

Question 37 of CBSE's 2026-27 Science sample paper uses V = IR twice. In the version for visually impaired students, 15 V drives current through 2 Ω, 3 Ω and 5 Ω in series. The 2026-27 Science marking scheme gives 1 mark for I = 15 V / 10 Ω = 1.5 A. It gives another for the voltage across the 3 Ω resistor: 1.5 A × 3 Ω = 4.5 V.

Each answer line in the scheme ends with its unit. Write the unit at every step. Read what the scheme says about units.

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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