Joule's law of heating

When current flows through a resistor, the source's energy turns into heat. This is the heating effect of electric current. It is why a heater coil glows and an electric iron gets hot. The law has three parts. The heat produced is directly proportional to:

• the square of the current, for a given resistance;
• the resistance, for a given current;
• the time for which the current flows.

H is in joules (J) when I is in amperes, R in ohms and t in seconds.

A small example

• A current of 2 A flows through a 10 Ω resistor for 30 s.
• H = I²Rt = (2 A)² × 10 Ω × 30 s = 1200 J.
• Double the current to 4 A: H = (4 A)² × 10 Ω × 30 s = 4800 J.

Twice the current gives four times the heat, because the current is squared.

Where marks go

Question 34 of CBSE's 2026-27 Science sample paper offers this law as one side of an internal choice. The 2026-27 Science marking scheme gives 1 mark for the statement, printed with all three parts and H = I²Rt. The second mark is for the effect of doubling R at the same current. The heat doubles too.

So a bare formula is not the full statement. The scheme lists the three conditions in words. Keep "for a given resistance" and "for a given current" in your answer.

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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