All core subjects 2026 — Question Paper

QJEE-P10

Did you know even a theoretically perfect engine is forced to waste energy? Meet the Carnot engine. It operates by taking heat from a hot source, doing some useful work, and dumping the leftover heat into a cold sink.

In our problem, this engine is operating between two specific temperatures. The hot source is at 400 Kelvin, and the cold sink is at 300 Kelvin.

To find the absolute maximum efficiency possible, we use the Carnot formula. It's simply one minus the ratio of the cold temperature to the hot temperature.

So, what happens when we plug in our actual numbers? The math gets surprisingly simple. We put our cold 300 K on top, and our hot 400 K on the bottom.

The units and the zeros cancel right out. This leaves us with one minus three-quarters, which evaluates to exactly one-quarter, or zero point two five.

Multiply by 100 to get a percentage, and we find our efficiency is 25%. So even this perfect engine wastes 75% of its heat, making option A the correct answer.