Question

What is the formula for thermal efficiency?

Answer 1

1. Thermal efficiency of a heat engine: A heat engine's thermal efficiency is defined as the ratio of work done to heat absorbed during the course of the cycle. The formula can be expressed as follows based on the definition:
2. The formula for the thermal efficiency : Efficiency $\left(\eta \right)=$ Work done by the engine/ heat absorbed

$$\begin{gathered} \eta =W/Q_1 \\ =(Q_1-Q_2)/Q_1 \\ =1-(Q_2/Q_1) \end{gathered}$$

Where $Q_1$ is the amount of heat absorbed, $Q_2$ is the amount of heat rejected and $W$ is the amount of work done by the system.

Note -If $Q_2$ is zero, the engine will have $100\%$ efficiency. That means there is no heat rejected to the sink.

Hence formula for the thermal efficiency is

$\eta =1-(Q_2/Q_1)$.