Answer to Question #143822 in Molecular Physics | Thermodynamics for Shayan Bhattacharjee

Question

in an internal combustion engine, during the compression stroke the heat rejected to the cooling water is 50kJ/kg and the work input is 100kJ/kg.
Calculate the change in internal energy of the working fluid stating whether it is a gain or loss.

Answer 1

We will use the first law of thermodynamics:

$$\Delta U=Q-W,$$

where Q and W are heat transferred and work done.

Since we see the work input, the work is negative. Since we see that the heat was given to water, it is negative as well. So:

$$\Delta U=(-50)-(-100)=50\text{ kJ/kg}.$$

This is a gain.