Thermodynamics exercises

Second Law of Thermodynamics

1. In a thermal machine 3kJ of heat is supplied by the hot source to the beginning of the cycle and 780J passes to the cold source. What is the work done by the machine, if we consider that all energy that is not transformed into heat goes to work?

The second law of thermodynamics states that:

So, substituting the values ​​in the equation, we have:

2. What is the yield of the thermal machine from the previous exercise?

Being the yield of a thermal machine given by:

Substituting the values ​​in the equation:

Or, as a percentage:

Carnot cycle

1. A Carnot cycle machine has a hot source temperature of 330 ° C and a cold source at 10 ° C. What is the performance of this machine?


Being the yield of a Carnot cycle thermal machine given by:


T1= hot source temperature;

T2= cold source temperature.

But the temperatures used must be in absolute scale, so we must convert them. Like this:

Applying these values ​​to the income equation gives: