2 Law of Thermodynamics (continued)

Yield of thermal machines

We can call the efficiency of a machine the relationship between the energy used as a form of work and the energy supplied:


= yield;

= work converted by the thermal energy supplied;

= amount of heat supplied by the heating source;

= amount of heat not transformed into work.

But as noted:

Therefore, we can express the yield as:

The minimum value for income is 0 if the machine does not do any work, and the maximum 1, if it were possible for the machine to turn all the heat received into work, but as seen, this is not possible. To know this percentage yield, multiply the result obtained by 100%.


A steam engine does a 12kJ job when it is supplied with a heat amount of 23kJ. What percentage capacity does the motor have to turn thermal energy into work?