11-3 1

In review, the 1 st law of thermodynamics indicates that all energy entering and leaving the system is accounted for and is conserved. 2

More work is gained by taking more energy at a higher temperature and giving up less energy at a lower temperature. 3

The transfer of energy as heat from the high temperature source to the engine would cause the engine to do work. This would not be a cyclic process. 4

In order for the cycle to be completed, the engine would have to transfer energy always as heat. 5

Because the only body to which this energy can be transferred is the high temperature source, the engine must do work to transfer this energy. 6

This is the same amount of work that was made available through the energy transferred as heat from the high temperature body in the first place. 7

Thus, no net work is obtained from the engine in a cyclic process. 8

The requirement that a heat engine gives up some energy at a lower temperature in order to do work does not follow the 1 st law of thermodynamics. 9

This requirement is the basis of what is called the second law of thermodynamics. 10

The second law of thermodynamics can be stated as follows: No cyclic process that converts heat entirely into work is possible. 11

According to the 2 nd law of thermodynamics, W can never be equal to Q h in a cyclic process. Some energy must always be transferred as heat to the system’s surroundings. (Q c >0) 12

A cyclic process cannot completely convert energy transferred as heat into work, nor can it transfer energy as heat from a low temperature body to a high temperature body without work being done in the process. 13

However, a cyclic process can be made to approach these ideal situations. A measure of how well an engine operates is given by the engine’s efficiency. 14

Efficiency is a measure of the useful energy taken out of a process relative to the total energy that is put into the process. 15

Efficiency = W net Q h net work done by engine heat energy added to engine 16

Efficiency = Q h – Q c Q h heat energy added – heat energy removed heat energy added 17

Notice that efficiency is a unitless quantity that can be calculated using only the magnitudes for the energies added to and taken away from the engine. 18

This equation confirms that a heat engine has 100% efficiency only if there is no energy transferred away from the engine as heat. (Q c = 0) 19

Real heat engines, though, will have efficiencies of less than 1.0. The smaller the fraction of usable energy that an engine can provide, the lower its efficiency. 20

The equation also provides some important information for increasing engine efficiency. 21

If the amount of energy added to the system as heat is increased or the amount of energy given up by the system is reduced, the ratio of Q c /Q h becomes smaller and the engine’s efficiency comes closer to

The efficiency equation gives only a maximum values for an engine’s efficiency. 23

Friction and thermal conduction in the engine hinder the engine’s performance and experimentally measured efficiencies are usually lower than the calculated efficiencies. 24