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Heat Engines and the Second Law of Thermodynamics
A "heat engine" is a machine which does work by using some of the internal energy of a body at a high temperature.
When the lid on a pan of boiling water is lifted by the steam inside, some of the internal energy of the steam is being converted to gravitational potential energy of the lid.
This is a very simple example of a heat engine (but one of historical importance as it is thought to have stimulated James Watt into inventing the steam engine!)

The conversion of energy from some other form of energy to internal energy of a substance can be done with 100% efficiency of conversion.
For example, 100J of electrical energy will be converted to 100J of internal energy by a resistor.

Conversion of energy from internal energy to some other form cannot be done with the same efficiency.
To show this, consider the possible heat engine shown below.
The diagrams are very simplified but are similar to some real heat engines found in cars etc.

 1. 2.

The engine is shown at two different stages in its cycle of operations:
1. hot gas expanding, doing work, pushing piston down
2. piston starting to move back up, compressing the gas.
It should be clear that this engine has a problem...
If we simply push the piston back up we will do just as much work as we obtained during the expansion stage; not a very satisfactory situation!

There are two obvious ways in which we can return the piston to the top doing less work than we obtained from the expansion stage:
- we could allow the gas to cool down before starting the compression stage, or
- we could allow this still quite hot gas to escape as the piston moves up and replace with other gas to be heated and compressed.
The second of these possibilities is more practical and is used in car engines etc.

However you choose to continue the cycle, you will always find that you need to allow the gas to give up some of its internal energy to the surroundings (as it cools down).
Therefore, the conversion of energy from internal energy of the hot gas can never be 100% efficient.

This is one illustration of the second law of thermodynamics which can be stated in many different ways.
One statement, based on the above logic, is as follows

 No heat engine, operating in a continuous cycle, can do work without transferring some internal energy from a hot body to a cold body.

The hot body is called the heat source and the cold body (often the surroundings) is called the heat sink.
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