Physics 102 - Thermodynamics

Chapter 18

Kelvin-Helmholtz instability





Thermodynamics is the study of heat in motion. In this section we will discuss the first and second laws of thermodynamics, heat engines and entropy.



First law of thermodynamics


The first law states that the heat added to a system = the increase in internal energy + external work done by the system. This is basically a statement of conservation of energy. It was first proven by an experimental setup like that below.

The system here consists of the weights in the Earth's gravity and the cylinder full of water with a paddle wheel. We consider the system to be isolated so the total energy remains constant. The gravitational energy makes the weight fall. The work done is the weight dropping. It turns the paddle wheel which increases the internal energy in the cylinder and raises the temperature of the water.

Say the total mass is 10 kg, it falls 1 m, and there is 1 kilogram of water being heated. How much does the temperature rise?




Adiabatic processes


An adiabatic process is one in which a gas is compressed or expanded, but no heat enters or leaves the system. Often this is a process that happens so fast that there is not time for heat exchange. Diesel engines work on this principle. The gas in the cylinder is rapidly compressed. It heats up to the point where it fires the glow plug and ignites the gas.



Second law of thermodyanmics


The second law states that heat flows from a hot object to a cold object. We will use this concept while investigating heat engines.


Heat engines


A heat engine is any device that converts internal energy into mechanical work. This could be a steam engine, an internal combustion engine or a jet engine, to name three common examples.


Work is done when heat transfers from a high temperature to a low temperature. The steps involved for a heat engine are:

1) Gains heat from a hot reservoir (source) - increasing the engine's internal energy

2) converts some of this energy to mechanical work

3) expels the remaining energy into a low temperature reservoir (sink)



A Stirling engine is a heat engine that uses a fixed quantity of gas. The steps of the cycle are:

1) compressing cool gas

2) heating the gas

3) expanding the hot gas

4) cooling the gas

repeating the cycle

stirling engine


One result of the second law is that not all of the heat energy can be converted to mechanical energy or work. Some energy must always be expelled in the process.

We can restate this in terms of efficiency of the heat engine. For an ideal Carnot engine, we write it like this:



Notice that the efficiency of the ideal engine goes to 100% if the cold reservoir is at 0 Kelvin. Real engines have considerably lower efficiency than a Carnot engine, since there is friction in the system, etc.







A refrigerator uses external work to draw energy from a low temperature reservoir to a high temperature reservoir.

1) The refrigeration unit compresses a gas, typically ammonia. The compression heats up the gas.

2) The hot gas dissipates heat through coils on the back of the refrigerator.

3) The cooled gas condenses into liquid at high pressure.

4) The liquid flows through a small hole into a low pressure area where is can expand. This makes the liquid boils and vaporize, lowering its temperature. This is what makes the inside of the refrigerator cold.

5) The cold gas is sucked back into the compressor to repeat the cycle.





Entropy is defined as the heat in a substance / its temperature. This is also a measure of disorder in the system. Another way of stating the second law of thermodynamics is to say that energy degrades into less useful forms. The entropy of a system can never decrease over time, if the whole system is considered.


If you were to mix the salt and pepper together in one shaker, and shake it up, the salt and pepper would become more and more mixed. No amount of shaking will return it to a separated state.




Some processes seem to make order out of disorder. To make sense of how entropy cannot decrease in these cases, you need to consider everything the whole system contains.