Physics Study Guide/Thermodynamics

Introduction[edit]

Thermodynamics deals with the movement of heat and its conversion to mechanical and electrical energy among others.

§Laws of Thermodynamics[edit]

§First Law[edit]

The First Law is a statement of conservation of energy law:

The First Law can be expressed as the change in internal energy of a system () equals the amount of energy added to a system (Q), such as heat, minus the work expended by the system on its surroundings (W).

If Q is positive, the system has gained energy (by heating).

If W is positive, the system has lost energy from doing work on its surroundings.

As written the equations have a problem in that neither Q or W are state functions or quantities which can be known by direct measurement without knowing the history of the system.

In a gas, the first law can be written in terms of state functions as

§Zero-th Law[edit]

After the first law of Thermodynamics had been named, physicists realised that there was another more fundamental law, which they termed the 'zero-th'.

This is that:

If two bodies are at the same temperature, there is no resultant heat flow between them.

An alternate form of the 'zero-th' law can be described:

If two bodies are in thermal equilibrium with a third, all are in thermal equilibrium with each other.

This second statement, in turn, gives rise to a definition of Temperature (T):

Temperature is the only thing that is the same between two otherwise unlike bodies that are in thermal equilibrium with each other.

§Second Law[edit]

This law states that heat will never of itself flow from a cold object to a hot object.

where  is the Boltzmann constant () and  is the partition function, i. e. the number of all possible states in the system.

This was the statistical definition of entropy, there is also a "macroscopic" definition:

where T is the temperature and dQ is the increment in energy of the system.

§Third Law[edit]

The third law states that a temperature of absolute zero cannot be reached.

§Temperature Scales[edit]

There are several different scales used to measure temperature. Those you will most often come across in physics are degrees Celsius and kelvins.

Celsius temperatures use the symbol Θ. The symbol for degrees Celsius is °C. Kelvin temperatures use the symbol T. The symbol for kelvins is K.

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