The First Law of Thermodynamics

There are three fundamental laws of thermodynamics. The first law is simply a statement of conservation of energy applied to the system. If an amount of thermal energyis given to the system, either

  • The internal energy U of the system can increase by an amount

  • It can do an amount of workon its' surroundings.

  • It can do a combination of the above.

Since energy is conserved,

The following must be noted:

  • Each of the quantitiesandin the equation above is considered from the systems point of view.

  • If is positive, then the system gains thermal energy and ifis negative the system loses thermal energy.

  • Ifis positive, the internal energy of the system increases. If the system is a gas, then the temperature of the gas increases. Conversely ifis negative the internal energy of the system is decreasing. If the system is a gas, the temperature of the gas is falling.

  • Ifis positive then the system is doing work on the surroundings – a gas is expanding for example. Ifis negative the surroundings are doing work on the system. W is negative.

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