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 work
on its' surroundings.
It can do a combination of the above.
Since energy is conserved,
The following must be noted:
Each of the quantities
and
in the equation above is considered from the systems point of view.
If
is positive, then the system gains thermal energy and if
is negative the system loses thermal energy.
If
is positive, the internal energy of the system increases. If the system is a gas, then the temperature of the gas increases. Conversely if
is negative the internal energy of the system is decreasing. If the system is a gas, the temperature of the gas is falling.
If
is positive then the system is doing work on the surroundings – a gas is expanding for example. If
is negative the surroundings are doing work on the system. W is negative.