First law of thermodynamicsFirst law of thermodynamics states that
"Energy can be neither created nor destroyed but one form of energy can be converted to another form."For example consider a ball is placed on the top of a table initially. It will have certain potential energy ( Energy possessed by virtue of its height ) as it is at a height from the ground. When it is allowed to fall from the table this potential energy will be converted into kinetic energy ( Energy possessed by virtue of its motion ). This kinetic energy will be converted to heat , sound etc., when it touches the ground.
In application of the first law to a given process, the sphere of influence of the process is divided into two parts namely system and surroundings. The region in which process occurs is the System and everything which the system interacts is the surroundings. First law of thermodynamics applies to both system and surroundings. In general,
Δ Energy of system + Δ Energy of surrounding = 0For the above example if you consider ball as a system initial energy is potential and final energy is kinetic,but the energy is gained by surroundings as heat and sound.
Systems are of two types.
- Open = System which exchange both mass and energy with surroundings.
- Closed = System which exchange only energy with surroundings.
Δ Energy of system = Change in internal energy = ± Q ± WOnly change in internal energies can be found as it is hard to know the energy associated with attractions and vibrations. Q is heat and W is work.
Sign Convention for heat and work.
Q and W always refer to system.
- Heat given by the system, Heat produced by the system = -Q
- Heat given to the system, Heat supplied to the system = +Q
- Work done by the system, work produced by the system = -W
- Work done on the system, work given to the system = +W
Δ Internal energy = Q - W
Heat is given to the system and work is done by the system.