Energy and its forms in thermodynamics

Capacity to do work is called energy. It is of two types.

1. Stored Energy

2. Energy in transition

1. Stored Energy - It is the form of energy with in the system. Example - Kinetic energy, potential energy and internal energy.

2. Energy in transition - This is the form of energy which appears at the boundary of the system and surrounding. This is of two types. Work transfer and Heat transfer. This type of energy is not the property of system.

a. Work transfer - In thermodynamics work is said to be done by a system when the sole effect on the things external to the system, is used in raising the weight. The weight may not actually be moved but the net effect should be to raise the weight.

For this let us consider a battery motor system which is affecting the surrounding by running a fan. If the fan in surrounding is replaced by weight pulley system then battery motor system will try to raise the weight.

Sign Conversions for work transfer -

Work done by the system = +ve

Work done on the system = -ve

Heat Transfer - The transfer of energy by virtue of temperature difference across the boundary is called heat transfer.

All other type of energy transfer are called work transfer.

Sign Conversion for heat transfer -

Heat given to the system = +ve

Heat taken out of the system = -ve

Displacement Work

The work done by the system or on the system by displacing the boundary of the system is called displacement work.

Let us consider a piston – cylinder arrangement.

Let P1 , V1 ,T1 are the initial properties.

Let piston is moving from inner state to the outer most state .

Let P2 , V2 ,T2 are the final properties.

Let a = Cross sectional area of the piston.

Fig. Piston and cylinder arrangement

Let us assume an intermediate position when the piston has moved small distance dx.

Force applied for this small movement df =P × a

The small work done for this displacement dw = df × dx

Dw = P × a × dx

Dw =P × dv | dv = a × dx

Total work done for displacing the boundary from initial state to final state can be given as

W = ʃdw = ʃP × dv

This work done is equal to the area covered under P-V diagram.