It is a capacity to perform a work or It is a capacity of system to exert a force for a certain interval. There are two types of energy:
|Stored energy||Transient energy|
|Energy which remains within a system boundary as an inherent property of system is known as stored energy.||Energy which can cross the system boundary during thermodynamics process is called transient energy.|
|It can be defined for state or instant.||It can defined for process.|
|They are thermodynamic properties.||They are not thermodynamics properties.|
|It is a state or point function.||It is a path function.|
|It has a single value at each equilibrium state.||It has not single value at each equilibrium state.|
|Cyclic integral is zero.||Cyclic integral is non-zero.|
|They are not boundary phenomenon.||They are boundary phenomenon.|
|Eg: kinetic energy, potential energy etc.||Eg: heat and work.|
It is a energy of system due to the molecular activity. It can be also defined as it is a sum of molecular potential energy and molecular kinetic energy. It is denoted by U. internal energy per unit mass called specific Internal energy. It is denoted by u and given by : u=internal energy/mass.
It is a energy due to the position of system. It is also defined as energy of system due to its elevation in gravitational field and expressed as: PE= mgz where m=mass, g= acceleration due to gravity and z= elevation of gravitational field.
It is a energy of system due to the motion. It is given by: KE= 1/2mv2.
It is sum of internal energy, potential energy and kinetic energy. It is given by:
E=U + 1/2mv2 + mgz
Specific total energy
E=u + 1/2v2 + gz
There are two types of energy transfer which are given below:
It is a transfer of energy without transfer of mass due to the temperature difference is called heat transfer. It is denoted by Q and its si unit is joule ( J ).
Sign convection for heat transfer: heat transfer from surrounding to system is taken as positive and from system to surrounding is taken as negative.
It is a transfer of energy without transfer of mass due to the any property difference except temperature difference. It is denoted by W and its si unit is joule ( J ).
Sign convection for work transfer: work done by the system is taken as positive but work done on the system is taken as negative.
Similarities of work and heat transfer:
- Both are forms of energy and transient types.
- Both are not thermodynamic properties.
- Both are path function.
- Both are boundary phenomenon.
- Both have same unit joule.
- Cyclic integral is not zero.
Difference between work and heat transfer:
|Work transfer||Heat transfer|
|1||It is a transfer of energy but not mass due to the any properties difference except temperature difference.||It is a transfer of energy but not mass due to the difference of temperature.|
|2||It is a high grade of energy.||It is a low grade of energy.|
|3||It is a organized form of energy.||It is unorganized form of energy.|
|4||For displacement work, movement of piston is required.||Movement of piston is not required.|
|5||Work done from the system is positive and to the system is negative.||Heat transfer from the system is negative and to the system is positive.|
|6||Area under P-V diagram.||Area under P-T diagram.|
Expression of displacement work transfer:
Consider a piston cylinder device containing a gas as shown in fig 1 . If piston is moves small distance dl by using a force F then work done for process 1-2 is given by:
Constant volume process
Constant pressure process:
Constant temperature process:
The process which fallowed the relation PVn = constant called polytropic process. The different process is depend on the value of n. so it is given below:
|Value of n||equation||process|
|0||P = constant||Constant pressure or isobaric|
|1||PV = constant||Constant temperature or isothermal|
|∞||V = constant||Constant volume process or isochoric|
|γ||PVY = constant||Adiabatic|
Work done in polytropic process:
The equation is applicable if n is not equal to 1.
Note: work done due to spring = 1/2KX2