Energy and energy transfer Thermodynamics Chapter-2

Energy:

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.

Internal energy:

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.

Potential energy:

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.

Kinetic energy:

It is a energy of system due to the motion. It is given by: KE= 1/2mv2.

Total energy:

It is sum of internal energy, potential energy and kinetic energy. It is given by:

E=U+KE+PE

E=U + 1/2mv2 + mgz

Specific total energy

E=u + 1/2v2 + gz

Energy transfer:

There are two types of energy transfer which are given below:

Heat transfer:

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.

Work transfer:

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:

  1. Both are forms of energy and transient types.
  2. Both are not thermodynamic properties.
  3. Both are path function.
  4. Both are boundary phenomenon.
  5. Both have same unit joule.
  6. 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:


Polytropic 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

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