applications of first law of thermodynamics

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

1

Applications ofFirst Law of

Thermodynamics

Sajjad Ahmed Memon

Senior Scientist

NIMRA

Heat can be supplied to a thermodynamic system under the following conditions:

Isobaric Process

Isochoric Process

Isothermal Process

Adiabatic ProcessSajjad Ahmed Memon S.S./

Health Physicist NIMRA

Isobaric Process

A thermodynamic process in which pressure of the system remains constant during the supply of heat.


Consider a cylinder fitted with a frictionless piston. The piston is free to move in the cylinder. An ideal gas is enclosed in the cylinder.


Q = U + W

But W = PV

Thus

QP = U + PV

As V = (V2 - V1)

QP = U + P (V2 - V1)


Isochoric Process

A thermodynamic process in which the volume of the system remains constant during the supply of heat.


Consider a cylinder fitted with a frictionless piston. An ideal gas is enclosed in the cylinder. The piston is fixed at a particular position so that the volume of cylinder remains constant during the supply of heat.


Let Q amount of heat is added to the system. Addition of heat causes the following changes in the system:

Q = U + W But W = PVThus Q = U + PV As V = 0Q = U + P (0) = U


This expression indicates that the heat supplied under isochoric process is consumed in increasing the internal energy of the system but no work is performed.


Isothermal Process

A thermodynamic process in which the temperature of the system remains constant during the supply of heat.


Isothermal Compression

Consider a cylinder of non-conducting walls and good heat conducting base. The cylinder is fitted with a frictionless piston. An ideal gas is enclosed in the cylinder. In the first stage pressure on the piston is increased and the cylinder is placed on a cold body. Due to compression, the temperature of the system increases but at the same time Q amount of heat is removed from the system and the temperature of the system is maintained.


Q = U + W

Since temperature is constant, therefore, there is no change in internal energy of the system. i.e. U = 0

As the work is done on the system, therefore, W is negative

Q = 0 + (-W)Q = -W


Isothermal Expansion

In another situation the cylinder is placed over a hot body and the pressure on the system is decreased. Due to expansion, the temperature of the system is decreased but at the same time Q amount of heat is absorbed from the hot body and the temperature of the system is again maintained.


Q = U + W

Since temperature is constant, therefore, there is no change in internal energy of the system. i.e. U = 0

As the work is done by the system, therefore, W is positive

Q = 0 + (W)Q = +W


Adiabatic Process

A thermodynamic process in which there is no heat transfer into or out of the system takes place. In other words Q = 0.

An adiabatic process is generally obtained by surrounding the entire system with a strongly insulating material or by carrying out the process so quickly that there is no time for a significant heat transfer to take place.


Q = U + W

Since there is no heat transfer into or out of the system. i.e. Q = 0.

0 = U + WU = - W


Since U is the change in internal energy and W is the work done by the system, therefore the possible outcomes:

1. A system that expands under adiabatic conditions does positive work, so the internal energy decreases.

2. A system that contracts under adiabatic conditions does negative work, so the internal energy increases.


applications of first law of thermodynamics

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