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.

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

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.

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

Q = U + W

But W = PV

Thus

QP = U + PV

As V = (V2 - V1)

QP = U + P (V2 - V1)

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

Isochoric Process

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

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

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.

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

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

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

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.

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

Isothermal Process

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

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

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.

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

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

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

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.

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

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

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

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.

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

Q = U + W

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

0 = U + WU = - W

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

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.

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA