## Tuesday, January 6, 2015

### Thermodynamics and First Law of Thermodynamics

Thermodynamics deals with the heat energy and its transformation to other formats of energy’s.It deals with different formats of heat energy is, conversion and its applications.

Coordinates of Thermodynamics of the fundamental physical quantities basing on which thermodynamics is steadied.Temperature, pressure, volume, internal energy and entropy are some of the coordinates of Thermodynamics.

Temperature is a measure of heat energy. Body with high heat energy will have more temperature than the body with a low heat energy.

Pressure is defined as the force acting per unit area.

Internal energy is the energy that the system poses inside. In the case of a gas internal energy is the sum of potential energy due to the position of the gas molecules, translational kinetic energy you to the motion of the gas molecules, vibrational kinetic energy due to the oscillation of particles and rotational kinetic energy to do the rotation of molecules.

Depending on the nature of the gas either all of these energies or some of the energies are going to contribute for the internal energy.

We cannot measure the absolute energy of a body or a system and we can only measure the change in it.

Internal energy cannot do any external work.

There are some fundamental laws of thermodynamics. Among them  zero'th  law thermodynamics is the first one.

Definition

If two bodies A and B are in thermal equilibrium with a third body C separately, then  A and B are also in thermal equilibrium.

Work done by a gas

Consider a cylinder having a fixed area of cross-section. Let a gas at a pressure P is available in the system where the three sides of the system are closed and the fourth side is a movable piston.

Because of the expansion of the gas heater place a certain force on the piston and most the piston in the forward direction by a small distance. Basing on the deformation of the work done we can calculate the value as shown below.

If the volume increases in this process then the work done is treated as positive and vice versa.

Work done by the system is treated as positive and the work done on the system is treated as negative. Work done by the system produces in the expansion in the system and vice versa.

If the pressure is variable we shall integrate the equation to get the work done.

First law thermodynamics

The heat energy is given to a system is equal to the sum of change in its internal energy and the external work done by it.

This is nothing but law of conservation of energy. Energy is neither created, nor destroyed and it just converts from one format to another format. Heat the heat energy supplied is converted into the format of increase in internal energy and the external work done.

While applying this la we shall apply certain sign convention.

Heat given to the system is treated as positive and heat given by the system is treated as negative.

Increase in internal energy is treated as positive and a decrease in internal energy is treated as negative.

Work done by the system is treated as positive and the work done on the system is treated as negative.

We are going to solve small problem basing on the concept of first law thermodynamics where we need to apply proper sign convention.

Problem and solution

In a thermodynamic process, the pressure of a fixed mass of a gas is changed. In this process gas releases 20 is also of heat energy and trade jolts of work is done on the gas. If initial internal energy of the system is 30 J  what is the final internal energy?

We can solve this problem basing on the first law thermodynamics. According to the law the total heat energy supplied is equal to the sum of change in internal energy and the external work done.

As the system releases the heat energy shall be treated as negative.

Change in internal energy has to be calculated.

As the work is done on the system it shall be treated as negative.

The above diagram consists of another problem also. That problem is also solved basing on the same concept.

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