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Monday, 27 June 2016


Today we will see here the concept of work and heat in thermodynamics. We will study here the basics of heat and work in the field of thermal engineering and also we will see further the sign conventions used for heat and work transfer in thermodynamics.

Work transfer in thermodynamics

During our discussion about mechanics, we have discussed that work is said to be done if a force is acting over an object and object moves through a displacement in the direction of force. Work will be calculated by the dot product of force and displacement and its unit of measurement will be joule and will be indicated by J.

Let us see the definition of work in thermodynamics

Work is basically defined as the transformation of energy by any process except from heat in the field of thermal engineering. In thermal engineering energy transfer in the form of work will be calculated by the product of force (F) and displacement (X). Displacement will be in the direction of the force.

Let us consider the following arrangement of cylinder and piston. Cylinder contained with gas and displayed here the two equilibrium state of the system by state 1 and state 2. At state 1, gas will be at high pressure and will have lower volume. While at state 2, gas will be at lower pressure and will have higher volume as compared to state 1.
In order to change the state of the system, we need to alter the condition of the system by altering the thermodynamic properties of the system such as temperature, volume or pressure. Above changes will be done by varying the volume by displacing the piston or by changing the pressure by removing or adding the weight over the piston or varying the temperature by heating the gas.

In simple words, in order to change the state of the system work will be done over the system or work will be done by the system and also heat will be supplied to the system or heat will be rejected from the system.

We use the basic concepts of thermodynamics in order to secure the information about the amount of heat energy and amount of work energy transfer across the system boundary for desired changes in the state of the system. Figure also indicates the curve between pressure and volume for indicating the condition of the system at state 1 and state 2.

As we can see in above figure that the state of the system initially is at state 1 and at this state pressure will be high and specific volume will be less at a temperature.

Weight placed over the piston is just balancing the force which is exerted in upward direction by gas. If we remove the weight from the piston, system will have unbalanced force and piston will move in upward direction due to force acting over the piston in upward direction by the gas.

Piston will move in upward direction and will be stopped once it will have again equilibrium position. Hence, we have changed the state of the system from state 1 to state 2 by decreasing the pressure of the gas by removing the weight from the piston and therefore volume will be increased according to Boyle’s law.

This condition of the system is expressed as final state and indicated by state 2 and at this state pressure will be less and volume will be high.

Work done over the system will be determined here by calculating the area below the curve which is displayed here by red shaded area.

Work done will be calculated as displayed in following figure.
We will see the “Heat transfer in thermodynamics” in our next post.
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Engineering thermodynamics by P.K. Nag
Engineering thermodynamics by R. K. Rajput