Hello students, we know according to Zeroth law, “If two systems are each in thermal equilibrium with third system, they are also in thermal equilibrium with each other”. Heat is the form of energy that always transfers from point of higher temperature to point of lower temperature. Heat given to substance, increases its temperature. The transfer of heat continues till the system and surrounding of system attains the same thermal state. In this article we are going to discuss about the changes that takes place in a substance when heat is given to system or taken out from the system.
Let’s understand concept in detail……………!
- Heat:
Heat is the form of energy which gives us the sensation of hotness or coldness. Temperature is used to determine the degree of heat. When two bodies are in contact with each other, temperature variation between the bodies decides the direction of transfer of heat. Heat always flows from higher temperature to lower temperature. Once the temperature of two points is same, then the transfer of heat is stopped.
- Internal energy:
Suppose the gas is filled in a container as shown in fig. We know that according to kinetic theory of gases, gas is composed of large number of extremely small sized particles known as molecules. The molecules of gas are randomly placed in the system, so that they are free to move anywhere in container. Every molecule possesses its own potential energy due to its mass and kinetic energy due to its random motion.
The sum of kinetic energy and potential energy of molecules is known as internal energy of system. It is denoted by ‘U’.
The parameters like pressure, temperature and volume are used to explain the changes that take place in internal energy under action of heat.
- Work:
When the heat is provided to system, the change in internal energy of system takes place. During this the system can either expands or contracts. This expansion or contraction of system under action of heat give rise the work.
The simplest form of calculating the work done is by measuring the changes in parameters like temperature, pressure and volume. Suppose that the gas is filled in cylinder chamber with a frictionless and mass less piston, and heat is given to cylinder, as shown below,
Here we consider that due to the heat pressure P acts on area A of piston, then the piston gets lifted upward so that the volume of gas changes from V1 to V2 by distance ‘h’ (see the fig)
By definition, the pressure P is given as,
Pressure, P = Force/Area
∴ P = F/A
∴F = P ×A
The work done during the process then can be written as,
∴ Work done, dW = Force × Displacement
∴ dW = F × h
∴ dW = P × A × h
But, A × h = volume, dV, then the above equation can be written as,
∴ dW = P × dV
Where dV is small change in volume. To find the total work done we need to integrate the above equation from V1 to V2
Using above method we can find the total work in the thermodynamic system.
Some important points to note:-
- When the volume of gas is increased, then work is done by system over surrounding, internal energy of system is increased. In this case work is taken as positive.
- When the volume of gas is decreased, then work is done by surrounding on system, internal energy decreased. In this case work is taken as negative.