Introduction To Heat, Internal Energy And Work

We already know what heat is. It is a form of energy but it always comes into picture when energy is being transferred from one system to another. Suppose we are at an initial state ‘a’ and want to go to a final state ‘b’. We can do that by various processes (as shown in the figure) and heat energy released or absorbed in all the processes is different. So we can see that heat does not depend on the state of system.

heat & State of system

So there was a need to define another term which was dependent on state of the system. This is termed as Internal Energy. It is generally represented as U. So now how can we define internal energy? Internal energy is the energy contained within the system associated to random motions of the particles along with the potential energies of the molecules due to their orientation. The energy due to random motion includes translational, rotational, and vibrational energy. So now we can say since internal energy is a state function and in all the processes shown above the change in internal energy from state ‘a’ to state ‘b’ will be same.

Similar to these quantities there is another term known as work. It is also associated with transfer of energy. Suppose a piston contains gas. If the piston moves outwards, we say work is done by the gas and it is positive. Similarly if it moves inwards, work is done on the gas and it is negative. Let’s take the following figure:

movement of piston

\(Work done\) = \(\int(F.dx)\)

= \(\int P(A.dx) \)

= \( \int P.dV \)

We can use the above equation to find work done by a gas. Also graphical approach suggests that work done is the area under P – V graph. So for the above described process we can see area is different for different processes (as shown in figure below) and hence work is also not a state function and depends on path.

work is state independent

Here we have covered the basics of heat, internal energy and work. In the next article we will see how these are related. So to learn more join us at BYJU’S.


Practise This Question

A system X is neither in thermal equilibrium with Y, nor with Z. The systems Y and Z -