CBSE Class 11-science Answered

In thermodynamics, the total energy of a system is called its internal energy and is denoted by U.

 Thermodynamics allows us to find expressions for how the internal energy of a system changes during a process. However, thermodynamics does not allow us to calculate U for a given state. The change in internal energy during a process is defined by ΔU and is equal to the difference between the energy of the system in the final state and the energy of the system in the initial state.

 ΔU = U_f – U_i

 Internal energy is a state function because it depends only on the state of the system and not on the process used to reach that state. Although we cannot calculate the absolute value of U, thermodynamics tells us (from experience) that the value of U depends only on the characteristics of the state of that system.

 For any process, the first law of thermodynamics states:

ΔU = w + q

Where ΔU is the change in energy of the system, w is the work done on or by the system and q is the heat exchanged between system and surroundings during the process. 

So it’s not necessary that we can calculate Internal energy only by knowing initial and final internal energy. There are other state functions also from which we can get it.

 Thus, you can get idea about initial internal energy and final internal energy from other state functions, from which you can calculate internal energy change.