We have learnt about heat, internal energy and work. We also know how these are related to states of the system. But before going any further we need to define thermodynamics. It is a process that brings about change in the state of a system. Now which quantities determine state of the system? These are pressure, volume, temperature, mass or composition, internal energy etc. These quantities are referred to as state variables and measured only at equilibrium. Why equilibrium? Suppose I heated a container containing gas from one end; initially the temperature will not be same everywhere but finally it will reach an equilibrium value. So we will consider that value. We can classify state variables further as intensive or extensive. Intensive variables are independent of the dimensions of the system like pressure and temperature, while extensive variables depend on dimensions of the system like volume, mass, internal energy etc.

First Law of Thermodynamics:

So now that we have learnt about thermodynamics we will move on and try to answer what is the first law of thermodynamics. This law relates heat, internal energy and work. According to this law, some of the heat given to system is used to change the internal energy while the rest is used in doing work by the system. It can be represented mathematically as,

\(\Delta Q=\Delta U+W\)


ΔQ = Heat given or lost

ΔU = Change in internal energy

W = Work Done

We can also represent the above equation as follows,

\(\Delta U=\Delta Q-W\)

So we can infer from the above equation that the quantity (ΔQ – W) is independent of the path taken to change the state. Further we can say that internal energy tends to increase when heat is given to the system and vice – versa.

Stay tuned with Byju’s to know more about application of first law of thermodynamics i.e. how they can be used to calculate work done or internal energy in different types of processes.

Practise This Question

Work done by a system under isothermal change from a volume V1 to V2 for a gas which obeys Vander Waal's equation  (Vβ n)(P+an2V)=nRT