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Besides, what happens to the internal energy of a system when it is heated?
When an object is heated, its particles move more vigorously and its internal energy increases. Unless the object changes state (eg melts or boils), its temperature will increase.
Similarly, what happens to internal energy when temperature increases?
The internal energy is the total amount of kinetic energy and potential energy of all the particles in the system. When energy is given to raise the temperature , particles speed up and they gain kinetic energy.
So, for constant volume heat addition there is only increase in internal energy but where as in case of constant preassure heat addition there is internal energy increase and work done by the system. So, In both the cases internal energy must have to increase.