Physics. Heat and thermodynamics.

3. Heat and work are both forms of energy.
They are related in forms, in the one that one can be transformed into
the other.
HEAT ENERGY (such a steam engines) can be used to do work
(pushing a train down the rack)
Work can be transformed into heat, such as might be experenced by
rubbing two objects together to warm them up.

7. IN THERMODYNAMIC SYSTEMS WHERE THE
TEMPERATURE AND VOLUME ARE HELD
CONSTANT, THE MEASURE OF “USEFUL
WORK” ATTAINABLE IS THE HELMHOLTZ FREE
ENERGY.
IN THERMODYNAMIC SYSTEMS WHERE THE
VOLUME IS NOT HELD CONSTANT, THE
MEASURE OF “USEFUL WORK” ATTAINABLE
IS THE GIBBS FREE ENERGY.

8. THE APPROPRIATE VALUE OF HEAT CAPACITY TO USE
IN A GIVEN PROCESS DEPENDS ON WHETHER THE
PROCESS PRODUCES A CHANGE IN VOLUME.
THE HEAT CAPACITY IS A FUNCTION OF THE AMOUNT
OF HEAT ADDED TO A SYSTEM.

9. IN THE CASE OF A CONSTANT-VOLUME PROCESS, ALL
THE HEAT AFFECTS THE INTERNAL ENERGY OF THE
SYSTEM (I.E., THERE IS NO PV-WORK, AND ALL THE
HEAT AFFECTS THE TEMPERATURE)
HOWEVER IN A PROCESS WITHOUT A CONSTANT
VOLUME, THE HEAT ADDITION AFFECTS BOTH THE
INTERNAL ENERGY AND THE WORK (I.E., ENTHALPY) ;
THUS THE TEMPERATURE CHANGES BY A DIFFERENT
AMOUNT THAN IN THE CONSTANT-VOLUME CASE AND A
DIFFERENT HEAT CAPACITY VALUE IS REQUIRED.