Write the basic difference between heat and internal energy.

(N/A) The state of a thermodynamic system is characterized by its internal energy,but not by heat. For example,a statement like 'a gas in a given state has a certain amount of heat' is as meaningless as the statement that 'a gas in a given state has a certain amount of work'. In contrast,'a gas in a given state has a certain amount of internal energy' is a valid statement.
Similarly,the statements 'a certain amount of heat is supplied to the system' or 'a certain amount of work was done by the system' are perfectly meaningful.
In short,heat and work in thermodynamics are not state variables. They are modes of energy transfer to a system resulting in a change in its internal energy. Heat is the mode of energy transfer,whereas internal energy is a state function.