Discuss modes (or remedies) for change of internal energy of any system.

Explanation

Heat and work are two distinct modes of energy transfer to a system that results in change in its internal energy
(a) Heat is energy transfer due to temperature difference between the system and the surrounding
(b) Work is energy transfer brought about by means (e.g. moving the piston by raising or lowering some weight connected to it) that do not involve such a temperature difference. According to figure (a) suppose a definite mass of gas system is in a cylinder.
State of gas (mean internal energy) U can change through two modes of changing state of system and hence changing internal energy of the system.
(i) To heat the cylinder containing the gas or keeping the cylinder in contact with a body at higher temperature some heat flows from hotter body to the gas on account of temperature difference. Therefore, internal energy of the gas increases.
(ii) To push the piston of the cylinder in the upward direction. Means, work is done on the system due to which the internal energy of the gas increases.