Heat: - It is the agent which produces in us the sensation of warmth and makes bodies hot. It is a form of energy. It flows from higher to lower temperature. In any exchange of heat, heat lost by the body is equal to the heat gained by the cold body. Substances generally expand when heated but the weight remains the same. A certain amount of heat known as latent heat is required to change the state of a body from solid to liquid or from liquid to gas without any change in temperature.
Internal energy:- Heat is not same as internal energy. Energy possessed by the atoms or molecules by virtue of their motion is called kinetic energy. There is a force of attraction between any two molecules; so they possess some potential energy. Sum of total kinetic and potential energies of atoms or molecules constituting a system are called the internal energy of the system. Thus heat is the part of internal energy which is transferred from one body to another an account of the temperature difference. Therefore the internal energy of a body was acquired by heat transfer.
If the internal energy of the system increases, then the sign of change in internal energy of the system is taken as positive and if the internal energy of the system decreases, then the sign of change in internal energy of the system is taken as negative.
Work: - Work is said to be done when force acting on a system displaces the body in its own direction. Work done W done on or by a system is the dot product of force and displacement.
If the gas expands, work is said to be done by the system and the sign of work done is taken as positive. If the gas is compressed, work is said to be done on the system and the sign of work done is taken as negative.