Heat is the form of energy, making a body 'hot'. The kinetic energy of the atoms and molecules of matter gives rise to heat energy. More precisely, this energy remaining inside the body is called the internal energy of the body. When this energy flows from one body to another as a result of their temperature difference the energy in flow is referred to as heat. Temperature indicates the thermal state of a body, i.e., whether a body is hot or cold. When the temperature of two bodies are the same, there is no flow of heat from one body to the other, and the bodies are said to be in thermal equilibrium. It bodies A and B are in thermal equilibrium with a third body C, the bodies A and B are in thermal equilibrium with each other. This is known as Zeroth law of thermodynamics. From this law, one can define the temperature as that property which is the same for all systems in thermal equilibrium and determines whether a given system is in thermal equilibrium with others. It can be used to differentiate the heat and temperature as follows:
(i) The application of heat generally raises the temperature of a body. Hence, heat is the cause and temperature is its effect.
(ii) The same amount of heat may rise the temperature of different bodies to different extents.
(iii) Just as the flow of a liquid from one vessel to another, connected with each other at the bottom, depends on the heights of the liquid and not on their amounts in the vessels, so also the flow of heat from one body to another depends on their temperatures and not the internal energies contained in the bodies. Heat always flows from a body at a higher temperature to a body at a lower temperature.
(iv) In a change of state, the internal energy of a body changes, but the temperature does not. The application of heat melts ice, but the temperature during melting *** constant.
(v) Nature tends to maintain the equality of the temperatures of different bodies, and not of the internal energies.
