3.4.1 Definition of Heat
Heat can be best described as that form of energy which can be relocated across the margin of the system under explicitly given temperature and in some other system or surroundings. Provided that the transfer of heat occur when there is a temperature difference between the two systems. Thus from here, it can be concluded that it is never a body that holds the heat. It can only be sensed when it crosses the boundary. So it is now evident that heat is energy in transition.
Sorting out an example might be helpful to understand the part. Suppose we take a heated piece of copper in one system and coldwater in the other in a bucket of another system, both containing heat. Now if the copper block is placed in water then the heat transfers from the copper body to the water to attain some thermal equilibrium. After reaching the heatbalance, there will be no transfer of heat as now there is no temperature variance.
3.4.2 Convention of Sign of Heat
It is always considered as positive when the heat flows into the system, and it is negative when taken out of the system. Therefore to denote the heat transfer at a particular time we use the symbol Q.
3.4.3 Adiabatic Process
An adiabatic process is that where we could find no heat transfer across the boundary of the system.
It is the wall which does not allow the heat to transfer. Moreover, those walls which allow heat to pass through is called a diathermic wall.
3.4.4 Unit of Heat
The S. I. unit of heat is joule. The rate of heat transfer or work transfer is represented in Watt.