3.4.1 Definition of Heat
Heat is defined as the form of energy that is transferred across the boundary of a system at a given temperature to another system (or the surroundings) at a lower temperature by virtue of the difference in temperature between the two systems. Thus heat transfer occurs solely because of the temperature difference between the two systems. This definition of heat tells that a body never contains heat. Rather, heat can be felt only when it crosses the boundary. Thus heat is energy in transition. If we consider a hot block of copper as one system and the cold water in the bucket as another system; none of them contains heat (of course, they do contain energy). When the copper block is placed in the water, heat flows from coppe.r body to the water and after sometime a thermal equilibrium is reached. At this point we no longer have heat transfer, because there is no temperature difference between the two systems.
3.4.2 Convention of Sign of Heat
The heat is positive when it flows into the systemnd heat flows out of a system is taken as negative. Here the symbol Q is used to denote heat transfer during certain time interval.
3.4.3 Adiabatic Process
A process in which no heat crosses the boundary of the system is called adiabatic process.
A wall which does not allow heat to cross it is called adiabatic wall and a wall which permits heat to cross it is called 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 given in watt.