Heat is energy transferred across the boundary of a system due to temperature difference between the system and the surrounding. The heat can be transferred by conduction, convection and radiation. The main characteristics of heat are:

1. Heat flows from a system at a higher temperature to a system at a lower temperature. 

2. The heat exists only during transfer into or out of a system. 

3. Heat is positive when it flows into the system and negative when it flows out of the system. 

4. Heat is a path function. 

5. It is not the property of the system because it does not represent an exact differential dQ. It is therefore represented as δQ.

Heat required to raise the temperature of a body or system, Q = mc (T₂ – T₁)

Where, m = mass, kg

T₁, T₂ = Temperatures in°C or K

c = specific heat, kJ/kg–K

Specific heat for gases can be specific heat at constant pressure (C_p) and constant volume (c_v)

Also; mc = thermal or heat capacity, kJ.

mc = water equivalent, kg.

WORK 

The work may be defined as follows: “Work is defined as the energy transferred (without transfer of mass) across the boundary of a system because of an intensive property difference other than temperature that exists between the system and surrounding.” Pressure difference results in mechanical work and electrical potential difference results in electrical work.

Or

“Work is said to be done by a system during a given operation if the sole effect of the system on things external to the system (surroundings) can be reduced to the raising of a weight”. The work is positive when done by the system and negative if work is done on the system