Thermodynamics- Basic Concepts of Heat, Work & Energy


Energy is a general term including stored energy and transition energy. The stored energy of a substance may be in the form of mechanical energy and internal energy. Part of the stored energy may take the form of either potential energy or kinetic energy. Rest of the part of the energy is the internal energy. In a non-flow process usually, there is no change of kinetic or potential energy hence change of mechanical energy will not affect during calculations. In the flow processes, however, there may be changes in both potential and kinetic energy and these must be taken into account while calculating the changes of stored energy.

Heat and work are the forms of energy in transition. These are only forms of energies that may be transferred from the boundary of a system. And Neither Heat nor work can be as stored energy.


Work is said to be done when a body is moved a distance by application of a force. If a part of the boundary of a system undergoes a displacement under the action of pressure, the work done W is the product of the force ( Pressure X Area) and the covered distance in the direction of the force.

Work is a transition quantity that only appears at the boundary while a change of state is taking place within the system.  Work is something that appears at the boundary when a system changes its state due to the movement of a part of the system under the application of the force.

Sign Convention

  • If the work is done by the system on the surroundings, e.g.when a fluid expands pushing a piston outwards, the work is said to be positive. i.e. Work output of the system = +W
  • If the work is done on the system by the surroundings,e.g.Rotating a handle or kick the paddle to start an engine, work is said to be Negative. i.e Work input to the system = -W


“Heat is something that appears at the boundary when a system changes its state due to the difference in temperature between the system and its surroundings.”

Heat is also a transition quantity like work that only appears on the boundary while a change is taking place within the system. It is apparent that neither δW or δQ is exact differentials and therefore any integration of the element quantities of work or heat which appear during the change from state 1 to state 2 must be written as

 \int_{1}^{2} \delta W = W_{1-2} and  \int_{1}^{2} \delta Q = Q_{1-2}

Sign Convention

If the heat flows into a system from the surroundings, the heat is said to be positive and if the heat flows from the system to the surroundings, heat is then said to be Negative.

In other words: Heat receives by the system = +Q and  Heat gave up by the system = -Q

Comparison of Heat and Work

We can increase the temperature of a body in two ways i.e. by directly heating the body or by doing mechanical work on that body. For example, we can warm our hands by immersing them into warm water or by rubbing our hands together. also, we can change the phase of an object by heating it or by doing mechanical work on it. For example, A piece of ice will melt by heating but even if we do not heat and rub the pieces of ice together, they will melt.

Similarities of heat and work

  • Both are boundary phenomenon i.e. both are recognized at the boundary of the system as they cross them.
  • Both are associated with a process, not a state. Unlike properties, work or heat has no meaning at a state.
  • Both are path functions and inexact differentials.

Dissimilarities of heat and work

  • In heat transfer, the temperature difference is required.
  • The sole effect external to the system could be reduced to rising of the weight but in the case of heat transfer, other effects are also observed.
  • In a stable system, there is not possible work transfer while there is no restriction for the transfer of heat.

Internal Energy

It is the heat energy stored in the gas. if a certain amount of heat is supplied to the gas. This can have three following different effects on gas.

  • The temperature of the gas will increase
  • The volume of the gas may increase thereby doing some external work
  • Both temperature and volume may increase.

But it will be decided by the conditions under which the heat is supplied to the gas. If during heating of the gas, the temperature of the gas increases, internal energy will also increase.

Joule’s Law of internal energy

Joule’s Law of internal energy states that the internal energy of a perfect gas is a function of temperature only. In other words, the internal energy of a gas depends only on the change of temperature, not by the change in pressure or volume.

Law of conservation of Energy

The law of conservation of energy states that energy can be neither created nor destroyed, it can only be transformed. So the total energy of an isolated system always is constant.