First law of thermodynamics | statements, equations, limitations and examples

First law of thermodynamics statements

(A) ” The heat and mechanical work are mutually convertible “. According to this statement, when a closed system undergoes a thermodynamic cycle, the net heat transfer is equal to net work transfer. In other words, the cyclic integral of heat transfers is equal to the cyclic integral of work transfers. 

                       i.e. ∮ẟQ = ∮ẟW

(B) ” The energy can neither be created nor destroyed though it can only be transformed from one form to another “.  According to this statement, when a system undergoes a change of state or a thermodynamic process, then both heat transfer and work transfer takes place. The net energy transfer is stored within the system and is known as the stored energy or total energy of the system.

Energy equation for unit mass-

q(1-2) – w(1-2) = ( gz2 – gz1 ) + (V2^2/2 – V1^2/2) + u2 – u1

NOTES. 1. When there is no change in potential energy of the system ( i.e. when the height of the system from the datum level is same), then PE1 = PE2. Thus the above equation is written as

Q(1-2) – W(1-2) = ( KE2 – KE1 ) + ( U2 – U1 )

2. When there is no change of potential energy and also there is no of the mass into or out of the system, then the above equation equation is written as

Q ( 1-2 ) – W (1-2 ) = U2 – U1 = dU

In other words, in a closed or non flow thermodynamic system,

PE = 0 and KE = 0

Thus, note number two equation is known as Non flow energy equation.

3. For an isolated system for which Q(1-2) = W(1-2) = 0, the equation becomes

E2 = E1

This shows that the first law of thermodynamics is the law of conservation of energy.


Some statements of  first law thermodynamics is like a ideal case which can not be possible in actual practice. It has limitations  As we have discussed earlier that according to the first law of thermodynamics:

1. “When a system undergoes a thermodynamic cycle, the net heat transfer is equal to the net work transfer”. This statement doesn’t specify the direction of flow of heat and work i.e. whether the heat flows from from a hot body to a cold body or from a cold body to a hot body. It also doesn’t give any condition under which these transfer take place.

2. “The heat energy and mechanical work are mutually convertible”. As we all know that the work is the superior form of energy and heat is less superior. That’s why work energy can be fully converted into heat energy but a little part of heat energy can be converted into mechanical work. By this statement we can easily conclude that heat energy and mechanical work are not fully interconvertible. In other words there is a limitation on the conversion of one form of energy into another form.

A machine which violates the first law of thermodynamics ( i.e. energy can neither be created nor destroyed but can be transformed from one form to another) is known as perpetual motion machine of first kind ( briefly written as PMM-1). It is defined as a machine which produces work energy without consuming an equivalent of energy from other source. Such a machine is impossible to obtain in actual practice, because no machine can produce energy of its own without consuming any other form of energy.


A bicycle pump is a great example of first law thermodynamics. When we pump on the handle frequently or rapidly then the cylinder of the pump becomes hot to mechanical work applied on it. This shows that the due the application of mechanical work the heat is generated which eventually increases the internal energy of the system.

We can also consider an example of human body. A human does so much work in daily life such as running, walking, playing, labor dancing etc. In such kind of works there’s a lot energy required. Energy also needed to make new cells in body and replace it with old cell which have died. Here we can see energy conservation follows.

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