First Law of Thermodynamics [Open and Closed Systems]

By | February 9, 2019

First law of thermodynamics

The first law of thermodynamics states that:

The algebraic sum of net heat and work interactions between a system and its surrounding in a thermodynamic cycle is zero


For a cyclic process



Q = Heat interaction

W = Work interaction

For a finite non-cyclic process

Q1-2 = W1-2 + ΔE


E = Internal energy

In general

E = U + KE + PE + any other kind of stored energy


U = Intermolecular energy

KE = Kinetic energy

PE = Potential energy

For a closed system in equilibrium KE, PE and other kinds of stored energy are zero.

Which means

E = U

Hence for a finite non-cyclic process first law of thermodynamics becomes

Q1-2 = W1-2 + ΔU

If we consider only P*ΔV work, above equation becomes

Q1-2 = P*ΔV + ΔU

Note: Conventionally work done by the system and the heat given to the system are always taken positive.

Internal energy

A property of a system whose change in a process executed by the system equal to the difference between the heat and work interactions by the system with its surrounding.


Enthalpy is a thermodynamic quantity which is equal to total heat content in a system.


H = U + PV

According to the first law of the thermodynamics

Q1-2 = P*ΔV + ΔU

Q1-2 = P(V2-V1) + U2 – U1

Rearranging the above equation

Q1-2 = U2 + P2V2 – (U1 + P1V1)

From the equation of enthalpy, it implies

Q1-2 = H2 – H1

Specific heat

Specific heat is the quantity of heat which is required to raise the temperature of unit mass by one degree Celsius.

There are two types of specific heat

  1. Specific heat at constant volume

Cv = ( ∂u/ ∂T)v=constant

  1. Specific heat at constant pressure

Cp = ( ∂h/ ∂T)p=constant

First law applied to the open system (or control volume)

Unlike a closed system mass flows in and out of an open system. Here we have to take conservation of mass into account.

First law of thermodynamics

Conservation of mass

(dm1/dt) – (dm2/dt) = dmcv/dt


dm1/dt = Rate of mass entering to the system

dm2/dt = Rate of mass leaving from the system

dmcv/dt = Rate of mass stored in the system

Conservation of energy

e = u + p*v + g*z + (V2/2)


e = stored energy in the stream of fluid

u = internal energy stored in the stream of fluid

V2/2 = kinetic energy of the stream of fluid

g*z = potential energy stored in the stream of fluid

p*v = pressure work

Mathematical expression of first law for open system

(dm1/dt)*e1 + (∂Q/∂t) – (dm2/dt)*e2 – (∂W/∂t) = dEcv/dt

At steady state

m1 = m2 = m

dEcv/dt = 0

Hence the equation becomes

(dm/dt)*e1 + (∂Q/∂t) – (dm/dt)*e2 – (∂W/∂t) = 0

Above equation is also known as steady flow energy equation.

Also read:

What is a thermodynamic state?

Macroscopic approach to study thermodynamics

What is two property rule?

What is dead state of a system?

What is a point function?

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