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Internal Energy Formula

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Last updated date: 19th Jul 2024
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Internal Energy Formula Details

The energy contained within the system is called internal energy. It is symbolized by the English letter ‘U.’ 

Internal energy is also called thermal energy. It is the energy of a substance due to kinetic and potential energies that are associated with the random motion of all the particles that make up the substance. 

In short, internal energy is the energy associated with the random motion or disordered arrangement of particles within the system, which is measured in KJ or Joule. We can calculate the same by the internal energy formula.

The change in internal energy equals the difference in the heat flow in a system and the work done by/on the system (PV) and the change in internal energy formula helps us calculate the same.

On this page, we will understand the internal energy formula, internal energy formula, ideal gas, specific internal energy formula, change in internal energy equation,  and the total internal energy formula.

Internal Energy Equation

As per the first law of thermodynamics, the energy of the universe is invariant. 

Also, the change in the internal energy of a system equals the total of the heat transferred and the work done. 

Besides this, the heat added/flown is equal to the sum of the change in the internal energy of the system and the PV work done.

The Internal Energy of Gas Formula is:

                   Q  =  ΔU  + W  ….(1)

                => ΔU  + PV

Rearranging the equation (1)  to get the formula for change in internal energy:

                  =>   ΔU  =  Q  -  PV …(2)

The equation (2) is also called the total internal  energy formula or the change in internal energy equation)

Total Internal Energy Formula

The total internal energy formula is the sum of transnational energies, rotational energies, binding energies, and chemical energies of all the particles in the system.

The internal energy also involves the potential energy. The potential energy remains stored in the form of chemical bonds, attraction, or repulsion.

Also, kinetic energy, which is because of the motion, translation, rotation, and vibration of particles within the system.

Change in Internal Energy Formula

The Change in Internal Energy Formula is:

                   ΔU  =  Q  +  W


U =  the total change in internal energy within the system

Q =  the heat exchanged between a system and its surroundings (outside the system)

W  =  work done by or on the system

Internal Energy Formula Ideal Gas

Despite the fact that in thermodynamics most frequently the change in internal energy ΔU is considered relevant for ideal gases unquestionably the absolute internal energy U can likewise be determined.

To do this, envision a gas enclosed in a chamber with a constant volume that is chilled off to absolute zero. In this expression, all the particles are stationary, and the gas, accordingly, has no internal energy. However, the heat Q is added at a constant volume to the gas until it arrives at a temperature of ‘T.’

All the heat that was necessary to heat the gas is ultimately present as internal energy U. Accordingly, at a temperature T the gas has the accompanying internal energy U is given as;                

    U  =  mcv.T    …..(3)


cv  =  the specific isochoric heat capacity, i.e., the specific heat capacity at constant volume

For ideal gases, cv can be temperature-dependent. 

Formula for Change in Internal Energy

The change of internal energy formula to derive the internal energy of gas formula is:

Taking equation (3) and writing it as per the formula of change in internal energy, we get:


ΔU  =  mcv.T    …..(4)


                  ΔT = T2 − T1

Here, equation (4) is the required specific internal energy formula.

Analogy Between Internal Energy and Gravitational Potential Energy

The internal energy of ideal gases can obviously measure up in similarity to the gravitational potential energy of an object While the gravitational potential energy addresses the energetic (gravitational) condition of an object at a given height ‘h,’ the internal energy addresses the energetic (kinetic) state of an ideal gas at a given temperature T. 

A given height h can be allocated specific potential energy by means of the mass m of the object. Similarly, specific internal energy can be allocated to a given temperature T by means of the mass m of the gas. 

The specific connection between the potential energy and the tallness ‘h’ is set up by the gravitational acceleration g (reliant upon the position!). On account of internal energy, the connection between internal energy U and temperature T is set up by the specific isochoric heat capacity cv (reliant upon the kind of gas!).


In the nutshell, the formula for change in internal energy relates to the heat flow and the work done by using the first law of thermodynamics.

FAQs on Internal Energy Formula

Q1: A Thermodynamic System Is Isochoric and the Heat Around the System Increases by 50 J. Then,

(i)  What will be the sign for δU for the system?

(ii)  Determine the value of the internal energy of the system in Joules?

(iii)  What will be the value of δU?


  1. Heat is lost to the surroundings, so Q = - ve = - 50 J.

  2. Since the work done, W = PδV = 0, so the work done is zero.

As per the equation, δU = Q + W, we get δU = Q

  1. Also, δU = Q  =  - 50 J

Q2: What is the Relation Between Change in Internal Energy and Temperature?

Ans: To determine the relationship between a change in internal energy (ΔU) and the change in temperature (ΔT), we don’t need to carry out elaborate investigations of the kinetic energy of the individual molecules. 

According to equation ΔU = Q, only the relationship between a heat (Q) added to an isochoric process (It directly corresponds to the change in internal energy ΔU) and the resulting temperature increase ΔT needs to be investigated.

Q3: Does Internal Energy Change in an Isothermal Process?

Ans: We know that internal energy is temperature-dependent, and in the isothermal process, the temperature of the system remains constant. From this, we infer that in an isothermal process, the internal energy of a system must be zero.