Enter the heat transferred to/from a system (Q) and the work done on that system (W) to calculate the change in internal energy (ΔU). This calculator can also determine the work or heat if you provide the other variables.

Change in Internal Energy Calculator

General (First Law)
Constant Pressure (PΔV work)

Enter any 2 values to calculate the missing variable

Change in Internal Energy Formula

The following formula is used to calculate the change in internal energy.

\Delta U = Q + W
  • Where ΔU is the change in internal energy (J)
  • Q is the heat transferred into the system (J) (positive into the system, negative out)
  • W is the work done on the system (J) (positive when done on the system, negative when done by the system)

To calculate the change in internal energy, add the heat transferred to the system and the work done on the system (using a consistent sign convention).

Change in Internal Energy Definition

A change in internal energy is defined as the net heat transferred to the system plus the net work done on the system (for the sign convention used above).

Change in Internal Energy Example

How to calculate the change in internal energy?

  1. First, determine the heat transfer.

    For this example we will say the heat transferred into the system was 20J.

  2. Next, determine the work done on the system.

    For this example, we will say the system performed 10J of work on something outside the system. With the sign convention used here (work done on the system is positive), that means W = -10J.

  3. Finally, calculate the change in internal energy.

    Using the formula we find the change in internal energy to be ΔU = Q + W = 20J + (-10J) = 10J.

FAQ

What is internal energy?

Internal energy (U) is the energy associated with a system’s microscopic degrees of freedom (molecular motion and intermolecular interactions). Heat is not “contained” in a system; heat (Q) is energy transferred due to a temperature difference. In typical thermodynamics, macroscopic kinetic and gravitational potential energy are treated separately from internal energy.

Can you calculate internal energy?

In general, it’s usually most appropriate to calculate the change in internal energy of a system, since absolute internal energy is not directly measurable.