What is the first law of thermodynamics?

It states that energy is conserved: the change in internal energy (ΔU) of a system equals the heat added (Q) minus the work done by the system (W). Written as ΔU = Q − W.

What does a positive ΔU mean?

A positive ΔU means the system gained internal energy — it absorbed more heat than the work it performed, so its temperature or stored energy increased.

What does a negative ΔU mean?

A negative ΔU means the system lost internal energy — it either released heat, did more work than it absorbed, or both.

When is W positive vs. negative?

By physics convention (used here), W is positive when the system does work on its surroundings (expansion), and negative when surroundings do work on the system (compression).

How do I handle an adiabatic process?

In an adiabatic process no heat is exchanged, so Q = 0. Set Q to zero and enter only the work value. ΔU will equal −W.

Which unit should I use?

Use joules (J) for standard SI problems, kilojoules (kJ) for engineering and larger-scale systems, or calories (cal) for chemistry and nutrition-adjacent problems. The calculator converts between them automatically.

Internal Energy Calculator

Name: Internal Energy Calculator
Availability: InStock
Author: Nham Vu

Enter heat added (Q) and work done by the system (W) to calculate the change in internal energy (ΔU = Q − W).

First Law of Thermodynamics

ΔU = Q − W

Unit

Heat added to system (Q)

Negative if heat is released by the system

Work done by system (W)

Negative if work is done on the system

Change in Internal Energy (ΔU)

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Enter values and click Calculate

Summary

Enter heat added (Q) and work done by the system (W) to calculate the change in internal energy (ΔU = Q − W).

How it works

Enter the heat added to the system (Q). Use a negative value if heat is released.
Enter the work done by the system (W). Use a negative value if work is done on the system.
Select your preferred unit (J, kJ, or cal).
The calculator applies ΔU = Q − W and displays the change in internal energy.
The energy flow diagram updates to show whether the system gained or lost internal energy.

Use cases

Solve first-law thermodynamics problems in physics and chemistry courses.
Analyze heat engines and refrigeration cycles.
Determine energy changes in gas compression and expansion processes.
Verify calculations for isothermal, adiabatic, and isochoric processes.
Support engineering coursework on thermodynamic system analysis.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu