Internal Energy
The total kinetic and potential energy of all the particles within a system.
Formula / equation
ΔU = q + w
Unit
J
In depth
Internal energy includes molecular translation, rotation, vibration, electronic energy and intermolecular potential energy. Its absolute value cannot be measured, but changes can, and for a reaction carried out at constant volume the change equals the heat exchanged.
Examples in the real world
Measuring ΔU directly in a rigid bomb calorimeter, where no expansion work is possible.