# Thermodynamics U vs E

1. Jan 23, 2016

### thinkcentre12

My question is what is the difference between ΔE and ΔU. Because I see both equations 2 and 3 interchangeably . But then I see equation 1. So there has to be a difference between them since they are using both symbols in one equation. I thought U was internal energy while E was total energy. So I am confused to what the difference is between 2 and 3 why can I use either equation?

Thanks I tried to explain my thinking

ΔE = ΔU + ΔKE + Δ PE (1)

ΔE = Q - W (2)

ΔU = Q - W (3)

2. Jan 23, 2016

### Andrew Mason

E is the symbol for the total energy of the system and U is the symbol for internal energy of the system.

E takes into account the macroscopic KE and PE of the system as well as the internal energy, U. The macroscopic kinetic energy and potential energy of the system are due to the motion and position of the centre of mass of the system. U can be thought of as the microscopic KE and PE - the kinetic and potential energies of all the constituent molecules in the system measured relative to the centre of mass of the system.

Your equation (2) is correct only if the macroscopic KE and PE do not change.

AM