Question

A system delivers 225 J of heat to the surroundings while delivering 645 J of work. Calculate the change in the internal energy, Δ E, of the system

1. -420 J
2. 420 J
3. -870 J
4. 870 J

Answer 1

The energy balance from the First Law of Thermodynamics states that the total internal energy $ΔU$ of a system is equal to the energy input minus energy output.

This is the heat $Q$ minus the work performed $W$. Mathematically,

$ΔU=Q−W$

In this system, heat is lost to the surrounding, which means that $Q$ is negative. Work being delivered to the surroundings means $W$ is positive.

Therefore, if $Q = − 225 J$ and $W=+645 J$ , then the change in internal energy is:

$ΔU=−225 J−(+645 J)= −870 J$

 

The change in internal energy of the system is (3). -870 J.