\(\def \u#1{\,\mathrm{#1}}\) \(\def \abs#1{\left|#1\right|}\) \(\def \ast{*}\) \(\def \deg{^{\circ}}\) \(\def \ten#1{\times 10^{#1}}\) \(\def \redcancel#1{{\color{red}\cancel{#1}}}\) \(\def \BLUE#1{{\color{blue} #1}}\) \(\def \RED#1{{\color{red} #1}}\) \(\def \PURPLE#1{{\color{purple} #1}}\) \(\def \th#1,#2{#1,\!#2}\) \(\def \lshift#1#2{\underset{\Leftarrow\atop{#2}}#1}}\) \(\def \rshift#1#2{\underset{\Rightarrow\atop{#2}}#1}}\) \(\def \dotspot{{\color{lightgray}{\circ}}}\)
Chapter 7: Thermodynamics
16. (Problem Solving)

Energy Flows

When thinking about energy flows, it is often useful to divide them into the flows into the system and the flows out of the system. The change to the system's total energy is then the difference between these two:
In this example, we have

There is more energy flowing out, so the total energy of the system drops by $100-70 = 30{\rm J}$. (We might also say that $\Delta E=70-100=-30{\rm J}$.)