First Law of Thermodynamics

The First Law of Thermodynamics


Energy cannot be created or destroyed; it can only change forms.
All energy must be accounted for.
For example, accumulation of energy, ΔE, occurs when energy entering (shown as heat) is greater than energy leaving (shown as work).
We need to define a

sign convention

for heat and for work.
The

First Law of Thermodynamics

is the key to analyzing all of the systems that we will consider in this chapter and the next chapter.
W
Sign convention arrow indicates that we have decided that work is positive when it is done by the system on the surroundings.
Q
Sign convention arrow indicates that we have decided that heat is positive when it is transferred from the surroundings into the system.
ΔE
This diagram show a system and system boundary containing several devices.
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Ch 4, Lesson C, Page 2 - First Law of Thermodynamics