Introduction to The 1st Law of Thermodynamics

We begin by drawing a boundary line to define the

system

we will analyze.
Our

system

can consist of many devices.
We are interested in the total energy within the

system

and energy that crosses the

system boundary

line: heat and work.
We need to define a

sign convention

for heat and for work.
The

First Law of Thermodynamics

is the relationship between heat, work and the total energy of the

system

.
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.
This diagram show a system and system boundary containing several devices.
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Ch 4, Lesson C, Page 1 - Introduction to The 1st Law of Thermodynamics