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Introduction to The 1st Law of Thermodynamics

We begin by drawing a boundary line to define the

we will analyze.

Our

can consist of many devices.

We are interested in the total energy within the

and energy that crosses the

line: heat and work.

We need to define a

sign convention

for heat and for work.

The

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

Lesson 4C: page 1 of 12

The 1st Law is a relationship between the total energy within a system and the amount of energy which enters and leaves the system.
The only two forms of energy that can cross the boundary of a closed system are heat and work.
Never forget the sign conventions that we have chosen.
Q is positive when is transferred from the surroundings into the system.
W is positive when the system does work on the surroundings.
Now, let’s get specific about the 1st Law.