# Introduction to The 1st Law of Thermodynamics

We begin by drawing a boundary line to define the

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
Q
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### Ch 4, Lesson C, Page 1 - Introduction to The 1st Law of Thermodynamics

• 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.

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