Lecture26 - PHYSICS 220 Lecture 26 Thermodynamics I...

Lecture 26 Purdue University, Physics 220 1
Lecture 26
Thermodynamics I
Textbook Sections: 16.1-16.5
PHYSICS 220

Lecture 26 Purdue University, Physics 220 2
First Law of Thermodynamics
Δ U = Q - W
Heat flow
into system
Increase in internal
energy of system
Work done by system
Equivalent ways of writing 1st Law:
Q = Δ U + W
The change in internal energy of a system ( Δ U) is
equal to the heat flow into the system (Q) minus the
work done by the system (W)
Energy Conservation

Lecture 26 Purdue University, Physics 220 3
Signs
You are heating some soup in a pan on the stove.
To keep it from burning, you also stir the soup.
Apply the 1 st law of thermodynamics to the soup.
What is the sign of
A) Q
B) W
C) Δ U
Positive, heat flows into soup
Negative, stirring does work on soup
Positive, soup gets warmer

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Lecture 26 Purdue University, Physics 220 4
A system is acted on by its surroundings in such a
way that it receives 50 J of heat while
simultaneously doing 20 J of work. What is its net
change in internal energy?
1. 70 J
2. 30 J
3. zero
4. -30 J

Lecture 26 Purdue University, Physics 220 5
W = P Δ V
W > 0 if Δ V > 0
expanding system does positive work
W < 0 if Δ V < 0
contracting system does negative work
W = 0 if Δ V = 0
system with constant volume does no work
Work Done by a System
M
M
Δ y
W = F d cos θ = P A d =
P A Δ y = P Δ V

Lecture 26