Thermodynamics: The
First Law and Heat
Transfer
From Warmup
 I would love to work problems in class. I think a synthesis problem
might be useful.
 I try to make the best use of our very limited class time. Class is the
only chance I get to identify and correct misconceptions, so we have to
have conceptual discussions. Some days, however, we will be more
mathematical (today somewhat, and even more on Friday).
 The text was very abstract, it would help to have a concrete
situation to visualize.
 For a lot of these problems you can imagine a gas in a piston.
 The PV diagram is rather confusing to me, I don't quite
understand what the significance of it is.
 It is a useful tool that let’s us think about arbitrary, ideal processes.
 Check the other box I guess? It seems kind of redundant to have
two question/comment boxes.
 Just a little.  Sorry about that.
The First Law of Thermodynamics
 What is the first law of thermodynamics in words?
 Where have you seen this principle before?
Work vs Heat
Work vs Heat
From Warmup
Are Q, W, and DE positive, negative, or zero for:
A. Rapidly pumping up a bicycle tire (the system in
question is the air in the pump)
B. Luke warm water in a pan on a hot stove (the system in
question is the water in the pan)
C. Air quickly leaking out of a balloon (the system in
question is the air that was originally in the balloon)
From Warmup
What is a state variable?
 They characterize what state the gas is in. They are about the
state the gas is in at that time, and don't worry about any
changes occurring to the system at that time or the changes that
were made to get the system to where it is. They fully describe
the behavior of an ideal gas at that time.
 Basically, they are called "state variables" because they refer to a
condition that happens at a moment in time where the conditions
are stable. Transfer variables are variables that arise while the
system is changing. At least that is what I am getting out of the
text...
Note: Does not necessarily refer to solid/liquid/gas states of matter
What are some variables that are NOT state variables? What are
they called?
PV Diagrams
Which of the following is NOT true of
the work done on a gas as it goes from
one point on the PV diagram to
another?
A. It cannot be calculated without
knowing n and T.
B. It depends on the path taken.
C. It equals minus the integral under
the curve.
D. It has units of Joules.
E. It is one of the terms in the First
Law of Thermodynamics.
PV Diagram practice
1. One cubic meter of an ideal gas at 300 K supports a
weight on a piston such that the pressure in the gas is
200,000 Pa (about 2 atm). The gas is heated up. It
expands to 3 cubic meters.
2. Plot the change on a graph of pressure vs volume (a PV
diagram)
3. How much work did the gas do as it expanded?
4. How do you know it did work?
PV Diagram practice
A gas expands, doing work on
a piston. Consider two
different processes shown by
paths 1 and 2 in the PV
diagram. On which path did
the gas do more work?
A. Path 1
B. Path 2
C. Same on both paths
D. No work was done.
PV Diagram practice
A gas expands, doing work on
a piston. Consider two
different processes shown by
paths 1 and 2 in the PV
diagram. On which path did
the gas absorb the most heat
from the outside world?
A. Path 1
B. Path 2
C. No heat was absorbed.
D. I don’t know.
Types of processes
Which path in the figure
corresponds to the following
processes?
 Isobaric
 Isovolumetric (aka isochoric)
 Isothermal
 Adiabatic
More on processes
For an ideal gas, what type of processes occurs when:
 DE = 0? (and therefore Q = -W)
Isothermal
 W = 0? (and therefore DE = Q)
Isochoric
 Q = 0? (and therefore DE = W)
Adiabatic
Heat Transfer
1. Conduction
2. Convection (No equations)
3. Radiation
Conduction
Convection
 Demo
 From warmup:
 The text mentioned energy transfer from convection but
didn't describe it mathematically. Is there any model we can
use to describe this?
 There are models of this, but they are very complicated,
mathematically (partial differential equations describing
compressible fluids responding to temperature….)
Radiation, Black Bodies,
Emissivity