Download Chapter 19 The First Law of Thermodynamics

Chapter 19
The First Law of Thermodynamics
Lecture by Dr. Hebin Li
Assignment
Due at 11:59pm on Sunday, December 7
 HW set on Masteringphysics.com
Final exam:
 Time: 2:15pm~4:15pm, Monday, December 8.
 Location: Green Library 100
 On all chapters
PHY 2048, Dr. Hebin Li
Physics II (spring 2015)
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Physics with calculus II in spring 2015.
PHY 2049-(U02 ~ U07)
Lecture: Mondays 8:00 am ~ 9:40 am, AHC3-110
Recitation: Wednesdays 2:00 pm ~ 3:50 pm, locations vary
Textbook: University Physics (13th edition) by Sears &
Zemansky, Volume II (Chapter 21 ~ 36)
 Masteringphysics will be needed for HW.
PHY 2048, Dr. Hebin Li
Goals for Chapter 19
 To represent heat transfer and work done in a thermodynamic
process and to calculate work
 To relate heat transfer, work done, and internal energy change
using the first law of thermodynamics
 To distinguish between adiabatic, isochoric, isobaric, and
isothermal processes
 To understand and use the molar heat capacities at constant
volume and constant pressure
 To analyze adiabatic processes
PHY 2048, Dr. Hebin Li
Thermodynamics systems
 A thermodynamic system is any
collection of objects that may
exchange energy with its
surroundings.
 In a thermodynamic process, changes
occur in the state of the system.
 Careful of signs! Q is positive when
heat flows into a system. W is the
work done by the system, so it is
positive for expansion.
PHY 2048, Dr. Hebin Li
Work done during volume changes
Macroscopic picture:
Microscopic picture:
The work done by the gas as the piston moves 𝑑𝑥
Since
Then
𝑑𝑊 = 𝐹𝑑𝑥 = 𝑝𝐴𝑑𝑥
𝑑𝑉 = 𝐴𝑑𝑥
𝑑𝑊 = 𝑝𝑑𝑉
𝑊=
𝑉2
𝑝𝑑𝑉
𝑉1
If the pressure is constant 𝑊 = 𝑝(𝑉2 − 𝑉1 )
PHY 2048, Dr. Hebin Li
Work on a pV diagram
 The work done equals the area under the curve on a pVdiagram.
 Work is positive for expansion and negative for
compression.
PHY 2048, Dr. Hebin Li
Work depends on the path chosen
 When a thermodynamic system changes from an initial state to
a final state, it passes through a series of intermediate states.
This series of states is the path of the process.
 The work done by the system depends not only on the initial
and final states, but also on the intermediate states, i.e. the path.
PHY 2048, Dr. Hebin Li
Internal Energy
 The internal energy of a system is the
sum of the kinetic energies of all
particles in the system, plus the sum
of all the potential energies of
interaction among these particles.
 It is not practical to calculate or
measure the absolute internal energy.
 The change in internal energy
depends on the heat transfer and
work done
𝑈2 − 𝑈1 = ∆𝑈 = 𝑄 − 𝑊
(First law of thermodynamics)
PHY 2048, Dr. Hebin Li
First Law of Thermodynamics
 First law of thermodynamics: The change in the internal
energy U of a system is equal to the heat added minus the
work done by the system:
U = Q – W.
 The first law of thermodynamics is just a generalization of
the conservation of energy.
 Both Q and W depend on the path chosen between states,
but U is independent of the path.
 If the changes are infinitesimal, we write the first law as
dU = dQ – dW.
PHY 2048, Dr. Hebin Li
Cyclic processes and isolated systems
• In a cyclic process, the system returns to its initial state. The internal
energy change is zero. Then
𝑈2 = 𝑈1 and 𝑄 = 𝑊.
• A isolated system does no work and has no heat flow in or out. That
is
𝑊 = 𝑄 = 0 and ∆𝑈 = 0
Heat transfer and work in the
process aba.
PHY 2048, Dr. Hebin Li
Four kinds of thermodynamics processes
 Adiabatic: No heat is
transferred into or out of
the system, so Q = 0.
 Isochoric: The volume
remains constant, so W = 0.
 Isobaric: The pressure
remains constant, so
W = p(V2 – V1).
 Isothermal: The
temperature remains
constant.
PHY 2048, Dr. Hebin Li
Internal energy of an ideal gas
• The internal energy of an ideal gas depends only on its
temperature, not on its pressure or volume.
• The temperature of an ideal gas does not change during a free
expansion.
PHY 2048, Dr. Hebin Li
Heat capacities of an ideal gas
 CV is the molar heat capacity at constant
volume.
 Cp is the molar heat capacity at constant
pressure.
 The figure at the right shows how we
could measure the two molar heat
capacities.
 To produce the same temperature
change, more heat is required at
constant pressure than at constant
volume. (∆𝑈 is the same)
 𝐶𝑝 = 𝐶𝑉 + 𝑅
PHY 2048, Dr. Hebin Li