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Topic B.2
Reference: pages 559 - 569
What IS Thermodynamics?

 A branch of physics that studies the effects of work, heat,
and energy on a system.
 The study of the conditions under which thermal energy
can be transferred through performing mechanical work
 Thermodynamics only focuses on large-scale observations
(in other words, we’re no longer focusing on the behavior
of individual molecules in a system, but, rather, the entire
sample)
Review…

 What is WORK?
 A force acting through a distance to transfer energy
 What are the Macroscopic properties that we use when
describing a thermal system of gas?
 Macroscopic Properties: Pressure, Volume and
Temperature—all used to determine the amount of work
that is/can be done by or to a sample of gas.
 HOW could we do work on an enclosed sample of gas?
Systems

 The complete set of objects being considered in a
particular scenario/problem
 Open System
 Mass is free to enter and/or leave the system
 Closed System
 Mass is not free to enter and/or leave the system. The
quantity of the gas will remain constant
 Isolated System
 No energy in any form can enter or leave the system
 No mass can enter or leave the system
State of a System

 The State of a system is known when particular quantifiable
characteristics of the system are known, such as the following:




Pressure
Volume
Temperature
Internal Energy
 These are each a State Function: a characteristic of the system.
 Note: If two gases, originally in different (thermodynamic)
states, are brought to the same state, the gases will have the
same internal energy—no matter how they got there.
Non-state functions

 Thermal Energy and Work
 Doing work, or adding or removing thermal energy
 Related to a CHANGE in the state, not in the state
itself
 A gas does not “contain” thermal energy—it can
transfer it when it changes state
 A gas does not “contain” work—it has work done to
it when compressed, or work done by it when
expanded
Thermodynamic Processes

 Thermodynamic Processes are any processes that
will result in the change of the state of a system
 Heating a gas
 Compressing the gas (doing work TO the gas)
 Expansion of the gas (work done BY the gas)
Internal Energy:
Reminders/Review
 Internal Energy:

 The sum of the total kinetic energy of the molecules in a
sample of a gas and the potential energy associated with the
intermolecular forces with that gas.
 Ideal Gases: assume that the intermolecular forces are nonexistent, so potential energy = 0
 Therefore the internal energy is solely related to the kinetic
energy (which is random…each molecule is likely different)
 Average Kinetic Energy:
𝐸𝐾 = 32𝑘𝑇
Internal Energy

 Internal energy of a fixed quantity of a gas (constant
number of moles) will only depend on the
temperature.
 It does NOT depend on volume or pressure
 Free-Expansion: when a gas is allowed to expand in
a way that is not constricted—both the volume an
pressure change in such a way that the temperature
will remain constant (in an ideal gas)
 Thus—the internal energy is constant for a given
temperature of ideal gas.
Work Done by/to a Gas

 Imagine a Piston—cross
sectional area A
 Change the position of
the piston by applying
a force to expand or
compress the gas
 Volume changes
W = P·ΔV
PV diagrams

PV Diagrams

 Total work done by the gas as it expands (or to the
gas as it’s compressed) = area under the curve
 Closed loop? Total (net) work done to/by the
system = enclosed area
Thermodynamic Processes

 Review: What do we call the following processes:
 When the pressure remains constant, but volume and
temperature change
 When the volume remains constant, but the pressure
and temperature change
 When the temperature remains constant, but the
pressure and volume change
Isobaric Processes

 Those processes in which the pressure of the system
remains constant while the volume and temperature
change
 Results in a horizontal line on a PV diagram (Isobar)
Isochoric Processes

 Those processes in which the volume remains
constant while the pressure and temperature change
 Results in a vertical line on the PV diagram (an
Isochore)
 No work is done during an isochoric process
Isothermal Process

 Those processes in which the temperature remains
constant (and, as a result, the internal energy)
 The pressure and volume will each change
Adiabatic Process

 Pressure, Volume, and Temperature all change in a
way so that no thermal energy (heat) is transferred
between the system and the surroundings
 Happens in a very well-insulated container
 Happens very quickly
 Energy does not have time to leave the system, so the
result is that the temperature will change (because
internal energy changes)
Adiabatic Processes

 Adiabatic Expansion:
 Pressure decreases
 Volume increases
 Temperature decreases
 Adiabatic Compression:
 Pressure increases
 Volume decreases
 Temperature increases
Energy Transfers

 Energy is conserved
 If work is done to a system, energy is transferred into
the system in some form
 If work is done by a system, energy is transferred
away from the system in some form
 Forms of energy:
 Internal Energy (related to the temperature)
 Thermal energy
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Law of Thermodynamics

𝑸 = ∆𝑼 + 𝑾
Q = Thermal energy / J
 When (+) , energy is transferred into the system
 Surroundings are at a higher temperature than the system
 When (-) , energy is transferred into the surroundings
 System is at a higher temperature than the surroundings
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Law of Thermodynamics

𝑸 = ∆𝑼 + 𝑾
DU = Change in Internal Energy
 When + , represents the increase in internal energy
 When - , represents the decrease in internal energy
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Law of Thermodynamics

𝑸 = ∆𝑼 + 𝑾
W  Work done
 When + , represents the work done BY the system
as the gas expands
 When - , represents the work done TO the system
 System causes the gas to compress.
Practice Problem

 An ideal gas is held in a cylinder by a moveable
piston and energy is supplied to the gas such that the
gas expands at a constant pressure of 1.50 x 105 Pa.
The initial volume of the cylinder is 0.040 m3 and its
final volume is 0.12 m3. The total energy supplied to
the gas during the process is 7.5 x 103 J.
 State and explain the type of change that the gas
undergoes
 Determine the work done by the gas
 Calculate the change in internal energy of the gas.