First law of thermodynamics

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First law of thermodynamics
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Thermal systems
Classical mechanics

• Thermal systems
• - deals with many individual objects
• - conceptually different from mechanical
  systems
• - dont know the position, velocity, and energy
  of
• any molecules or atoms or objects
• - cant perform any calculation on them
• Sacrifice microscopic knowledge of the system,
• using macroscopic parameters instead
• - volume (V)
• - temperature (T)
• - pressure (P)
• - number of particles (N)
• - energy (E), etc.
• Macroscopic systems with many individual
  objects
• - processes are often irreversible
• - arrow of time does exist
• - energy conservation is not enough to
  describe

Thermal system
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Thermodynamic systems
Isolated systems can exchange neither energy nor
matter with the environment.
reservoir
Heat
Work
Open systems can exchange both matter and energy
with the environment.
Closed systems exchange energy but not matter
with the environment.
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Idea gas model
Lattice model for solid state materials

• The ideal gas model
• all the particles are identical
• the particles number N is huge
• the particles can be treated as point masses
• the particles do not interact with each other
• the particles obey Newtons laws of motion, but
  their motion is random
• collisions between the particles are elastic

The ideal gas equation of state
kB 1.38 ? 10-23 J/K
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Internal energy

• The internal energy of a system of N particles,
• U, is all the energy of the system that is
• associated with its microscopic components
• when view from a reference frame at rest
• with respect to the object.
• Internal energy includes
• - kinetic energy of translation, rotation, and
  
• vibration of particles
• - potential energy within the particles
• - potential energy between particles
• Internal energy is a state function it depends
  
• only on the values of macroparameters (the
• state of a system)

For a non-ideal gas
For an ideal gas (no interactions)
Monatomic
Diatomic
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Heat
Heat and work are both defined to describe energy
transfer across a system boundary.

• Heat (Q) the transfer of energy across the
  boundary of a system due to a temperature
  difference between the system and its
  surroundings.
• - Q gt 0 temperature increases heating process
  
• - Q lt 0 temperature decreases cooling process
• - (C heat capacity)
• Heat transfer mechanisms
• - conduction exchange of kinetic energy
  between
• microscopic particles (molecules, atoms, and
• electrons) through collisions
• - convection energy transfer by the movement
  of a
• heated substance such as air
• - radiation energy transfer in the form of
  electromagnetic
• waves
• Work (W) any other kind of energy transfer
  across boundary

heat
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Quasi-static processes

• Quasi-static (quasi-equilibrium) processes
• Sufficiently slow processes, and any
  intermediate state can be considered as at
  thermal equilibrium. The macro parameters are
  well-defined for all intermediate states.
• The state of a system that participates in a
  quasi-equilibrium process can be described with
  the same number of macro parameters as for a
  system in equilibrium.
• Examples of quasi-static processes
• - isothermal T constant
• - isovolumetric V constant
• - isobaric P constant
• - adiabatic Q 0

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Work done during volume changes
Quasi-static process at each infinitesimal
movement
Work done by the gas as its volume changes from
Vi to Vf
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Work done during volume changes (cont.)

• dV gt 0 the work done on the gas is negative
  
• dV lt 0 the work done on the gas is positive
  

In thermodynamics, positive work represents a
transfer of energy out of the system, and
negative work represents a transfer of energy
into the system.
P-V diagram
The work done by a gas in the expansion is the
area under the curve connecting the initial and
final states
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Work and heat are not state functions
c
a
b

• a. isobaric
• b. isovolumetric

a. isovolumetric b. isobaric
isothermal

• Because the work done by a system depends on the
  initial and final states and
• on the path followed by the systems between the
  states, it is not a state function.
• Energy transfer by heat also depends on the
  initial, final, and intermediate states
• of the system, it is not a state function
  either.

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• When heat enters a system, will it increase the
  systems internal energy?
• When work is done on a system, will it increase
  the systems internal energy?

It depends on the path!
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The first law of thermodynamics

• Two ways to exchange energy between a system
• and its surroundings (reservoir)
• heat and work
• Such exchanges only modify the internal energy
  of
• the system
• The first law of thermodynamics conservation of
  energy

Q gt 0 energy enters the system Q lt 0 energy
leaves the system W gt 0 work done on the system
is negative energy leaves the system
W lt 0 work done on the system is positive
energy enters the system

• For infinitesimal processes

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Several examples
Isolated systems
Adiabatic processes
Cyclic processes
Insulating wall
initial state final state
Expansion U decreases Compression U increases
The internal energy of an isolated
systems remains constant
Energy exchange between heat and work
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Idea gas isovolumetric process
Isovolumetric process V constant
P
2
1
(CV heat capacity at constant volume)
V1,2
V
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Idea gas isobaric process
P
Isobaric process P constant
2
1
(CP heat capacity at constant pressure)
V1
V2
V
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Idea gas isothermal process
1
P
Isothermal process T constant
2
V1
V2
V
During an isothermal expansion process, heat
enters the system and all of the heat is used by
the system to do work on the environment.
During an isothermal compression process,
energy enters the system by the work done on
the system, but all of the energy leaves the
system at the same time as the heat is removed.
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Idea gas adiabatic process
Adiabatic process Q 0
P
2
1
V2
V1
V
Idea gas
Adiabatic process
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Idea gas adiabatic process
P
2
1
, and divided by
let
V2
V1
V
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Idea gas adiabatic process
P
2
1
V2
V1
V
For monatomic gas,
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Idea gas adiabatic process
P
2
1
V2
V1
V
or
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Idea gas adiabatic process
P
2
1
V2
V1
V
During an adiabatic expansion process, the
reduction of the internal energy is used by the
system to do work on the environment. During
an adiabatic compression process, the environment
does work on the system and increases the
internal energy.
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Summary

• Internal energy, heat, and work
• - internal energy is the energy of the system
  a state function
• - heat and work are two ways to exchange energy
  between the system
• and the environment. They are not state
  functions and depend on the path
• The first law of thermodynamics connects the
  internal energy with heat and
• work

Quasi-static process
Character
isovolumetric
V constant
isobaric
P constant
isothermal
T constant
adiabatic