Ch15 Thermodynamics

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Ch15 Thermodynamics

• Zeroth Law of Thermodynamics
• If two systems are in thermal equilibrium with a
  third system, then they are in thermal
  equilibrium with each other.

First Law of Thermodynamics The Internal Energy
of a closed system will be equal to the energy
added to the system by heating minus the work
done by the system its surrounding
Second Law of Thermodynamics Heat flows out from
hot objects to cold heat does NOT flow from cold
to hot
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Internal Energy

• The sum of all the energy of all the molecules in
  an object (thermal energy)
• Internal Energy of an Ideal Gas

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Ch15 Thermodynamics

• Heat Transfer of energy due to ?T
• Work Transfer of energy NOT due to ?T
• Q Heat
• W Work
• W done on the system is negative (Giancoli)
• W done by the system is positive (Giancoli)
• ?U Change in energy

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Ch15 Thermodynamics

• First Law of Thermodynamics

Law of Conservation of Energy
AP equation sheet
Heat added is Heat lost is
- Work on system is Work by system is
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Ch15 Thermodynamics
The distinction between work done on the gas and
work done by the gas is one that is often made on
the AP Exam
The area under the P-V curve will always be the
work done by the gas during the process
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First Law of Thermodynamics

• 2500J of heat is added to a system, and 1800J of
  work is done on the system. What is the changed
  in internal energy of the system?

(Q) 2500J of heat will increase the Internal
Energy (W) 1800J of work done ON the system will
Is the work positive or negative? Why?
Did the temperature increase or decrease?
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Ch15 Thermodynamics

• Isothermal process Constant temperature
• The system is in contact with a heat reservoir
• Change of phase

• The work done by the gas in an isothermal process
  equals the heat added to the gas

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Isothermal process

• Constant temperature, i.e. PV is constant

Which Isothermal process is at a higher
Temperature? Which Isothermal process does more
work?
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Adiabatic

• Adiabatic Process No heat in or out of the
  system
• Well insulated (like a thermos)
• The process happens very quickly (firing of a car
  cylinder

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Work

• Given the following two processes Isothermal and
  Adiabatic.
• Both processes start at 10Pa and end with a
  volume of 10m3
• During which process is more work done?
• Estimate the work done in each process.

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Ch15 Thermodynamics

• Isovolumetric (Isochoric) No change in volume
• Inside a ridged container

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Ch15 Thermodynamics

• Isobaric No change in pressure
• Movable piston

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Internal Energy ?U

• 1 mole of an ideal gas is brought from point a to
  point c by 3 different process paths. Which
  path has the highest change in internal energy?
• 1)
• 2)
• 3)
• 4) All the same
• 5) Unknown

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Work (W)

• 1 mole of an ideal gas is brought from point a to
  point c by 3 different process paths. During
  which path did the gas do the most work?
• 1)
• 2)
• 3)
• 4) All the same
• 5) Unknown

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Heat (Q)

• 1 mole of an ideal gas is brought from point a to
  point c by 3 different process paths. During
  which path was the most heat added?
• 1)
• 2)
• 3)
• 4) All the same
• 5) Unknown

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• One mole of monatomic ideal gas is enclosed under
  a frictionless piston. A series of processes
  occur, and eventually the state of the gas
  returns to its initial state with a P-V diagram
  as shown below. Answer the following in terms of
  P0, V0, and R.

• Find the temperature at each vertex.
• Find the change in internal energy for each
  process.
• Find the work by the gas done for each process.
•                                                   
      

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• An ideal gas is slowly compress at constant
  pressure (2.0 ATM) from 10.0L to 2.0L
• Heat is then added to the gas holding the volume
  constant and the pressure and temperate are
  allowed to rise until the temperature reaches its
  original value.
• Calculate the total work done by the gas
• Calculate the total heat flow into the gas

Pressure Pa
Isothermal line
2
10
Volume m3
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• In an engine 0.25 moles of an ideal gas in the
  cylinder expands rapidly and adiabatically
  against the piston. In this process, the
  temperature of the gas drops from 1150K to 400K.
• What type of process is this?
• b) How much work does the gas do?

Is the work done by the gas positive or negative?
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Efficiency

• Efficiency (e) the ratio of work W done by the
  system to the input heat QH

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• An automobile engine has an efficiency of 20 and
  produces an average of 23,000J of mechanical work
  per second.
• How much input heat is required?
• How much heat is discharged as wasted per
  second?

b)
a)
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• Is the cars efficiency higher or lower as the
  car warms up?

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