Physics 108 lec 11: 1st Law of Thermodynamics

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Physics 108 lec 11 1st Law of Thermodynamics
Waves

• 1st Law of Thermodynamics
• The 1st Law of Thermodynamics states that the
  total amount of energy in the universe is
  constant, or that energy is conserved.
• We have already seen a special case of the 1st
  Law of Thermodynamics in the conservation of
  mechanical energy (K U Ko Uo).
• Now we will look at another form of conservation
  of energy, for thermal systems.
•  
• 1st Law of Thermodynamics (for thermal systems)

?Uinternal change in internal (thermal) energy
of system due to temperature W P?V work done
by system Q heat added to system
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Example (1st Law of Thermo) 250,000J of heat
are added to 1kg of water for producing steam in
a steam engine at a pressure of 101350Pa. What
is the amount of volume expansion during this
process if the temperature remains constant?
Given                    Path                  
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Thermal Efficiency

• The ratio of work output (W) to heat input (Q)
  describes the efficiency of a heat based engine.
• If all heat becomes useful work
• Q W and ?Uinternal 0, no wasted heat.
• Then, W/Q1, which is a 100 efficient engine.
  NOT!!
• Thermal efficiency

Note The efficiencies quoted on furnaces, hot
water heaters, etc. are neither Thermal nor
Carnot. They are approximately the ratio of
actual efficiency (thermal) to maximum possible
efficiency (Carnot). That way they look more
effective.
Carnot Efficiency

• The maximum possible (Carnot) efficiency of an
  engine depends on its combustion (in) and exhaust
  (out) temperatures.
• engine that burns hotter ? more heat energy
  available
• engine that has cool exhaust ? little wasted
  energy
• Carnot efficiency

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Example (Thermal/Carnot Efficiency)A new
furnace burns natural gas at a temperature of
1050K and exhausts its flu gases at 430K. Also,
the furnace produces useful work energy of 240J
for each 450J of heat energy contained in the
natural gas fuel. What are the thermal and
Carnot efficiencies of the furnace? What would
the company advertise for an efficiency ?
Given                    Path                  
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Waves

•  

• Waves consist of disturbances that propagate
  through a substance or region of space.
• Waves typically consist of either a single
  disturbance (wave pulse) or a series of
  disturbances one after another (periodic wave).
• Two types of waves
•             Mechanical    - travel through
  matter                                    -
  observed as a disturbance in matter
•                        Examples
•  
•             Electromagnetic     - travel through
  matter or empty space                            
                   - observed as a disturbance in
  electric and magnetic fields
•                        Examples
•  
• Two ways waves propagate
•             Longitudinal Waves - disturbances
  move parallel to direction of wave
•                     Examples
•             Transverse Waves - disturbances move
  perpendicular to direction of wave
•                     Examples

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Wave Properties

• wavelength (      ) - length between two similar
  points on a wave
• amplitude (      ) - maximum movement from
  disturbance by wave
• period (      ) - time for one full wavelength to
  pass by a fixed point
• frequency (      ) - number of wavelengths
  passing by per unit of time

Wave Equations
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Example (Waves) Find wavelength for X-ray with
f1018Hz in tissue and in air.
Given                    Path                  
Want           Conversions/Equations
Example (Waves) Find speed of ocean wave w/
period 15s and wavelength 25m.
Given                    Path                  
Want           Conversions/Equations