L 17 - Thermodynamics [2]

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L 17 - Thermodynamics 2

• Science that studies the relationships between
  heat and work
• it applies to all living and non-living things
• it predicts the direction of natural processes
• why ice melts (rather than getting colder!)
• why gases expand to fill entire volumes

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Thermal Expansion

• Expansion occurs in all dimensions
• Expansion must be taken into account when
  designing roads and bridges in climates that vary
  significantly from winter to summer all
  materials expand, steel, concrete, asphalt . . .

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winter/summer expansion gaps
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Areas and volumes expand too!
cold
hot
cold
hot
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Does the whole get bigger or smaller when heated?
cold
hot
Both the inner and outer diameters increase when
the ring is heated
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Bi-Metal strips

• thermal expansion of metals is put to good use in
  a bi-metallic strip.
• this is two strips of different metals bonded
  together

metal A
metal B
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Heating a Bi-metal strip

• when heat is applied to the bi-metallic strip,
  both metals expand, but by different amounts!
• result

this can be used as a switch in your coffee
maker!
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Bi-Metal strip thermal switch
when a preset temperature is reached, the
bi-metallic strip opens up
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Thermocouples- measure Temp.

• The temperature sensor you stick into your turkey
  is a thermocouple
• it is composed of 2 wires composed of different
  metals welded together - when it gets hot it
  makes electrical current flow

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Thermocouples protect you!

• a thermocouple is used in gas heaters and dryers
  to protect against explosions
• a thermocouple is placed in the pilot light
• as long as the pilot light is on, the
  thermocouple is hot and current flows
• a circuit senses the current and allows the main
  gas valve to open
• if the pilot light is out, the circuit prevents
  the main gas valve from opening

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Internal energy, Temperature and Heat

• in a gas the molecules
• have energy because
• they are moving.
• the sum of all the energies of all the molecules
  is the systems internal energy
• the temperature of the system is a measure of how
  much internal energy it has, Temperature
  Internal Energy

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Heat ? what is it ? ? ?

• Heat is the energy that flows from one system to
  another because of their temperature difference.
• Heat stops flowing when the two systems come to
  the same temperature.
• Heat was considered to be an actual fluid
  (caloric), but it is NOT a fluid- it is energy!

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Heat Flow and the laws of thermodynamics

• System A has a certain amount of internal energy
  and so does system B
• If energy is transferred and the internal energy
  of B decreases by some amount then internal
  energy of A must increase by the same amount.
• ?the first law
• If the temperature of A is less than the
  temperature of B then heat flows from B to A (hot
  to cold). ? the second law

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1st and 2nd Laws of Thermodynamics

• the 1st law says that energy is conserved ?
  whatever energy system A gains must be accounted
  for by the energy that B lost. (assuming that
  the systems are isolated so that they do not
  interact with any other systems)
• The 2nd law specifies the direction of heat flow?
  hot to cold (ice melts!)

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Reverse heat flow- why ice melts
Notice that the 1st law does not say that ice
must melt. The first law does not prohibit that
the ice from getting colder and the water warmer!
The 2nd law specifies the direction of heat flow
in natural (spontaneous) processes.
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But what about refrigerators?

• A refrigerator is a device that moves heat from
  something cold (inside) to something hot (the
  room).

needs to be plugged in to work! energy in
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Refrigerators and the 2nd Law

• Does this violate the 2nd law? NO, because it is
  not a spontaneous process
• Refrigerators require energy input (work)
  (electricity) to operate.
• Heat does not flow spontaneously from cold to
  hot, but it can be made to flow backwards if
  there is an input of WORK.
• It uses electrical energy to pump heat from cold
  to hot.

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Heat flow

• HEAT ? the energy that flows from one system to
  another because of temperature differences.
• But how does it flow? Three ways
• convection
• conduction
• radiation

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Convection

• heat is carried from place to place by the bulk
  movement of either liquids or gases
• does not apply to solids
• when water is boiled, hot liquid rises and mixes
  with cooler liquid, thus the heat is transferred
• Hot air rises
• want heat into lower level of house (winter)
• cooled air into upper levels (summer)

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Conduction

• heat is transferred directly through a material,
  with no bulk movement of stuff
• only energy moves

iron is a particularly poor conductor of heat
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heat conduction
HOT
COLD
Heat Flow
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Thermal Conductivity
Material Thermalconductivity
Copper 400
Silver 420
Steel (stain.) 14
wood 0.15
glass 0.8
wool 0.04
Goose down 0.025
styrofoam 0.01

• The effectiveness of a material in conducting
  heat is characterized by a parameter called the
  thermal conductivity
• there are good thermal conductors
• (metals) and poor ones (insulators)

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Grandmas silver spoons