Temperature, Heat and Expansion

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Temperature, Heat and Expansion

• Physics 1010
• Dr. Don Franceschetti
• October 25, 2007

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Temperature

• We have a temperature sense which can be
  quantified.
• Of an object is a measure of the average random
  kinetic energy of atoms or molecules that make up
  that object.
• Almost all substances expand when the temperature
  is raised.
• Three temperature scales in use.

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Thermometer

• Invented by Galileo.
• Based on expansion of (most) materials with
  increasing temperature.
• --or another material property, like electrical
  resistance.

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Celsius (centigrade) Temperature

• Water freezes at 0 C
• Water boils at 100 C
• Problem negative numbers are hard to avoid

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Fahrenheit Temperature

• G. D. Fahrenheit 1686-1736
• Pick your story
• 0 F is temperature of ice/salt bath
• 0 F is coldest winter temperature
• 100 F is hottest summer temp
• 100 F is human body temp
• 100 F is cow body temp (rectal)
• Water freezes at 32 F boils at 212 F
• 180 degrees between freezing and boiling
• T( C)(T( F)-32)5/9

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Kelvin (Absolute) Temperature

• All gases in limit of high temp and low density
  loose 1/273 rd of their volume at 0 C for each
  one C decrease in temperature.
• If substances remained as ideal gases, there
  would be no volume left at -273 C.
• T(K) T(C) 273.15, absolute zero is coldest
  possible temperature.
• Even though gases all condense when cold enough,
  this is still true.

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More precisely

• Temperature is a measure of the average motional
  kinetic energy of molecules in the material.
• Microwave increases rotational kinetic energy of
  water molecules, these bump into others.
• A thermometer measures its own temperature!

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Heat

• Heat is energy in transit from one body to
  another.
• Internal energy is the grand total of all
  energies inside a substance.
• Is measured in joules or calories (people used to
  think of heat as a fluid--caloric)
• I calorie is heat required to raise temperature
  of one gram of water by 1 degree Celsius
• 1 calorie is 4.184 joules
• Food Calorie 1000 calories, is energy released
  on total combustion of food

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Count Rumford, a.k.a. Benjamin Thompson
(1753-1791)

• Born in Woburn MA but went to Britain in 1776.
• Disproved caloric theory by drilling cannons.
• Invented many things for the kitchen.
• 1791 named Count Rumford of the Holy Roman
  Empire.
• Estate funded Rumford lectures at Harvard-still
  going on!

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Specific Heat (thermal inertia)

• The specific heat capacity of any substance is
  the quantity of heat required to change the
  temperature of a unit mass of the substance by 1
  degree.
• Water has a high capacity, important effects on
  climate.
• The higher the specific heat capacity, the slower
  the rise in temperature.

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Thermal Expansion

• Proportional to temperature change and original
  length of material.
• Bimetallic strips will curve when temperature
  changes, basis of thermostat.
• Gaps in roads and sidewalks must be provided.
• Most materials expand at a nearly constant rate
  with temperature. Water does not.

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Water

• Maximum density (M/V) at 4C.
• As pond cools to 4C water falls to bottom.
• Whole pond must cool to 4C before water can
  freeze on top.
• Ice floats.

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Water thermal expansion
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Review

• Why does a penny become warmer when it is struck
  by a hammer?
• On which temperature scale does the average
  kinetic energy of molecules double when the
  temperature doubles.
• Desert sand is very hot during the and very
  cool at night. What does this tell you about its
  specific heat.
• Why are incandescent bulbs typically made of
  very thin glass?

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Heat Transfer

• Physics 1010
• Dr. Don Franceschetti
• March 30, 2004

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Conduction

• Energy transfer from atom to atom. Loose
  electrons also contribute
• Thermal Conductivity
• Best conductors are metals silvergtcoppergtaluminum
  gtiron
• Insulators include wool, wood, straw, paper,
  cork, air, styrofoam

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You can walk on hot coals!

• Wood is a very good insulator, even when hot
• Air is a good thermal insulator. You can stick
  your hand in a hot oven, but dont touch the
  side.
• Snow is also a good insulator.

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Convection

• Fluids transport heat as they move in response to
  density changes.
• Important in weather. Air expands as it rises,
  and it cools as it expands.
• As air expands molecules are more likely to
  collide with molecules receding from them.

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At the seashore

• See page 310, text.

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Radiation

• Energy can pass through empty space in the form
  of electromagnetic radiation (light, radio waves,
  microwaves)
• The frequency of peak energy emission is directly
  proportional to the absolute temperature.

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Emission of Electromagnetic Energy

• Everything emits radiant energy at a rate
  proportional to its area and the fourth power of
  the absolute temperature.
• Everything absorbs energy, too.
• Good emitters are good absorbers

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Cavity Radiation Spectrum

• See page 312.

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Reflection of Radiant Energy

• Absorption and reflection are opposites.
• Best absorber doesnt reflectis black.
• Pupil of eye allows light to enter with very
  little reflection
• Snow melts slowly in sunlight because its white

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Earth and Sun

• Day Suns 6000K emits a lot more than Earths
  300K or so.
• Night Earth has net loss of heat
• An object left out in the open at night radiates
  energy into space and receives much less in
  return.
• Good conductors draw heat from the ground
• Outer space has a temperature of 2.7K!

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Newtons law of cooling

• Rate of cooling proportional to temperature
  difference.
• Wind chill, adjusts for effect of convection.

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Greenhouse Effect

• Much of the Suns radiation is absorbed by the
  Earths Surface. It is at high frequencies to
  which the atmosphere is transparent.
• The Earth radiates more at lower frequencies
  which is absorbed and re-emitted by water vapor
  and carbon dioxide in the atmosphere.
• Earth would be at 18 degrees C were it not for
  the greenhouse effect.

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Solar Power

• Solar constant 1.4 kw/m2
• In U. S. average over all time is 0.18 kw/m2
• Enough for home heating/cooling falls on roof of
  typical house.
• Electricity generation is a challenge

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Thermos bottle (Dewar Flask)

• A double-walled glass container with a vacuum
  between the walls. Glass surfaces are silvered
• No conduction through vacuum
• No convection through vacuum
• Radiation reflected through walls.

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Review

• Why are materials such as wood, fur, feathers and
  even snow good insulators?
• Does a good insulator prevent heat from getting
  through it, or slow its passage?
• How does buoyancy relate to convection?

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Review

• What happens to the volume of air as it rises?
  What happens to its temperature?
• When an air molecule is hit by an approaching
  fast-moving molecule, does its rebound speed
  increase or decrease? How about when it hits a
  receding molecule?