Temperature, Heat and Expansion

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Temperature, Heat and Expansion

• Physics 1010
• Dr. Don Franceschetti
• March 28, 2005

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Temperature

• 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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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 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

• Born in 1753 in Woburn MA but went to Britain in
  1776
• Disproved caloric theory by drilling cannons
• Invented many things for the kitchen

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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

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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 thermal expansion
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Homework 4, due April 4, 2005

• RQ15.1 Why does a penny become warmer when it is
  struck by a hammer?
• RQ15.8 On which temperature scale does the
  average kinetic energy of molecules double when
  the temperature doubles.
• E15.15 Why would a watermelon stay cooler for a
  longer time than sandwiches when both are removed
  from a cooler on a hot day?

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Homework 4, contd

• E15.21 Desert sand is very hot in the dat and
  very cool at night. What does this tell you
  about its specific heat.
• E15.38 Why are incandescent bulbs typically made
  of very thin glass?