Heat

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Heat

• Temperature

2
Temperature

• Related to average kinetic energy of molecules
• Measured on a scale based on some standard
• Read with thermometer containing material which
  expands or contracts with temperature change

3
Temperature Scales

• Fahrenheit used in USA, based on freezing of sea
  water
• Celsius (centigrade) used in most of world,
  based on freezing and boiling pts of water
• Kelvin SI unit of temperature for all scientific
  work, based on zero point volume of ideal gas

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Temperature Scales
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Heat

• Energy that transfers from one object to another
  because of temperature difference
• Thermal contact -- heat flows due to objects
  touching
• Temperature difference controls direction of heat
  flow, not total energy

6
Thermal Equilibrium

• When no heat flows between objects in thermal
  contact
• Thermometer reaches thermal equilibrium to
  measure temperature

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Internal Energy

• Total energy of molecules inside substance,
  potential and kinetic
• Objects dont contain heat, but internal energy
• Transfer of heat changes internal energy

8
Heat Quantities

• Since heat is energy, it is measured in joules,
  the SI unit of work and energy
• Quantity of heat transferred is measured by its
  effect -- the temp change
• Temp change depends on amount and type of
  substance as well as quantity of heat

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Heat Quantities

• Common unit for heat measurements is the calorie
  amount of heat needed to raise one gram of water
  one degree Celsius
• Kilocalorie (1000 calories) is food Calorie,
  spelled with capital C
• Both are units of energy 1 cal 4.187 J

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Specific Heat Capacity

• Quantity of heat needed to raise temp of unit
  mass of substance 1 oC
• Different substances have different capacities
  for storing heat
• Same amount of heat will cause different temp
  changes in different stuff
• Acts like thermal inertia

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Heat Quantities

• Old idea of heat said that substances contain
  invisible fluid called caloric, released when
  heat given off
• English unit of heat is British thermal unit
  (Btu), the heat required to raise the temp of 1
  lb. of water 1 oF

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Specific Heat of Water

• Very high water can store much energy without
  much temp rise
• Also means water cools slowly
• Controls climate of coastal areas, moderating
  temperature changes
• Central continental areas have more drastic temp
  changes

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Thermal Expansion

• Almost all matter expands when heated, contracts
  when cooled solid, liquid, and gas
• Must be accounted for in construction of
  buildings, bridges, roads, etc.
• Different materials expand at different rates

14
Thermal Expansion

• Bimetallic strip two metal strips joined
  together -- bends when heated due to different
  expansion rates -- used in thermostats,
  thermometers
• Liquids usually expand more than solids, gases
  more than liquids

15
Expansion of Water

• Water has abnormal expansion between 0 oC and 4
  oC -- expands when cooled, contracts when heated
• At all other temps, water expands and contracts
  normally
• Water has smallest volume and greatest density at
  4 oC

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Expansion of Water

• Solid form (ice) less dense than liquid, so water
  expands when it freezes
• As water cools below 4 oC, open structure of ice
  starts to form, causing expansion
• Very important in nature -- keeps lakes from
  freezing from the bottom upward and killing all
  aquatic life.