15 Temperature, Heat, Expansion
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15 Temperature, Heat, Expansion
• Temperature & Heat
• Internal Energy & Specific Heat
• Homework:
• RQ: 1, 2, 7, 8, 10, 12, 15, 21, 24.
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Temperature
• T ~
• Ex: hammered metal
• Expansion with T increase
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Superheating
• Mythbusters
• Instant coffee
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Water Expansion• Water expands from 4°C to 100°C (as does
most materials)• However, water contracts when warmed from
0°C to 4°C. (transient ice melting)
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Heat
• ‘heat’ is a transfer of thermal energy due to temperature difference
• Examples:
• ice in warm liquid: heat flows from liquid to the ice
• warm liquid is put in a refrigerator: heat flows from drink to the cold air in refrigerator
• //
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Internal Energy
• the total of all molecular energies, kinetic plus potential, that are internal to a substance. It is ~ (mass) x (temperature).
• Heat ≠ Internal Energy,
• however, for a thermal-only process,
• Heat = (Internal Energy)
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mixing: ‘hot’ and ‘cold’ = ‘warm’
• for liquids & solids: heat ~ (mass)x(temperature change)
• when ‘hot’ and ‘cold’ are mixed, heat lost by ‘hot’ = heat gained by ‘cold’
• Ex. 1kg water at 0°C is mixed with 1kg water at 20°C:
• (1kg)(T - 0) = (1kg)(20 – T)
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Calories and Joules
• 1 Calorie = _______ calories
• 1 calorie = ________ joule
• Calorie is the common food unit.
• Btu’s are also used, e.g. gas range
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Specific Heat
• quantity of heat required to raise the temperature of 1kg by 1°C. //
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• c = Q/(mT) [J/(kg·K)]• c = heat/(massx temperature change)• Water: 4186 J/kgC• heat needed per kg to raise temperature by
1 degree C or K.
specific heat
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summary
• T ~ KE/molecule
• most substances undergo thermal expansion (note the water exception)
• heat is an exchange of thermal energy
• superheating
• specific heat = heat needed to raise temp. of 1kg by 1C (substance dependent)