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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

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)