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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)