15 Temperature Heat Expansion Temperature Heat Internal Energy

15 Temperature, Heat, Expansion • Temperature & Heat • Internal Energy & Specific Heat • Homework: • RQ: 1, 2, 7, 8, 9, 10, 15, 21, 24. 1

Temperature • T ~ average kinetic energy • T increase = increase in speed of molecules, e. g. hammered metal increases in temperature. • liquids, gases, and solids usually expand when T increased 2

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) 3

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What happens to the hole size? 5

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 air and refrigerator • // 6

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 = D(Internal Energy) 7

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. 1 kg water at 0°C is mixed with 1 kg water at 20°C: • (1 kg)(T - 0) = (1 kg)(20 – T) T = 20 – T 2 T = 20 T = 10°C 8

more mixing • Ex. 2 kg water at 40°C is mixed with 1 kg water at 20°C: • (1 kg)(T - 20) = (2 kg)(40 – T) T – 20 = 80 – 2 T 3 T = 100/3 = 33. 3°C 9

Calories and Joules • 1 Calorie = 1000 calories • 1 calorie = 4. 18 joule • Calorie is the common food unit. • Btu’s are also used, e. g. gas range 10

Specific Heat • the specific heat of a substance is the quantity of heat required to raise the temperature of 1 kg of a substance 1°C. // 11

specific heat • c = Q/m. DT [J/(kg·K)] • heat needed per kg to raise temperature by 1 degree C or K. • slope warming water = DT/Q = 1/(mc) 12

example c’s • in J/(kg-C): • • aluminum 920 copper 390 ice 2100 water 4186 13

Example: • A student wants to check “c” for an unknown substance. She adds 230 J of heat to 0. 50 kg of the substance. The temperature rises 4. 0 K. 14

summary • T ~ avg. KE • most substances undergo thermal expansion (note the water exception) • heat is an exchange of thermal energy • specific heat = heat needed to raise temp. of 1 kg by 1 C (substance dependent) 15

Thermodynamics • flow and effect of thermal energy on matter (solids, liquids, gases) • temperature and internal energy • heat engines 16