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Heat and Temperature Related to energy § 16.1
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Heat Q Energy transferred because of a temperature difference high T
low T heat Q Zero heat flow at thermal equilibrium
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“0th Law of Thermodynamics”
Thermal Equilibrium Insulation A B At equilibrium TA = TB “0th Law of Thermodynamics”
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We could measure it before we knew what it was
Temperature Scales We could measure it before we knew what it was § 16.2
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Fahrenheit Scale 0 °F: salt-ice slush 100 °F: body temperature
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Celsius Scale 0 °C: water freezes 100 °C: water boils
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Converting Linear relation, 2 points known (0 °C, 32 °F), (100 °C, 212 °F) °F°C: x = 9 °F 5 °C (y – 32 °F) °C°F: y = 9 °F 5 °C x + 32 °F
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Poll Question There is a temperature at which the readings in °C and °F are the same. True. False.
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Kelvin Scale Absolute temperature
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Temperature and Gas Pressure
(constant volume) different samples Absolute zero Source: Griffith, Physics of Everyday Phenomena, Fig. 10.6
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Absolute Zero Gas p decreases with T Absolute zero T at which p = 0
Minimum total molecular kinetic energy
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Kelvin scale Unit: K K = same size as °C but 0 K = absolute zero
(T in K) = (T in °C)
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Poll Question How do the temperatures 273.15 °C and 273.15 K compare?
°C is the same as K. °C is half K. °C is twice K. °C is absolute zero.
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Thermal Expansion size does matter § 16.3
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Linear Expansion L0 T0 L0 + DL T0 + DT DL = a DT L0
a = coefficient of linear expansion valid for small DT
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CPS Question A metal ring is heated. What happens to its outer diameter? It increases. It stays the same. It decreases. Impossible to determine.
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CPS Question A metal ring is heated. What happens to its inner diameter? It increases. It stays the same. It decreases. Impossible to determine.
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Volume Expansion T0 + DT V0 + DV V0 T0 V0 DV = b DT
If expansion is isotropic, and DT is small, b = 3a
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Water is Weird Density maximum disappears above ~2.5% salt
Source: Young and Freedman, Figure 17.12 Density maximum disappears above ~2.5% salt
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Poll Question Between 0 °C and 4 °C, warmer fresh water
Rises above colder water. Sinks beneath colder water. Has neutral buoyancy. It is impossible to determine.
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Thermal Stress Results from: Uneven composition Uneven temperature
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Mechanical Equivalent of Heat
energy is energy § 16.4
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Mechanical Equivalent of Heat
James Joule’s life-long obsession Identical effects of adding heat to a system and doing work on it. Source: Griffith, The Physics of Everyday Phenomena
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Heat Units Joule Calorie (cal): heat needed to raise 1 gram of water 1 degree C (or K) = J. British Thermal Unit (BTU): heat needed to raise 1 pound of water 1 degree F = J
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Another Heat Unit U.S. Food Calorie: Cal = 1000 cal
Food energy values are often presented in kJ in other countries
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Board Work Human food intake is about 2000 C /day.
What is this in joules / day? What is this in watts?
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