Heat and Temperature Related to energy § 16.1

Heat Q Energy transferred because of a temperature difference high T low T heat Q Zero heat flow at thermal equilibrium

“0th Law of Thermodynamics” Thermal Equilibrium Insulation A B At equilibrium TA = TB “0th Law of Thermodynamics”

We could measure it before we knew what it was Temperature Scales We could measure it before we knew what it was § 16.2

Fahrenheit Scale 0 °F: salt-ice slush 100 °F: body temperature

Celsius Scale 0 °C: water freezes 100 °C: water boils

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

Poll Question There is a temperature at which the readings in °C and °F are the same. True. False.

Kelvin Scale Absolute temperature

Temperature and Gas Pressure (constant volume) different samples Absolute zero Source: Griffith, Physics of Everyday Phenomena, Fig. 10.6

Absolute Zero Gas p decreases with T Absolute zero  T at which p = 0 Minimum total molecular kinetic energy

Kelvin scale Unit: K K = same size as °C but 0 K = absolute zero (T in K) = 273.15 + (T in °C)

Poll Question How do the temperatures 273.15 °C and 273.15 K compare? 273.15 °C is the same as 273.15 K. 273.15 °C is half 273.15 K. 273.15 °C is twice 273.15 K. 273.15 °C is absolute zero.

Thermal Expansion size does matter § 16.3

Linear Expansion L0 T0 L0 + DL T0 + DT DL = a DT L0 a = coefficient of linear expansion valid for small DT

CPS Question A metal ring is heated. What happens to its outer diameter? It increases. It stays the same. It decreases. Impossible to determine.

CPS Question A metal ring is heated. What happens to its inner diameter? It increases. It stays the same. It decreases. Impossible to determine.

Volume Expansion T0 + DT V0 + DV V0 T0 V0 DV = b DT If expansion is isotropic, and DT is small, b = 3a

Water is Weird Density maximum disappears above ~2.5% salt Source: Young and Freedman, Figure 17.12 Density maximum disappears above ~2.5% salt

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.

Thermal Stress Results from: Uneven composition Uneven temperature

Mechanical Equivalent of Heat energy is energy § 16.4

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

Heat Units Joule Calorie (cal): heat needed to raise 1 gram of water 1 degree C (or K) = 4.184 J. British Thermal Unit (BTU): heat needed to raise 1 pound of water 1 degree F = 1054.35 J

Another Heat Unit U.S. Food Calorie: Cal = 1000 cal Food energy values are often presented in kJ in other countries

Board Work Human food intake is about 2000 C /day. What is this in joules / day? What is this in watts?