# Thermal Physics (Thermodynamics) Zeroth Law of Thermodynamics Zeroth Law of Thermodynamics Thermometers Temperature Scales

## Thermal Physics (Thermodynamics) Zeroth Law of Thermodynamics Zeroth Law of Thermodynamics Thermometers Temperature Scales

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Thermometers Thermometers are devices used to measure the temperature of an object or a system Example is mercury thermometer Make use of physical properties that change with temperature Many physical properties can be used volume of a liquid length of a solid pressure of a gas held at constant volume volume of a gas held at constant pressure electric resistance of a conductor color of a very hot object Temperature Scales Thermometers can be calibrated by placing them in thermal contact with an environment that remains at constant temperature Environment could be mixture of ice in addition to water in thermal equilibrium Also commonly used is water in addition to steam in thermal equilibrium An ear thermometer measures infrared radiation from the eardrum  why is this useful Celsius scale: Temp. of icewater (watersteam) mixture defined as 0°C (100°C) Freezing point vs. boiling point of water Distance between these 2 points divided into 100 equal segments Temperature Scales Fahrenheit scale: Most common scale used in the U.S. Employs a smaller degree than Celsius scale Uses a different zero of temperature than Celsius scale Temperature of the freezing point of water is set at 32°F Temperature of the boiling point of water is set at 212°F 180 divisions between these 2 points Conversion between Celsius (TC) in addition to Fahrenheit (TF) temperatures:

Gas Thermometer Ideally, the readings of a thermometer should not depend on material used Gas thermometer comes close to this ideal Principle is that pressure of a gas at constant volume increases with temperature Gas placed in constant-volume container in addition to pressure is measured (manometer in figure above) Calibrated by measuring pressure at 2 temperatures Temperature readings are nearly independent of the gas Pressure varies with temperature when maintaining a constant volume Gas Thermometer If temperature measurements are per as long as med with gas in flask at different starting pressures at 0°C, the data looks like the graph at right: In each case, regardless of the gas used, the curves extrapolate to the same temperature (absolute zero) at zero pressure Gases liquefy in addition to solidify at very low temperatures, so we cant actually observe this zero-pressure condition The absolute-zero reference point as long as ms basis of Kelvin temperature scale Kelvin Temperature Scale Named as long as British physicist Lord Kelvin (18241907) Units same as those on Celsius scale, but zero point is shifted so that 0 K = 273.15°C: Modern definition (since 1954) of Kelvin scale defined in terms of two points First point is absolute zero Second point is the triple point of water Triple point is the single point where water can exist as solid, liquid, in addition to gas Single temperature in addition to pressure Occurs at 0.01°C in addition to P = 4.58 mm Hg 1 K = 1/273.16 of temperature of triple point of water

Some Kelvin Temperatures in addition to Temperature Scale Comparisons Thermal Expansion The thermal expansion of an object is a consequence of the change in the average separation between its constituent atoms or molecules At ordinary temperatures, molecules vibrate with a small amplitude As temperature increases, the amplitude increases This causes the overall object as a whole to exp in addition to For relatively small changes in temperature, the linear dimensions of object change according to: Coefficient of linear expansion, a, depends on the material (see Table 10.1) These are average coefficients (they can vary with temperature) Thermal Expansion Since the linear dimensions of an object change with temperature, there is also a change in surface area: And a change in volume: g = 2a in addition to b = 3a only if a is the same in all directions Many applications of thermal expansion Pyrex glass Expansion joints in bridges in addition to buildings Rising sea levels due to ocean warming (g = 2a = coefficient of area expansion) (b = 3a = coefficient of volume expansion as long as solids; as long as fluids, see Table 10.1)

Thermal Expansion Thermostats Bimetallic strips in thermostats (2 metals exp in addition to differently) (abrass > asteel) CQ1: The Statue of Liberty is 93 m tall on a summer morning when the temperature is 20°C. If the temperature of the statue rises from 20°C to 30°C, what is the order of magnitude of the statues increase in height Choose the best estimate, treating the statue as though it were solid copper (a = 17 × 106 °C1 ). 0.1 mm 1 mm 1 cm 10 cm 1 m Example Problem 10.23 Solution (details given in class): 2.7 102 N The b in addition to in the figure at right is stainless steel (coefficient of linear expansion a = 17.3 106 °C1 ; Youngs modulus Y = 18 1010 N/m2). It is essentially circular with an initial mean radius of 5.0 mm, a height of 4.0 mm, in addition to a thickness of 0.50 mm. If the b in addition to just fits snugly over the tooth when heated to a temperature of 80°C, what is the tension in the b in addition to when it cools to a temperature of 37°C

Ideal Gas Law Boyles Law At a constant temperature, pressure is inversely proportional to the volume Charles Law At a constant pressure, the temperature is directly proportional to the volume Gay-Lussacs Law Ideal Gas Properties At a constant volume, the pressure is directly proportional to the temperature These 3 laws are summarized by the Ideal Gas Law: R = universal gas constant = 8.31 J/molK = 0.0821 Latm/molK (P = absolute pressure, T = temp. in Kelvin) (note that if n = constant, PV/T = constant) CQ2: If the volume of an ideal gas is doubled while its temperature is quadrupled, what happens to the pressure of the gas It remains the same. It decreases by a factor of 2. It decreases by a factor of 4. It increases by a factor of 2. It increases by a factor of 4. Example Problem 10.32 Solution (details given in class): 884 balloons A tank having a volume of 0.100 m3 contains helium gas at 150 atm. How many balloons can the tank blow up if each filled balloon is a sphere 0.300 m in diameter at an absolute pressure of 1.20 atm

CQ3: Interactive Example Problem 10.35 A weather balloon is designed to exp in addition to to a maximum radius of 20 m at its working altitude, where the air pressure is 0.030 atm in addition to the temperature is 200 K. If the balloon is filled at atmospheric pressure in addition to 300 K, what is its radius at liftoff 4.2 m 7.1 m 18 m 49 m 358 m

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