Chapter 19An alcohol thermometer and a mercury thermometer may agree only at the calibration points...
Transcript of Chapter 19An alcohol thermometer and a mercury thermometer may agree only at the calibration points...
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Chapter 19 Temperature 19.1 Temperature and the Zeroth Law of Thermodynamics 19.2 Thermometers and the Celsius Temperature Scale 19.3 The Constant-Volume Gas Thermometer and the Absolute Temperature Scale
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Temperature l We associate the concept of temperature
with how hot or cold an object feels l Our senses provide us with a qualitative
indication of temperature l Our senses are unreliable for this purpose l We need a reliable and reproducible method
for measuring the relative hotness or coldness of objects l We need a technical definition of temperature
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Thermal Contact
l Two objects are in thermal contact with each other if energy can be exchanged between them l The exchanges we will focus on will be in the form
of heat or electromagnetic radiation l The energy is exchanged due to a temperature
difference
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Thermal Equilibrium
l Thermal equilibrium is a situation in which two objects would not exchange energy by heat or electromagnetic radiation if they were placed in thermal contact l The thermal contact does not have to also be
physical contact
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Zeroth Law of Thermodynamics
l If objects A and B are separately in thermal equilibrium with a third object C, then A and B are in thermal equilibrium with each other l Let object C be the thermometer l Since they are in thermal equilibrium with each
other, there is no energy exchanged among them
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Zeroth Law of Thermodynamics, Example
l Object C (thermometer) is placed in contact with A until they achieve thermal equilibrium l The reading on C is recorded
l Object C is then placed in contact with object B until they achieve thermal equilibrium l The reading on C is recorded again
l If the two readings are the same, A and B are also in thermal equilibrium
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Temperature – Definition
l Temperature can be thought of as the property that determines whether an object is in thermal equilibrium with other objects
l Two objects in thermal equilibrium with each other are at the same temperature l If two objects have different temperatures, they
are not in thermal equilibrium with each other
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Thermometers
l A thermometer is a device that is used to measure the temperature of a system
l Thermometers are based on the principle that some physical property of a system changes as the system’s temperature changes
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Thermometers, cont l These properties include:
l The volume of a liquid l The dimensions of a solid l The pressure of a gas at a constant volume l The volume of a gas at a constant pressure l The electric resistance of a conductor l The color of an object
l A temperature scale can be established on the basis of any of these physical properties
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Thermometer, Liquid in Glass l A common type of
thermometer is a liquid-in-glass
l The material in the capillary tube expands as it is heated
l The liquid is usually mercury or alcohol
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Calibrating a Thermometer l A thermometer can be calibrated by placing it in
contact with some natural systems that remain at constant temperature
l Common systems involve water l A mixture of ice and water at atmospheric pressure
l Called the ice point of water l A mixture of water and steam in equilibrium
l Called the steam point of water l Once these points are established, the length
between them can be divided into a number of segments
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Celsius Scale
l The ice point of water is defined to be 0o C l The steam point of water is defined to be
100o C l The length of the column between these two
points is divided into 100 increments, called degrees
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Problems with Liquid-in-Glass Thermometers l An alcohol thermometer and a mercury thermometer
may agree only at the calibration points l The discrepancies between thermometers are
especially large when the temperatures being measured are far from the calibration points
l The thermometers also have a limited range of values that can be measured l Mercury cannot be used under –39o C l Alcohol cannot be used above 85o C
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Absolute Temperature Scale
l Absolute zero is used as the basis of the absolute temperature scale
l The size of the degree on the absolute scale is the same as the size of the degree on the Celsius scale
l To convert: l TC = T – 273.15
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Absolute Temperature Scale, 2
l The absolute temperature scale is now based on two new fixed points l Adopted by in 1954 by the International
Committee on Weights and Measures l One point is absolute zero l The other point is the triple point of water
l This is the combination of temperature and pressure where ice, water, and steam can all coexist
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Absolute Temperature Scale, 3
l The triple point of water occurs at 0.01o C and 4.58 mm of mercury
l This temperature was set to be 273.16 on the absolute temperature scale l This made the old absolute scale agree closely
with the new one l The units of the absolute scale are kelvins
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Absolute Temperature Scale, 4
l The absolute scale is also called the Kelvin scale l Named for William Thomson, Lord Kelvin
l The triple point temperature is 273.16 K l No degree symbol is used with kelvins
l The kelvin is defined as 1/273.16 of the difference between absolute zero and the temperature of the triple point of water
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Fahrenheit Scale
l A common scale in everyday use in the US l Named for Daniel Fahrenheit l Temperature of the ice point is 32oF l Temperature of the steam point is 212oF l There are 180 divisions (degrees) between
the two reference points
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Comparison of Scales
l Celsius and Kelvin have the same size degrees, but different starting points l TC = T – 273.15
l Celsius and Fahrenheit have different sized degrees and different starting points
F C9 325
T T F= +
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Comparison of Scales, cont
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HomeWork l 1. Liquid nitrogen has a boiling point of -195.81°C at atmospheric pressure.
Express this temperature (a) in degrees Fahrenheit and (b) in kelvins. l 2. Convert the following to equivalent temperatures on the Celsius and
Kelvin scales: (a) the normal human body temperature, 98.6°F; (b) the air temperature on a cold day, -5.00°F.3.
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1.
2.