Transcript Simple Kinetic Molecular Model of Matter - HSphysics

Temperature
Name: ________________
Class: _________________
Index: ________________
Learning Objectives
• explain how a physical property which varies with temperature
may be used to define temperature scale and state examples of such
properties.
• explain the need for fixed points and state what is meant by ice
point and steam point.
• discuss the action of a thermocouple thermometer, showing an
understanding of its use for measuring high temperatures and
temperatures which varies very rapidly.
Temperature
The atoms and molecules in
a gas are in constant motion.
Temperature is a measure of
the speed with which they
move. (More exactly it is a
measure of their average
kinetic energy.) The higher
the temperature, the faster
the molecules move.
Designing Thermometers
Properties used for designing thermometers
are:
– it is easy to read,
– it is inexpensive,
– it is sensitive
– it has a big range of temperatures
Physical Properties of Thermometers
1) Liquid-in-glass Thermometers: The most common
type of thermometer is the liquid-in-glass thermometer.
It consists of a narrow, sealed glass tube with a bulb
filled with mercury, alcohol (usually dyed red, to make
it easily visible), or some other liquid at its lower end.
As the temperature rises, the liquid expands and rises in
the tube. A drop in temperature causes the liquid to
contract, so that its level in the tube falls.
2) Bimetal Thermometers: A bimetal thermometer
contains a strip consisting of two fused strips of
different metallic substances (metals or alloys). The
strip is usually in the form of a spiral or rod fixed at one
end. When the temperature changes, one of the metallic
substances expands or contracts more than the other,
causing the strip to wind or unwind, or to bend. A
pointer attached to the free end of the strip indicates the
temperature. Bimetal strips are widely used in dial
thermometers and in thermostats. Another type of
thermometer using dissimilar metals is the
thermocouple.
3) Resistance Thermometers: determine
temperature by measuring the change in electrical
resistance of a metal wire or of a thermistor, a
resistor composed of a semiconducting material.
The wire or thermistor is usually enclosed in a
slender rod, or probe. The temperature is indicated
on a meter or as a digital display.
4) Infrared Thermometers: measure the infrared
(heat) radiation given off by an object to determine
its temperature. The pyroelectric thermometer
contains a type of crystal that produces an
electrical signal whose strength is proportional to
the infrared radiation to which it is exposed. The
most familiar use of such thermometers is in ear
thermometers that measure infrared radiation from
the tympanic membrane (ear drum) to determine
body temperature. Other types of infrared
thermometers include bolometers and pyrometers.
5) Gas Thermometers: The most common type, the
constant-volume gas thermometer, determines
temperature by measuring the pressure needed to keep
the volume of a gas constant as the temperature varies.
The pressure of a gas confined to a constant volume
varies with temperature in a known manner; if the
pressure of the gas is measured, its temperature can be
easily calculated.
6) Liquid-crystal Thermometers: contain liquid
crystals that change colours over a certain range of
temperatures. Various mixtures of liquid crystals are
used, each mixture in a separate container and each
becoming light in color at a specific temperature. In
digital types, each container is covered with a stencil of
a number that corresponds to the temperature at which
the mixture in the container becomes light in color.
Fever thermometers designed to be placed on the
forehead and most indoor digital thermometers are of
this type.
Temperature Scales
• Celsius scale also called the centigrade scale.
• About 200 year ago, Mr Anders Celsius selected two
fixed points, namely ice point and steam point.
• Ice point is the temperature of pure melting ice
at standard atmospheric pressure called 00C.
• Steam point is the temperature of steam at
standard atmospheric pressure called 1000C.
Calibrating an unmarked thermometer
• Put thermometer into pure melting ice.
After few minutes, marks the position of mercury level; thus
00 C is obtained.
• Put thermometer into steam, hence 1000C is obtained.
• Divides the length obtained into 100 divisions.
The Fahrenheit Scale:
• Ice point is 320 F, and steam point is 2120F.
• This scale is of historical importance.
The Kelvin Scale:
• Kelvin scale is also called as Absolute Scale.
• Kelvin (K) is the SI unit of temperature.
• Ice point = 273 K and steam point = 373 K
Temperature Scales
Conversion
TK = TC + 273.15
where TK = Degrees in Kelvin and TC = Degrees in Celcius
Convert Kelvin Scale to Celsius Scale:
1. 255 K
2. 137 K
Convert Celsius Scale to Kelvin Scale:
1. 430 C
2. - 380 C
Standard Thermomet
Mercury Thermometers
• The mercury thermometer is a common type of
thermometer in everyday use.
• Narrow bore of capillary tube makes the
thermometer more sensitive.
• Range : -100C to 1100C (or 00C to 1000C).
• Round or oval glass stem save as magnifying
lens. (Why ?)
• the bulb is made by thin glass. (Why ?)
Example
The length of the mercury thread is found to be 20 mm
and 100 mm at 00C and 1000C respectively on an
ungraduated thermometer. What is the temperature
corresponding to the length of mercury thread of 40 mm ?
Solution:
Let x is the required temperature.
0
100 C
Ɵ = (lƟ – l0) / (l100 – l0) x 100°C
= (40 – 20) / (100 – 20) x 100°C
= 25 °C
The above formula can also be used
for other units like resistance.
x
100 mm
40 mm
00C 20 mm
Clinical Thermomete
Clinical Thermometer
• Typical clinical thermometer is
liquid-in-glass thermometer.
• Range : 350C to 420C.
• It has a constriction for
preventing liquid fall back to
the bulb immediately after
taking the reading.
• When taking reading, the bulb
is gently held under the
patient’s tongue.
A range of Clinical Thermometers
Mercury and Alcoh
Mercury Thermometer
Most of the liquid-in-glass widely use mercury, because
1.
2.
3.
4.
5.
more uniform expansion,
does not stick to glass.
visible meniscus,
react quickly when to temperature changes,
boiling point: 3570C, freezing point = -390C.
But its weak points are:
1. expensive,
2. poisonous liquid
3. High freezing point
Alcohol Thermometers
This is a very cheap liquid-in-glass thermometer.
It can measure very low temperature.
Alcohol Thermometer
Alcohol is used for making thermometers because:
1. Low freezing point ---- -1150C,
2. Safe liquid,
3. Cheap.
Its disadvantages:
1.
2.
3.
4.
Non-uniform expansions,
Sticks to glass,
Slow reaction to temperature changes,
Low boiling point --- 780C.
Thermocouple
Thermocouple Thermometers
A thermocouple is a junction between two different metals that
produces a voltage related to a temperature difference.
Thermocouples are a widely used type of temperature sensor for
measurement and control. They are inexpensive and interchangeable,
are supplied fitted with standard connectors, and can measure a wide
range of temperatures. The main limitation is accuracy: system errors
of less than one degree Celsius (C) can be difficult to achieve.
Thermocouple Thermometers
Thermocouple
thermometers with two
different metals are
placed in contact, a
voltage develops
between them.
The voltage varies with
temperature.
One of the junctions is
kept in melting ice at
00C while the other one
is used as temperature
probe.
Thermocouple is used for
measuring very high
temperature and
temperatures which varies
greatly. The voltmeter can be
calibrated directly in 0C.
Others
Maximum and Minimum Thermometer
The maximum thermometer records the highest
temperature and minimum thermometer the lowest
for the day (or a period).
Maximum
Thermometer
Minimum
Thermometer
Maximum and Minimum Thermometer
This thermometer
records the highest
and the lowest
temperature for
the day. (How ?)
Note:
The metal index can
be moved up or down
by a bar magnet.
References
http://whs.wsd.wednet.edu/Faculty/Busse/MathHomePage/busseclasses/apphysics/studyg
uides/chapter11_2008/Images/11_08Figure.jpg
http://science.howstuffworks.com/thermometer-info.htm/printable
http://img.diytrade.com/cdimg/95109/7146156/0/1224341069/Glass_thermometers.jpg
http://image.made-in-china.com/2f0j00VvCEebZnSHkj/Bimetal-Thermometer.jpg
http://image.made-in-china.com/2f0j00PbQapgtMITCw/Infrared-Thermometer.jpg
http://www.pc-oscilloscopes.com/images/pt100_sensor.jpg
http://www.temperature.com.au/Portals/0/File/image/Thermometers/TCWECM60.jpg
http://www.connecticutvalleybiological.com/images/pa142.jpg
http://www.magnet.fsu.edu/education/tutorials/magnetacademy/superconductivity101/ima
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