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.

Properties used for designing thermometers are:

– it is easy to read,

– it is inexpensive,

– it is sensitive

– it has a big range of temperatures

Designing Thermometers

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 ScalesTemperature 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.

• 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.

Calibrating an unmarked thermometer

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 ScalesTemperature Scales

Convert Kelvin Scale to Celsius Scale:1. 255 K

2. 137 K

Convert Celsius Scale to Kelvin Scale:

1. 430 C

2. - 380 C

Conversion

TK = TC + 273.15

where TK = Degrees in Kelvin and TC = Degrees in Celcius

Standard ThermometerStandard Thermometer

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 ?)

ExampleExampleThe 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 ?

1000C

00C

x

100 mm

40 mm

20 mm

Solution:Let x is the required temperature.

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.

Clinical Thermometer 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.

Clinical ThermometerClinical Thermometer

A range of Clinical Thermometers

Mercury and AlcoholMercury and Alcohol

Most of the liquid-in-glass widely use mercury, because

1. more uniform expansion,2. does not stick to glass.3. visible meniscus,4. react quickly when to temperature changes,5. boiling point: 3570C, freezing point = -390C.

But its weak points are:

1. expensive,2. poisonous liquid3. High freezing point

Mercury Thermometer

Alcohol Thermometers

This is a very cheap liquid-in-glass thermometer.It can measure very low temperature.

Alcohol is used for making thermometers because:

1. Low freezing point ---- -1150C,2. Safe liquid,3. Cheap.

Its disadvantages:Its disadvantages:

1. Non-uniform expansions,2. Sticks to glass,3. Slow reaction to temperature changes,4. Low boiling point --- 780C.

Alcohol Thermometer

ThermocoupleThermocouple

Thermocouple ThermometersThermocouple 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 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.

Thermocouple Thermometers

OthersOthers

MaximumThermometer

MinimumThermometer

The maximum thermometer records the highesttemperature and minimum thermometer the lowestfor the day (or a period).

Maximum and Minimum ThermometerMaximum and Minimum Thermometer

This thermometerrecords the highestand the lowesttemperature forthe day. (How ?)

Note:The metal index can be moved up or down by a bar magnet.

Maximum and Minimum ThermometerMaximum and Minimum Thermometer

References

http://whs.wsd.wednet.edu/Faculty/Busse/MathHomePage/busseclasses/apphysics/studyguides/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/images/superconductivity-temperature.jpg

http://4.bp.blogspot.com/_hHKNFGOospc/SVsy8de5yEI/AAAAAAAAAFU/nWBL8_fetjE/s320/thermometer13.jpg

http://mooigoed.files.wordpress.com/2010/04/s_celsius.jpg

http://www.allproducts.com/manufacture100/thermometrics/product1.jpg