Measurement of temperature
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Thermal Physics

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2.2

Thermal Properties

  

B

Measurement of temperature

  
Core-1 Appreciate how a physical property which varies with temperature may be used for the measurements of temperature and state examples of such properties.  
Core-2 Recognize the need for identify a fixed point  
Core-3 Describe the structure and action of liquid-in-glass thermometers  
Sup-1 Demonstrate understanding of sensitivity, range, and linearity   
Sup-2 Apply a given property to the measurement of temperature   
Sup-3 Describe the structure and action of a thermocouple and show an understanding of its use for measuring high temperature and those which vary rapidly  

2.2.b Measurement of temperature

Temperatures

  1. What is temperature?

  2. Define temperature?
    Temperature is a measure of the average kinetic energy possessed by each molecule of the substance.

  3. What are the physical properties that are used to measure temperatures?

    1. Expansion of materials

      1. Liquids: mercury, alcohol

      2. Solids: Thermocouple

    2. Electromotive force: Thermocouple

    3. Change in electrical resistance

Thermometers

  1. List the types of temperature meters?

    1. Liquid-based thermometers

    2. Solid-based thermometers

  2. List liquid-in-glass thermometers?

    1. Mercury thermometer

    2. Alcohol thermometer

    3. Clinical  thermometer

  3. Describe the structure of liquid-in-glass thermometers?
    1- Glass tube  2-Bulb 3-Liquid 4-Thread 5-Scale


  4. Explain how liquid-in-glass thermometers operate?
    The bulb of thermometer contains liquid (e.g. mercury, alcohol) which expands when temperature rises and pushes a thread of liquid along the scale where temperature can be read

  5. List the basic properties of thermometers?
    sensitivity, range, and uniformity

  6. What is meant by the following terms: sensitivity, range, and uniformity?
    sensitivity:     is the ability of a thermometer to detect small changes in temperature.
    range:     is the range of temperatures which can be measured by the thermometer
    linearity (uniformity):     the expansion of the liquid should be linear (uniform, the same) for all different temperatures measured.

  7. How do we increase the sensitivity of a thermometer?
    1- increase in the volume of the bulb
    2- decreases the diameter of the tube (bore)

  8. Explain how a decrease in tube diameter of a liquid-in-glass thermometer will increase sensitivity?

  9. Explain how an increase in volume of a liquid-in-glass thermometer will increase sensitivity?

  10. How do we increase the range of a thermometer?
    1- decrease in the volume of the bulb
    2- increases the diameter of the tube (bore)

  11. Explain how a decrease in volume bulb of a liquid-in-glass thermometer will increase range?

  12. Explain how an increase in tube diameter of a liquid-in-glass thermometer will increase range?

  13.  

Celsius Scale

  1. List the three basic scales for temperature measurements?
    Celsius Scale
    Kelvin Scale

  2. What are the two fixed points in Celsius scale?
    1- The lower fixed point
    2- The upper fixed point

  3. What is meant be "lower fixed point" and "upper fixed point"?

    1. The lower fixed point is the temperature of pure melting ice.  This is called the 0°C.

    2. The upper fixed point is the temperature of steam above pure boiling water under standard atmospheric pressure.  It is call the 100°C. 


Kelvin Scale

  1. Describe the Kelvin scale?
    The Kelvin scale is the same as the Celsius scale except it starts from absolute zero.  The a one degree Kelvin is equal to one-degree Celsius.

  2. What is meant by "absolute-zero" temperature?
    Absolute-zero is the lowest temperature in the universe and at this temperature the molecules of a substance have the minimum possible energy.  It occurs at -273°C.

  3. What is the relation between Kelvin temperature and Celsius temperature?
    TK=TC+273 where TK is temperature in Kelvin and TC is temperature in Celsius.

Mercury Thermometers

  1. List four advantages of mercury as a liquid?

    1. Does not stick to the sides of tube

    2. thread easy to use

    3. conducts heat well

    4. responds quickly to changes in temperature

  2. List three disadvantages of mercury?

    1. Freezes at -39°C.  Not suitable for low temperatures

    2. Poisonous

    3. Expensive

Alcohol Thermometer

  1. Describe alcohol thermometer.

  2. List two advantages of Alcohol?

    1. Freezes at -115°C.  Suitable for low temperature measurements.

    2. Has expansion more than mercury.

  3. List three disadvantages of alcohol

    1. Has to be colored to be seen easily

    2. Clings (sticks) to the sides of tube

    3. Thread has tendency to break

Clinical Thermometers

  1. Why a clinical thermometer has a narrow constriction just above its bulb 
    to stop the mercury thread from falling back into the bulb when the thermometer cools down

  2. List the properties of a clinical-thermometer.

    1. Is used to measure the temperature of human body

    2. Its range is 35°C to 43°C

    3. Has a narrow constriction just above its bulb 

  3.  

Thermocouple Thermometers

  1. Describe the structure  of a thermocouple meter?
    It is made by joining two different metals assembled (e.g. copper& iron) into two junctions.  One junction is placed in pure melting ice (Cold junction) and the other junction is placed in the hot object (called hot junction).

     

  2. Describe the operation of a thermocouple meter?
    When there a difference in temperature between the hot and cold junctions an electric current is generated.  This current is used to deflect a galvanometer which is calibrated to temperature degrees.

  3. List the advantages of a thermocouple thermometer?

    1. Easy to read by using a pointer or a digital display.

    2. Scale can be placed a way from the temperature detector  

    3. Can be used automatic electrical control circuits.

    4. Wide range of temperature -200°C to 1600°C

    5. It has has a small thermal capacity and is used to measure rapidly changing temperatures.

Practical

  1. List three precautions a person must take when measuring temperature?

    1. Mercury thread should be kept along the scale the parallax errors

    2. Line of sight should be perpendicular to scale.

    3. Reading should be at the top of mercury thread.

  2.  

 

Test your Skills

1- Why Mercury or alcohol thermometers are not suitable to measure temperatures deep inside a furnace?

For more information write: abumsamh@emirates.net.ae
Created SEP 1, 2000
Copyright © 2000
Designed by: R. K. Abu-Msameh
Last modified: December 06, 2000