Measurement of Temperature

Measurement of Temperature

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Which Instrument is Used to Measure the Temperature?

 

  1. Temperature is a measure of the degree of hotness of an object.
  2. A hot object has a higher temperature than a cold object.
  3. When two objects are in thermal contact, heat energy is transferred from the object at the higher temperature to the object at the lower temperature.
    Measurement of Temperature
  4. Table shows a comparison between temperature and heat.
    TemperatureHeat
    Degree of hotness of an objectEnergy transferred from a hot object to a cold object.
    A base quantityA derived quantity
    SI unit is kelvin (K)SI unit is joule (J)
    A scalar quantityA scalar quantity
  5. Temperature is measured by a thermometer. The most commonly used thermometer is the liquid- in-glass thermometer.
  6. A liquid-in-glass thermometer consists of a capillary tube with a thin-walled bulb at one end. It usually contains a fixed mass of mercury or alcohol.
  7. When the temperature increases, the volume of the mercury increases.
  8. This causes the mercury to expand into the capillary tube. Therefore, the length of the mercury column increases as temperature increases.
  9. The change in the length of the mercury column is proportional to the change in the temperature.
  10. The laboratory liquid-in-glass thermometers use the Celsius scale.
  11. The Celsius scale is defined by two fixed points of temperature – the ice point and the steam point. Table gives the definition and values of the two fixed points.
    Fixed pointDefinitionValue
    Lower fixed point: Ice pointThe temperature of pure melting ice0°C
    Upper fixed point: Steam pointThe temperature of steam from water that is boiling under standard atmospheric pressure100°C
  12. The liquid-in-glass thermometer uses the concept of thermal equilibrium to measure temperature.
    (a) When a thermometer is placed into an object, energy is transferred between the thermometer and the object until thermal equilibrium occurs.
    (b) The temperature of the thermometer is now equal to the temperature of the object.
    (c) Therefore the reading of the thermometer is the temperature of the object.

Calibration of Thermometers

A new thermometer without a scale can be calibrated by placing the bulb in ice and then in steam, as shown in Figure.
Measurement of Temperature 1Step 1

  • Place the bulb into melting ice.
  • Wait until the mercury column stops moving.
  • There is now thermal equilibrium at 0°C.
  • Mark 0°C on the stem.

Step 2

  • Place the bulb into steam.
  • Wait until the mercury column stops moving.
  • There is now thermal equilibrium at 100°C
  • Mark 100°C on the stem.

Step 3

  • Divide the region between the 0°C mark and the 100°C mark into 100 equal divisions.
  • Label the scale of the thermometer from 0, 10, 20, …, to 100.
  • The thermometer is calibrated and ready to measure temperature.
  1. If the scale of the thermometer is not marked, the temperature of an object can be determined from the formula or calibration graph, as shown in Figure.
    Measurement of Temperature 2
  2. Lower temperatures can be measured by liquid- in-glass thermometers filled with alcohol, which freezes at -115°C and boils at 78°C. The alcohol used needs to be coloured.
  3. For extra low temperature up to -200°C, liquid pentane is used instead of alcohol.
  4. Water is not suitable for use in thermometers because
    (a) it freezes at 0°C,
    (b) it does not expand regularly.
  5. The sensitivity of a thermometer can be increased by:
    (a) Decreasing the diameter of the capillary tube in the stem. This will cause the mercury column to move through a longer distance in the tube as it expands.
    (b) Using a bulb with thinner walls. Heat energy can be transferred at a faster rate into the mercury. The thermometer will respond faster to changes in temperature.

Calibration of Thermometers Example Problems with Solutions

Example 1. Figure shows a thermometer being used to measure the temperature of some sand.
Measurement of Temperature 3What is the temperature of the sand?
Solution:
Measurement of Temperature 4

Example 2. Figure shows the calibration graph of a mercury thermometer.
Measurement of Temperature 5
What is the temperature, y ?
Solution:
Measurement of Temperature 6

What is Temperature and Can Temperature be Measured Directly

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What is Temperature and Can Temperature be Measured Directly

Temperature

The degree of ‘hotness’ or ‘coldness’ of a body or a place is called temperature.
One of the methods of measuring the degree of hotness and coldness is by the sense of touch. But our sense of touch can sometimes be misleading. Let us look at the following experiment to understand why!

Activity
Aim: To show that the terms ‘hot’ and ‘cold’ as we feel them are only relative terms
Materials needed: Three glasses, hot water (as hot as you can bear to dip your finger in), ice-cold water, and water at room temperature
Method:
1. Pour hot water, water at room temperature, and the ice-cold water into three separate glasses.
2. Keep the three glasses side by side on a table with the glass containing water at room temperature in the middle.
What is Temperature and Can Temperature be Measured Directly 1
3. Dip the index finger of one hand in the ice-cold water and that of the other hand in the hot water. Hold the fingers in the water for a minute.
4. Then dip both the index fingers into the water at room temperature.
Observation: You will find that to one finger, the water feels hot and to the other it feels cold. Conclusion: This is because the finger that was in ice-cold water has become cold, and when it was placed in the water at room temperature, it felt that the water was warm, relative to the earlier experience of ice-cold water. Similarly, the finger that was first in hot water will feel the water at room temperature to be relatively cold.

So, you see the terms ‘hot’ and ‘cold’ are only relative terms. There must always be a reference body with respect to which we can say a given body is hot or cold.
Therefore, it is important that we use an instrument, rather than our own judgement, to measure temperature.

Measurement of Temperature
We measure temperature with an instrument called the thermometer. All thermometers measure temperature by making use of some property of a substance that varies with temperature. One such property is the change in volume of a substance (expansion and contraction) with temperature.

Temperature Scales

Just as we use different units to measure length (inches, centimetres, etc.), we use different units to measure temperature. These different units are represented by different temperature scales.
Here is a simplified description of how a temperature scale is defined. Two reference temperatures are chosen and the difference between these two temperatures are further broken down into a certain number of divisions. Each division is called one degree.
The most commonly used reference temperatures are the melting point of ice and the boiling point of water.
The three commonly used scales are the Celsius, the Fahrenheit scales, and the Kelvin scale.

  1. Celsius Scale
    This scale is indicated by °C [read as degree Celsius (in honour of Anders Celsius) or degree centigrade]. On this scale, the melting point of ice is taken as 0°C and the boiling point of water, as 100°C. The difference between these two points is divided into 100 degrees.
  2. Fahrenheit Scale
    This scale is indicated by °F (read as degree Fahrenheit). On this scale, 32°F is taken as the melting point of ice and 212°F as the boiling point of water. The difference between these two points is divided into 180 degrees.What is Temperature and Can Temperature be Measured Directly 2

Conversion of Temperature

Conversion Formula
Two simple equations given below will help us to convert one temperature scale into another.
F = (C x \(\frac{9}{5}\)) + 32, and
C = \(\frac{5}{9}\) (F – 32), where F is for Fahrenheit and C is for Celsius.

What is Temperature and Can Temperature be Measured Directly 3

Fet us look at the following examples to understand the conversion properly.

Example 1: Convert 86°F to °C.
Solution: We know that
C = \(\frac{5}{9}\) (F – 32)
= \(\frac{5}{9}\) (86 – 32)
= \(\frac{5}{9}\) × 54
Therefore, 86°F = 30°

Example 2: The temperature during a hot day in Delhi was 45°C. Express this temperature in degree Fahrenheit.
Solution:
We know that F = (C x \(\frac{9}{5}\)) + 32
We are given that C = 45°C, therefore, the day temperature in degree Fahrenheit
= (45 x \(\frac{9}{5}\)) + 32°F = (81+ 32)°F = 113°F

Types of Thermometers

The substance that is generally used in a conventional thermometer is either mercury or alcohol. Based on this, there are two types of thermometers: mercury thermometer and alcohol thermometer.

Mercury Thermometer
In 1714, Gabriel Fahrenheit invented the first mercury thermometer.
Mercury is used in thermometers because of the following reasons:

  • Mercury remains in the liquid state for a wide range of temperatures. It melts at -38.87°C and boils only at 356.58°C.
  • It is relatively easy to see because of its silvery grey colour.
  • It does not stick to glass.
  • It has a fairly uniform rate of expansion for a wide range of temperatures.

Alcohol Thermometer
Thermometers using alcohol have some advantages over mercury thermometers.

  • Alcohol is cheaper and less harmful than mercury.
  • Alcohol thermometers can measure much lower temperatures (up to -115°C) than mercury. However, the main disadvantage is that alcohol thermometers cannot measure temperatures higher than 78°C, whereas mercury thermometers can measure much higher temperatures.

Reading a Conventional Thermometer
The thermometers we use in the laboratory are called laboratory thermometers and the ones used to check body temperature are called clinical thermometers.

A laboratory thermometer is shown in Figure. It consists of a thin glass tube which is sealed at one end and has a bulb at the other end. The bulb is generally filled with mercury or alcohol, depending on whether it is a mercury or an alcohol laboratory thermometer. Mercury appears as a silvery grey line and alcohol looks like a red line in the thermometer. To read the temperature on this thermometer, you just read the number on the scale at the tip of the red or silvery-grey line. Remember to mention the unit used (Celsius or Fahrenheit).

The range of temperatures which a laboratory thermometer can measure is from -10°C to 110°C.

What is Temperature and Can Temperature be Measured Directly 4
Types of Thermometer

A clinical thermometer is generally a mercury thermometer though digital thermometers are becoming quite popular.
There are two characteristic features of a clinical thermometer:

  • There is a little arrow (at 98.4 or 98.6°F) showing the normal body temperature. .
  • There is a constriction or ‘kink’ in the tube near the bulb. This kink has been made to ensure that the mercury in the thermometer does not contract (and flow back into the bulb) before the temperature has been read.

To read the body temperature of any person with a clinical thermometer, follow the steps given below:
What is Temperature and Can Temperature be Measured Directly 5
Here are some precautions that one must follow while handling a mercury clinical thermometer.

  • Wash the thermometer well and dip it in an antiseptic liquid before and after using it.
  • Be careful while using a thermometer. It can break easily. Remember, mercury is a toxic substance.

The temperature of our body is closely related to a physical quantity called ‘heat’ or ‘heat energy’ possessed by the body. Let us learn more about this physical quantity.