Chapter 5: Temperature Measurements

By the end of this chapter, you will be able to:

a) understand the difference between heat and temperature.

b) understand how temperature scales are established.

c) Calibrate a thermometer and use it to measure temperature.

d) compare the qualities of thermometric liquids.

e) describe the causes and effects of the daily variations in atmospheric temperature.

Keywords

1. heat
2. temperature
3. temperature
4. scale
5. thermometer

5.1: Introduction

On a cold day, we feel uncomfortable and wear warm clothes such as sweaters, jackets and so on to minimise loss of heat from our bodies. On the other hand, on a hot day, we feel uncomfortable and therefore we wear light, cool clothes that allow us to release unwanted heat. These are related to temperature.

In this chapter, you will learn that temperature change is a result of heat effects in a body and that daily temperature changes have an effect on our lives. 5.2: Temperature and Heat

Activity 5.1 Understanding temperature and heat

What you need Bursen burner, a bicycle spoke, water in a beaker, a digital thermometer.

What to do

a) Hold a bicycle spoke on a lit Bunsen burner for about 5 minutes.

b) State how you feel at the end of this timer.

c) Put water in a beaker on a lit Bunsen burner and wait for about 3 minutes.

d) Put the thermometer in water at intervals of 5 minutes for 3 times. Note the differences on the thermometer readings. Explain what you observe.

DID YOU KNOW Temperature is a measure of the degree of hotness of a body, and that heat is a measure of the energy content of a body?

Temperature Measure

DID YOU KNOW

Temperature is measured using a thermometer?

There are different kinds of thermometers. Some are analog while others are digital like the one shown in Figure 5.1.

5.3: Temperature Scales

The Celsius Scale (°C)

There are three temperature scales, namely: The Celsius scale (°C), the Kelvin scales (K) and the Fahrenheit scale (°F).

Assignment 5.1 Conversions of scales of temperature

Explore in textbooks how to convert temperatures from one scale to the other e.g:

1. °C to °F

2. °F to °C

3. °C to K

4. K to °C

5. °F to K

6. K to °F

Advantages of the Kelvin scale

There are no negative temperatures.

Pressures and volumes of gases will change in proportion to Kelvin temperature. Absolute zero is 273 degrees below O° C.

5.4: Types of Thermometers

A thermometer is required to measure temperature. The thermometer makes use of a physical property of a substance which changes continuously with temperature. This physical property is referred to as thermometric property.

A good thermometric property of matter should:

• Vary continuously with temperature.
• Vary uniquely over the range of temperatures to be measured.
• Have a measurable change.

The following table shows some of the thermometric properties of matter that are used in the various types of thermometers:

Thermometric Property

Volume of a gas

Volume of a liquid

Pressure of a fixed mass of gas

Thermometer

Gas thermometer

Laboratory or clinical thermometer

Constant-volume gas thermometer

Table 5.1: Thermometric properties used in various thermometers

Thermometric liquids

These are liquids used in thermometers to help measure temperature.

Activity 5.2 Discussing the types of liquids used in the thermometers

a) In pairs, state two commonly used liquids in thermometers other than water.

b) Suggest the advantages of the liquids mentioned above over water.

c) Present your work to the rest of the class.

The other reasons why water is not used as a thermometric liquid are:

Water:

Sticks on the glass surface.

Has non-uniform expansion.

Is colourless and thus reading of meniscus is difficult.

A good thermometric liquid should:

Not freeze nor boil in the working range.

Not be corrosive to the container.

Not be poisonous or otherwise dangerous if the thermometer breaks.

Have a different thermal expansion than the container.

Be clearly visible and easily readable in the tube.

Have a uniform thermal expansion.

Have a low freezing point.

Advantages and disadvantages of mercury as a thermometric liquid

Advantages

It is a good conductor of heat (high thermal Poisonous conductivity)

Does not wet (cling to the sides) of the tube

High boiling point (357°C)

Uniform expansion

Responds quickly to temperature changes

Visible meniscus

Disadvantages

Small thermal expansion

Expensive High freezing point, -39°C (cannot be used in places that are very cold)

Table 5.2: Mercury as a thermometric substance

Advantages and disadvantages of alcohol as thermometric liquid

Advantages

Safe liquid

Large expansivity

Cheap

Low freezing point (-115°C)

Disadvantages

Wets the tube

Low boiling point (78°C))

Does not react quickly to temperature changes

Needs to be dyed since it is colourless

Non-uniform expansion

Table 5.3: Advantages and disadvantages of alcohol as a thermometric liquid

Assignment 5.2 Reading thermometers

Using internet and other textbooks, research on:

a) How thermometers are used in measuring temperature and how they can be read. b) Whether digital thermometers are better than other kinds of thermometers and why.

NOTE:

The common forms of thermometers that are easily found are 1- in glass thermometer. A common example is the clinical thermometer used in hospitals. liquid

A Clinical thermometer is used to measure the temperature of the human body. The range that it can assess is from 35 degrees Celsius (35°c) to 42 degrees Celsius (42°c). For hygiene and safety reasons, they should first be sterilised before use.

Figure 5.2: Clinical thermometer

5.5: Calibration of a Thermometer

Calibration gives you peace of mind from knowing that the temperature you are reading is the correct temperature of the media you’re measuring. That can be extremely important for procedures which require precise temperature control, including industrial chemical processes and food preparation.

Calibrating a thermometer

To calibrate a thermometer means to put the correct mark of reading at the correct place so that other temperatures can be deduced from these marks.

To do this, two extreme points are chosen to mark its scale and these points must be able to be reproduced accurately.

Usually, we take the steam point of pure water as 100°C and the ice point of water as 0°C.

To calibrate a thermometer, it is inserted into a funnel filled with crushed ice. As it melts, the water drips off into the beaker. When the mercury thread, seen just above the ice, remains steady for sometime, its position is marked on the stem. This mark gives the lower fixed point and is marked as 0°C.

A hypsometer is used to obtain the upper fixed point of the thermometer. The experimental set up is as shown below.

The bulb of the ‘thermometer’ should NOT be allowed to dip into the boiling water. This is because if the water is not pure, it will be boiling at a different temperature from that of the steam.

1. Water is steadily boiled to generate steam that surrounds the bulb.
2. When the thread, seen above the cork, remains steady for some time, its level is marked on the stem. This mark gives the upper fixed point and is marked as 100°C.
3. After determining the position of the ice point and steam point, the temperature, 0, of an object can be determined by using the formula:

EXERCISE 5.1

The figure above shows the length of the mercury thread of a thermometer at the melting point and the boiling point of water. What is the length of the mercury thread when the thermometer is dipped into a hot liquid of temperature 70°C?

5.6: Variation of Atmospheric Temperature

Atmospheric temperature is a measure of temperature at different levels of the Earth’s atmosphere.

It is governed by many factors, including incoming solar radiation, humidity and altitude. It also changes with time, varying from season to season, and from day to night, as well as irregularly due to passing weather systems.

Reason for the daily temperature variation

1. The solar energy received by any region varies with time of day, with seasons, and with latitude that causes temperature variations.

2. Variation with latitude: The angle of incident of the solar radiation varies with the shape of the Earth. The Sun is more overheaded in equatorial regions as compared to the higher latitudes. This is the only reason why temperature varies from Equator to the poles.

3. Cloud obstructs the receipt and loss of insolation causes low temperature in clouded region.

4. A clear sky in desert regions causes high temperature because insolation is received without obstruction in the day and lost without obstruction in the night.

5. Snow-bound regions absorb less and reflect more insolation causing low temperature.

6. A high altitude region has high temperature range because in this region, air is thin, which brings great loss of insolation in the night and no obstruction in the receipt of insolation in the day.

7. Distance from the sea: The interior regions of the landmasses receive higher temperature whereas regions nearest to the sea have moderate temperatures due to sea breezes.

8. Warm and cold winds disturb the temperature rang

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