Class 6 Science Chapter 7 Notes Temperature and its Measurement

→ The temperature of a body tells us how hot or cold it is.

→ The three most-used scales of temperature are (a) the Celsius scale, (b) the Fahrenheit scale, and (c) the Kelvin scale. The units of temperature in these scales are (a) degree Celsius, denoted by °C, (b) degree Fahrenheit, denoted by °F, and (c) kelvin, denoted by K.

→ The SI unit for temperature is kelvin.

→ A clinical thermometer is used for measuring body temperature.

→ Normal temperature of a healthy human adult is taken to be 37.0 °C or 98.6 °F.

→ Laboratory thermometers typically have a temperature range from -10 °C to 110 °C.

Lambok and his elder sister Phiban live in Shillong. One day they came home from school. Their parents were away at work and Lambok complained that he was feeling feverish. Phiban touched his forehead and felt that he might be having a fever. To confirm this, she took out the thermometer kept in the almirah. She washed its tip with soap and water and measured Lambok’s temperature. To her relief, she found that his temperature was normal. She washed the thermometer tip again, dried it, and put it back. She then asked Lambok to change his school uniform, eat his lunch, and rest for some time.

Hot or Cold?

We know from experience that some bodies are hotter than others. For example, during summer, the tap water may be hotter than the cold water from a matka (earthen pot) or a refrigerator. We can realize this by merely touching the two samples of water. But can we always rely upon our sense of touch? Let us find out.

Activity 7.1 Let us Investigate
Take three large containers and label them A, B, and C, as shown in Figure.

Pour warm water into container A, tap water in B, and ice-cold water in C.
We will conduct this activity in two parts – prediction and observation.
Firstly, predict what will you feel if you

• dip your right hand in A and left hand in C and keep them there for 1-2 minutes.
• take out your hands from containers A and C, and place both hands simultaneously in B.

Write down your predictions.

• What will my right hand feel on dipping in B?
• What will my left hand feel on dipping in B?

Now, conduct the activity and write your observations.

Compare whether your observations match with your predictions. Did your right hand feel that the water in container B is cool, while your left hand felt that the same water is warm? What do you infer from these observations? We cannot always rely upon our sense of touch to decide correctly whether a body is hot or cold.

Temperature

A reliable measure of the hotness (or coldness) of a body is its temperature. A hotter body has a higher temperature than a colder body. The difference in temperature between the two bodies tells us how hot a body is in comparison to another body. A device that measures temperature is called a thermometer. There are two kinds of thermometers that you are likely to come across-clinical thermometers and laboratory thermometers. Clinical thermometers are used to measure human body temperatures whereas laboratory thermometers are used for many other purposes. Let us now learn more about thermometers and how to use them to measure temperature.

Measuring Temperature

Clinical Thermometer
You might be familiar with a thermometer, like that shown in Figure, which is used for measuring our body temperature. It is called a clinical thermometer. Such thermometers show temperatures digitally. These are also known as digital clinical thermometers and run on batteries. These measure temperature when the thermometer is placed in contact with a person’s body. For measuring temperature, the clinical thermometers generally use a scale called the Celsius scale. On this scale, the unit of temperature is degree Celsius and is denoted by °C.

Earlier, mercury thermometers were used for measuring the body temperature. However, mercury is an extremely toxic substance and is difficult to dispose of if the thermometer breaks accidentally.

Digital thermometers pose no such risk and also the numbers in their display are easier to read. Therefore, mercury thermometers are being replaced by digital thermometers. Temperature in a digital thermometer is determined with the help of heat sensors.

Precautions to be taken while using a Digital Clinical Thermometer

• To be used after reading the instruction manual of the thermometer.
• The tip of the thermometer is to be washed with soap and water before and after use.
• While washing, care to be taken to keep the digital portion such as the display out of water.
• Do not hold the thermometer by the tip.

Activity 7.2 Let Us Measure
Let us now use a digital clinical thermometer to measure body temperature. You may measure your temperature as well as the temperature of some of your friends. Talk to your friends to find out who would be willing to get their body temperatures measured by you.

Wash your hands and the tip of the digital thermometer with soap and water. Reset the thermometer by pressing the reset button. Place the thermometer under the tongue and close your mouth. Wait till the thermometer makes a beeping sound or flashes a light. Take it out of the mouth and read the temperature on the digital display. Record the temperature in Table.

Clean the tip of the thermometer with soap and water, and dry it. Repeat the above steps for measuring the temperatures of your friends.

The normal temperature of a healthy human body is taken to be 37.0 °C. But in this activity, did you find that the normal temperature of every person was 37.0 °C? Or did you find the temperature slightly higher or lower for some people? The temperature of every person may not be 37.0 °C. What we call normal temperature is the average body temperature of a large number of healthy people. A perfectly healthy person may, therefore, have a normal temperature slightly different from 37.0 °C. The body temperature is influenced by several factors, such as age, time of the day, and activity level.

You may try measuring your temperature at different times of the day and on different days. Record the thermometer readings in your notebook. After a month, analyze your temperature record and see if there are any variations. If yes, try to think what might be the reasons for that. The temperature of human beings does not normally go below 35 °C or above 42 °C. For measuring the body temperature of small children or old people, the digital thermometer can also be placed in the armpit. The temperature measured this way is about 0.5 °C to 1 °C lower than the actual body temperature.

There is another scale of temperature known as the Fahrenheit scale. On this scale, the unit of temperature is degree Fahrenheit and is denoted by °F. A temperature measured as 37.0 °C on the Celsius scale is equivalent to 98.6 °F on the Fahrenheit scale. The Fahrenheit scale is not used in most scientific studies anymore. In scientific work, there is another scale of temperature known as the Kelvin scale. On this scale, the unit for temperature is kelvin and is denoted by K. The SI unit of temperature is kelvin.

All three temperature scales, Celsius, Fahrenheit, and Kelvin are named in honor of the scientists who developed these scales. We can easily convert the temperature from Celsius scale to Kelvin scale by using:

Temperature in Kelvin Scale = Temperature in Celsius Scale + 273.15

The names of temperature scales—Celsius scale, Fahrenheit scale, and Kelvin scale—start with a capital letter. For the units for temperature, degree Celsius, and degree Fahrenheit, the word degree starts with a lower-case letter while Celsius and Fahrenheit start with a capital letter. The unit kelvin starts with a lowercase letter. The symbols of all units (°C, °F, K) are capital letters. Note that the degree sign (°) is not written with K. A full stop is not written after the symbol, except at the end of a sentence. While writing the temperature, a space is left between the number and the unit. For temperatures more than one degree, use the plural of ‘degree’, that is, ‘degrees’ while writing the full form of the unit.

How was fever detected before thermometers were developed? Fever affects the pulse rate of a person. This was known even in olden days in India. However, apart from fever, some other situations also affect the pulse rate. Hence, pulse rate alone is not a reliable indicator of fever.

Laboratory Thermometer
There are many types of laboratory thermometers but the one that might be available in your school laboratory may look like the one shown in Fig. a.

It consists of a long, narrow, uniform glass tube that is sealed. At one end of the tube is a bulb which contains a liquid. Outside the bulb, in the tube, a narrow column of liquid can be seen. There is a Celsius scale marked along the tube. The liquid column rises or falls with temperature change. The mark of the Celsius scale with which the top level of the liquid column coincides is the temperature reading. The liquid used in the laboratory thermometer is generally alcohol (colored red to make it easily seen) or mercury.

Precautions to be taken while using a Laboratory Thermometer

• Handle with care. If it hits against some hard object, it can break.
• Do not hold it by the bulb.

Activity 7.3 Let us Observe
Let us try to find the temperature range of a given laboratory thermometer. Take a laboratory thermometer and observe it. Note down the following:

• What is the lowest temperature it can measure? ________
• What is the highest temperature it can measure? ________
• So, the range of this thermometer is ________

Now look at the thermometer shown in Fig. a carefully. Can you tell me its range? Its range is from -10 °C to 110 °C.

Activity 7.4 Let us Observe and Calculate
Let us now try to find the smallest value that a given laboratory thermometer can read. Again, take the same laboratory thermometer that you used in Activity 7.3 and observe it. Note down the following:

• How much is the temperature difference indicated between the two bigger marks? ________
• How many divisions (shown by smaller marks) are there between these two bigger marks? ________
• How much temperature does one small division indicate? ________
• So, the smallest value that the thermometer can read is ________

Fig. b shows a closeup of a part of the thermometer shown in Fig. a. Can you now find the smallest value that this thermometer can read?

For the thermometer shown in Fig. b, the temperature difference indicated between 0 °C and 10 °C or between 10 °C and 20 °C is 10 °C. The number of divisions between these marks is 10 divisions. So, one small division can read 10/10 = 1 °C. That is, the smallest value that this thermometer can read is 1 °C.

Your school laboratory may have thermometers for which the range and the value of the smallest division may be different. It is, therefore, always necessary to look carefully at the thermometer you are about to use. You have learned how to find the temperature range of a given laboratory thermometer. You have also learned how to find the smallest value that a given laboratory thermometer can read. We will now learn how to measure temperature using a laboratory thermometer. But, let us first learn how to use a laboratory thermometer correctly.

The correct way of Measuring Temperature using a Laboratory Thermometer
When the thermometer is immersed in water, its bulb should not touch the bottom or the sides of the beaker. The thermometer should be held vertically (Figure). It should not be tilted. The temperature must be read while the thermometer is immersed in water. While reading the thermometer, the eye should be directly in line with the level of the liquid column to be read (Figure).

Activity 7.5 Let Us Measure
Take some warm water in a beaker. Dip the thermometer in water so that the bulb is immersed in water (Figure). Observe the rise of the liquid column in the thermometer. Wait till the column stops rising and note the temperature (do not wait too long; otherwise, the water itself will start to cool). What is the temperature of water measured by you? Compare it with the readings of your friends. Do you notice that, as soon as you take the thermometer out of the water, the level of the liquid column begins to fall? This means that the temperature must be read while the thermometer is immersed in water.

The following two experimental setups are to be arranged by your teacher. One by one, as per your turn, go closer to the setups and observe the readings of both the thermometers. What are the temperatures of ice and boiling water?

Read the temperatures of ice and boiling water again after some time. Are the temperatures the same or have changed? You may have noticed that the temperature of water remains constant while it is boiling. Also, the temperature of ice remains constant while it is melting.

Caution
This activity should be performed strictly only under the supervision of the teacher. Do not touch the experimental setups.

Activity 7.6 Let Us Compare
Phiban’s Science teacher arranged the experimental setup for measuring the temperature of boiling water. The temperature readings of the boiling water taken by Phiban and her classmates in Shillong are given in Table.

Compare the temperatures of boiling water recorded by different students. Why are there differences in their readings? Discuss the possible reasons amongst yourselves. Maybe, the correct way of reading temperature was not followed by all the students.

Air Temperature
You might have seen thermometers, such as the one shown in Figure, hung on walls of your school laboratory, doctor’s clinic, and hospitals. These give an approximate idea of the room temperature. Have you seen weather reports in newspapers, TV news, or the internet? These reports also mention the maximum and minimum air temperature of the day.

Activity 7.7 Let us Analyse
Read or listen to the weather reports for a place for 10 successive days. Record the maximum and minimum air temperature for each day in the Table. Analyze the data in the Table.

Does the maximum and minimum temperature stay at the same level during these days? Because weather depends on several factors, these temperatures usually vary every day. Generally, as we approach the summer season, the temperature rises and during the winter season, it falls. There are many techniques for measuring air temperature. Air temperature is an important weather parameter and is monitored at weather stations all over the world. The data gathered on air temperature along with various other parameters are used for making weather forecasts.

Anna Mani (1918 – 2001) was an Indian scientist, also known as the ‘Weather Woman of India’. She invented and built a large number of weather measurement instruments. This reduced the reliance of India on other nations for such instruments. She also explored the possibilities of using wind and solar energy in India. Her work helped India to become one of the global leaders in renewable energy.

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