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Measurement and Effects of Heat

Temperature and Temperature scale

On returning from school, when Ravi touched the iron gate of his house, he found it to be extremely hot. Later, he touched other things made of iron present inside his house and found that they were not hot. Then, he touched other substances (not made of iron) present in his house to determine whether they were hot or cold. He listed the various substances observed in the table given below.

 Substances Hot/ Cold Tea Hot Coffee Hot Ice Cold Ice cream Cold Cooked rice Hot Frozen meat Cold

Try to make a table listing some other substances, which are present in your house and classify them as hot or cold. But how do you decide whether a substance is hot or cold? Can you always tell whether a substance is hot or cold simply by touching it? The following activity will help you understand better.

Measurement of Temperature

Activity:

Take three containers and label them as A, B, and C. Take hot water in container A, and cold water in container C. In container B, mix hot and cold water in equal amounts. Now, place your left hand in container A and right hand in container C for two minutes. Then, dip both your hands in container B.

What can you say about the water present in all three containers? Note your observations in the table given below.

 Container Hot/Cold A B C

What is your observation for container B? Is the water in container B hot or cold?

When you dip your hands in container B, your left hand will tell you that the water is cold, while your right hand will tell you that the water is hot. Thus, you will not be able to distinguish whether the water present in container B is hot or cold.

From this activity, we can conclude that we cannot decide whether a substance is hot or cold just by touching it. Thus, we need something more reliable than our sense of touch to decide whether a substance is hot or cold.

The measure that can be used to detect the degree of hotness and coldness of a substance is called temperature. More the temperature of a substance, the hotter it will be.

Thermometer

The device that is used to measure the temperature is called a thermometer.

The scales used to measure temperature can either be degree Celsius or degree Fahrenheit.

The thermometer in which liquid is used as a thermometric fluid is called a liquid thermometer. The two most commonly used liquids are mercury and alcohol.

Use of mercury in thermometers

1. Mercury has a very high boiling (357 ºC) and freezing point (-39 ºC). Thus it can be used over a wide range of temperature.
2. Mercury has a large expansion over a small change in temperature.
3. It does not stick to the glass of the capillary tube.
4. It is an opaque and shiny liquid and thus it can be easily seen through the glass tube.

Use of alcohol in thermometers

1. The freezing point of alcohol is very low (-100 ºC). Thus, it can be used to measure temperature in regions such as Arctic and Antarctic.
2. Alcohol expands more quickly than mercury.
3. Alcohol is brightly coloured. Hence, it can be easily seen in the capillary glass tube.

Types of thermometer

There are two types of thermometers: clinical thermometers and laboratory thermometers. The table given below tells us the difference between them.

 Types of Thermometers Clinical thermometer Laboratory thermometer This thermometer is used in homes. It is basically used to measure the temperature of humans. A clinical thermometer has a temperature range of only 35 ºC to 42 °C. Can you tell the reason why? This is because our body temperature never goes below 35 °C or above 42 °C. This thermometer is used to measure the temperature of all things, except the human body. It is a complex device. A laboratory thermometer has a temperature range of -10 ºC to 110 ºC.

Why can we not use a laboratory thermometer to measure the body temperature of humans? Let us perform a small activity to understand.

Activity:

Take a beaker full of water. Now, dip a laboratory thermometer in it. Make sure that it touches neither the bottom of the beaker, nor the walls of the beaker. You will see that the mercury line rises for some time, but then ceases to rise. Note the reading where the mercury stagnates. This is the temperature of water. Take out the thermometer.

What do you observe? The mercury starts falling rapidly. This means that with a laboratory thermometer, temperature has to be read when placed in water. On the other hand, to measure the body temperature, the thermometer has to taken out of the mouth to note the reading. Thus, it is not convenient to use a laboratory thermometer.

Do You Know?

Mercury is a very toxic substance. Hence, now-a-days digital thermometers have become more popular, which do not contain mercury.

There is another thermometer which is particulary used to measure maximum and minimum temperatures of a day. Thia thermometer is known as Maximum-minimum thermometer.

Temperature Scale

The three temperature scales that are in use are Celsius scale, Fahrenheit scale, and Kelvin scale. However, the SI unit of temperature is Kelvin (K).

0 K =  -273 oC

273 K = 0 °C

373 K = 100 °C

Conversion of Temperature Scale

• Degree celsius to Kelvin scale

Temperature in Kelvin = Temperature in °C + 273

For example 40°C = (40 + 273) K = 313 K

Conversely, 343 K = (343 − 273) K = 70 °C

• Degree celsius to Fehrenheit

Temperature in Fehrenheit (°F) = °C $×$ 9/5 + 32

or, Temperature in degree celsius (°C) = (°F  -  32)  $×$  5/9

For example human body temperature is 37 °C = 37 $×$ 9/5 + 32 = 98.6 °F

Precautions while using thermometer

• Thermometer used be washed well before and after every use with an antiseptic like alcohol.
• Washing should not be done in hot water.
• Keep the mercury level straight or along the line of si…

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