Class 6 Science Chapter 7 Notes Temperature and its Measurement
In this chapter, we will learn about a reliable measure of the hotness or coldness of anybody. We come across various types of thermometers in our day-to-day life. Here, we will develop an understanding of which ones to use where, and how to use them effectively.
Hot or Cold
In our everyday life, we come across various objects, out of which some are felt hot while others can be felt cold. For example
- A steel bench becomes hot on a bright sunny day but a clay brick remains cold.
- A cane of cold drink feels cold to touch but touching ice feels colder.
Hence, among these hot or cold objects, some objects may be hotter or colder than the other. So, we can say that the relative hotness or coldness of objects can be decided by simply touching the objects. But our sense of touch is not enough to decide whether the object is hot or cold. A reliable measure of the hotness (or coldness) of a body is its temperature.
Temperature and its Measurement
The degree of hotness or coldness of an object is known as the temperature of the object. A hotter body has a higher temperature than a colder body. A device that measures temperature is called a thermometer. There are two types of thermometers that you are likely to come across
- Clinical thermometers
- Laboratory thermometers
Clinical thermometers are used to measure human body temperatures, whereas laboratory thermometers are used for many other purposes.
Different Scales of Temperature
→ The temperature of an object should always be stated with its unit. The most common unit for measuring temperature is degree Celsius (°C). The clinical thermometers generally use the Celsius scale for measuring 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.
→ We can easily convert the temperature from Celsius scale to Kelvin scale by using T(K) = T(°C) + 273.15
→ 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.
Way of Writing Units
→ The names of temperature scales-Celsius scale, Fahrenheit scale, and Kelvin scale 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.
→ 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.
Measuring Temperature through Clinical Thermometer
A clinical thermometer is given in the figure below which shows temperatures digitally. It is generally used for measuring our body temperature. These are also known as digital clinical thermometers and run on batteries. Temperature in a digital thermometer is determined with the help of heat sensors.
Reading a Digital Clinical Thermometer
→ Wash your hands and the tip of the digital thermometer with an antiseptic solution or clean water.
→ Reset the thermometer by pressing the reset button and placing it under the tongue.
→ Wait till the thermometer makes a beeping sound.
→ Take it out of the mouth and read the temperature on the digital display.
→ Clean the tip of the thermometer again as done before using it.
Precautions to be taken While Using a Digital Clinical Thermometer
→ Use it after reading the instruction manual of the thermometer.
→ Wash the tip of the thermometer with soap and water before and after use.
→ Make sure to keep the digital portion (i.e. display) of the thermometer out of water while washing it.
→ Do not hold the thermometer by the tip.
The normal temperature of a healthy human body is taken to be 37.0 °C, which is equivalent to 98.6 °F on a Fahrenheit scale. However, body temperature is influenced by several factors, such as age, time of the day, and activity level. The temperature of human beings does not normally go below 35 °C or above 42 °C.
In the olden days, before thermometers were developed, fever was detected by checking the pulse rate of a person. However, apart from fever, some other situations also affect the pulse rate. Hence, pulse rate alone is not a reliable indicator of fever.
Mercury Thermometer
Mercury thermometers are also used for measuring the body temperature. But 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.
The non-contact thermometers, used during the Covid-19 pandemic, are called infrared thermometers. Such thermometers can measure temperature without touching a person’s body and thus, reduce the risk of spreading disease.
Measuring Temperature through Laboratory Thermometer
A laboratory thermometer consists of a long, narrow, uniform glass tube that is sealed. There is a bulb at one end of the tube 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 a temperature change.
→ The range of a laboratory thermometer is from -10 °C to 110 °C.
→ Before reading a laboratory thermometer, it is very important to find the smallest value that a given laboratory thermometer can read.
→ To find that, divide the temperature difference between two bigger marks by the number of divisions present between them.
→ The temperature reading can be recorded by looking at the mark on the Celsius scale with which the top level of the liquid column coincides.
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 as shown in the 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.
→ The temperature of water remains constant while it is boiling. Also, the temperature of ice remains constant while it is melting.
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.
→ The liquid used in the laboratory thermometer is generally alcohol or mercury.
Air Temperature
The temperature of air at a particular time is called air temperature. Weather reports shown on TV, newspapers, and the Internet show the maximum and minimum air temperature of the day.
Thermometers that give an approximate idea of the air temperatures are called maximum and minimum thermometers. These can be generally seen on the walls of school laboratories, doctor’s clinics, and hospitals. Weather reports also mention the maximum and minimum air temperature of the day.
These temperatures usually vary every day, because weather depends on several factors. There are many techniques for measuring air temperature. The data gathered on air temperature along with various other parameters are used for making weather forecasts. The temperature at the core of the sun reaches as high as 15 million degrees Celsius and there is no limit on the highest temperature that can exist.
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.
Scottish-Irish physicist, William Thomson aka Lord Kelvin is best known today for inventing the international system of a unit of temperature that bears his name, Kelvin. He discovered that in this temperature scale, -273 °C (or 0 K) is considered as absolute zero. He noted that molecules stop moving at absolute zero. It is therefore the lowest possible temperature, or the absolute zero on any temperature scale.
→ Temperature: A measure of the degree of hotness or coldness of an object is its temperature.
→ Thermometer: A device that measures temperature is called a thermometer.
→ Clinical thermometer: A thermometer used to measure human body temperature is a clinical thermometer.
→ Laboratory thermometer: A thermometer used to measure temperatures other than human body temperature is a laboratory thermometer.
→ Kelvin: Kelvin is the SI unit of temperature.
→ Celsius Scale: For measuring temperatures, clinical thermometers generally use a scale called the Celsius scale.
→ Degree Celsius: On the Celsius scale, the unit of temperature is degree Celsius and is denoted by °C.
→ Degree Fahrenheit: On the Fahrenheit scale, the unit of temperature is degree Fahrenheit and is denoted by °F.
→ Fahrenheit Scale is a temperature scale that uses the degree Fahrenheit (Symbol: °F).
→ Air temperature: Air temperature is a measure of how hot or cold the air is.