Topics to be learn : Hot or Cold? Temperature Measuring Temperature Clinical thermometer Laboratory thermometer Air temperature

Hot or Cold? :

• Humans often rely on touch to judge temperature.
• We know from experience that some bodies are hotter than others — for example, tap water may feel hotter than water from a matka (earthen pot) or a refrigerator. We usually judge this simply by touching the objects.
• The same body feels different to two hands because our sense of touch depends on the temperature our hand was previously exposed to.

Limitations of Touch

• Sense of touch is relative to the observer's prior state.
• Touch cannot provide a numerical or reliable measure of hotness.

We cannot always rely on our sense of touch to correctly judge whether a body is hot or cold. We need a reliable, unbiased instrument.

Temperature :

Definition : Temperature is defined as the reliable measure of the hotness or coldness of a body, A hotter body has a HIGHER temperature than a colder body. The difference in temperature between two bodies tells us how hot one body is compared to another.

Definition : Thermometer — a device used to measure temperature.

Two kinds of thermometers are commonly used:

Type	Used for
Clinical thermometer	Measuring human body temperature
Laboratory thermometer	Measuring temperature of substances for many other purposes (water, air, experiments, etc.)

Measuring Temperature :

(i) Clinical thermometer :

A clinical thermometer is used to measure body temperature.

Digital Thermometers : Modern clinical thermometers are digital — they show the temperature reading on a small screen and run on batteries.

• Scale used: Celsius scale — unit is degree Celsius, denoted °C.
• Digital thermometers determine temperature using heat sensors.

Mercury vs Digital Thermometers :

• Earlier, mercury thermometers were widely used for measuring body temperature.
• Mercury is an extremely toxic substance and is difficult to dispose of safely if the thermometer breaks.
• Digital thermometers pose no such risk, and their display is easier to read.

→ Mercury thermometers are therefore being replaced by digital thermometers.

Non-contact (Infrared) Thermometers :

• During the COVID-19 pandemic, special thermometers were used that could measure a person's temperature from a distance, without touching them.
• These are called non-contact thermometers or infrared thermometers.
• They reduce the risk of spreading disease because no physical contact with the person is needed.

Normal Body Temperature :

The normal temperature of a healthy human body is taken to be 37.0 °C (98.6 °F). However, this is an average — the temperature of every healthy person may not be exactly 37.0 °C.

• 'Normal temperature' = average body temperature of a large number of healthy people.
• A perfectly healthy person may have a normal temperature slightly different from 37.0 °C.
• Body temperature is influenced by age, time of day, and activity level.
• Human body temperature normally does not go below 35 °C or above 42 °C.
• For small children or old people, the thermometer may be placed in the armpit — this reading is about 0.5 °C to 1 °C LOWER than the actual body temperature.

Know This — Detecting Fever Before Thermometers Before thermometers were developed, fever was often judged by checking a person's pulse rate, since fever affects pulse rate. However, pulse rate can also change due to other reasons (exercise, excitement, etc.), so pulse rate alone is NOT a reliable indicator of fever.

Writing Conventions for Temperature Units :

• Names of scales (Celsius, Fahrenheit, Kelvin) start with a CAPITAL letter.
• 'degree Celsius' and 'degree Fahrenheit' — the word 'degree' starts with a lower-case letter.
• The unit 'kelvin' starts with a lower-case letter (no 'degree' before it).
• Symbols °C, °F, K are always CAPITAL letters. Note: the degree sign (°) is NOT written with K.
• A full stop is not written after the symbol (except at the end of a sentence).
• Leave a space between the number and the unit, e.g., 37.0 °C (not 37.0°C or 37.0 C).
• For temperatures greater than one, use the plural 'degrees' when writing the full form, e.g., '37 degrees Celsius'.

Know This — Can a clinical thermometer measure boiling water or ice? No. The temperatures of boiling water and ice lie OUTSIDE the range of a clinical thermometer (which is designed only for the narrow human body-temperature range, roughly 35 °C–42 °C).

(ii) Laboratory thermometer :

A laboratory thermometer is used to measure temperatures beyond the range of a clinical thermometer — such as boiling water, ice, or air.

Structure of a Laboratory Thermometer

• A long, narrow, uniform, sealed glass tube.
• One end has a bulb containing a liquid (the liquid can be alcohol, coloured red for visibility, or mercury).
• A Celsius scale is marked along the tube.
• The liquid column rises or falls with a change in temperature.
• The mark on the scale where the top of the liquid column stops indicates the temperature reading.

Range: The lowest and highest temperature a thermometer can measure. Example: a common laboratory thermometer has a range of −10 °C to 110 °C.

Least count (smallest readable value): Found using this method —

• Note the temperature difference between two large (numbered) marks, e.g., between 0 °C and 10 °C → 10 °
• Count the number of small divisions between these two large marks, e.g., 10 divisions.
• Least count = \(\frac{\text{Temperature difference}}{\text{number of divisions}}\) = \(\frac{10 °C}{10}\) = 1 °C.

Worked Example : Between 0 °C and 10 °C, if there are 10 small divisions, each small division reads 10/10 = 1 °C. So the smallest value this thermometer can read (its least count) is 1 °C.

• Different laboratory thermometers may have different ranges and least counts — always examine the thermometer you are about to use.

Phase Changes and Temperature Stability :

A critical scientific observation noted is that the temperature of a substance remains constant during a change of state:

• Boiling: The temperature of water remains constant while it is boiling. Water stays at 100°C while boiling, even as heat is continuously added.

• Melting: The temperature of ice remains constant while it is melting. Ice remains at 0°C while melting. The heat energy supplied during these transitions is used to overcome intermolecular forces of attraction rather than raising the temperature.

(iii) Air temperature :

Thermometers hung on walls of school laboratories, doctors' clinics, and hospitals give an approximate idea of room temperature — these are called room thermometers.

Weather reports (in newspapers, TV, or the internet) mention the maximum and minimum air temperature of the day.

• Air temperature is monitored at weather stations across the world and used to prepare weather forecasts.
• Maximum and minimum temperatures usually vary from day to day since weather depends on several factors.
• Generally, temperature rises as summer approaches and falls during winter.

Know a Scientist — Anna Mani (1918–2001) Anna Mani was an Indian scientist, also known as the 'Weather Woman of India'. She invented and built a large number of weather-measurement instruments, reducing India's reliance on other nations for such instruments. She also explored the possibilities of using wind and solar energy in India. Her work helped India become one of the global leaders in renewable energy. Quote: "Wrong measurements are worse than no measurements at all." Scottish-Irish physicist, Willlam Thomson aka Lord Kelvin is best known today for inventing the international system of unit of temperature that bears his name, Kelvin. He discovered that in this temperature scale, -273°℃ (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.

Know This — Extreme Temperatures The temperature at the core of the Sun reaches as high as 15 million degrees Celsius. There is no known upper limit on how high temperature can go. However, there IS a limit to the lowest temperature that can be achieved — it is called absolute zero, close to −273.15 °C (0 kelvin).

Available in Notes PDF :

• Glossary — Key Terms
• Memory Aid (Mnemonic)
• Chapter Summary — Key Points
• Exam Preparation Tips
• Practice Questions

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