Chapter Notes

Temperature and its Measurement

Our sense of touch can give us a general idea of whether something is hot or cold. For instance, we can feel if tap water is hotter than water from an earthen pot. However, our sense of touch is not always reliable for accurately determining the exact hotness or coldness of an object. This is because our perception of hot or cold can be influenced by what our hands have been exposed to previously.

For example, if you place your right hand in warm water and your left hand in ice-cold water for a few minutes, then place both hands into tap water, your right hand will feel the tap water as cool, while your left hand will feel it as warm. This shows that we cannot always rely on our sense of touch to correctly decide 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 possesses a higher temperature than a colder body. The difference in temperature between two bodies helps us quantify how much hotter one body is compared to another.

A thermometer is a device specifically designed to measure temperature accurately.

There are primarily two types of thermometers that are commonly used:

1. Clinical thermometer
2. Laboratory thermometer

Measuring Temperature

Clinical Thermometer

A clinical thermometer is used to measure human body temperatures. Modern clinical thermometers are often digital, running on batteries, and display temperatures numerically. These thermometers measure temperature when placed in contact with a person's body.

The Celsius scale is generally used by clinical thermometers for measuring temperature. On this scale, the unit of temperature is degree Celsius, denoted by $\text{°C}$.

• Always read the instruction manual before use.
• Wash the tip of the thermometer with soap and water before and after each use.
• Be careful to keep the digital display portion out of water during washing.
• Do not hold the thermometer by its tip.

The normal temperature of a healthy human body is considered to be $37.0 \text{°C}$. However, this is an average body temperature derived from a large number of healthy individuals. Therefore, a perfectly healthy person might have a normal temperature that is slightly different from $37.0 \text{°C}$. Several factors, such as age, time of day, and activity level, can influence an individual's body temperature.

The temperature of human beings typically stays within a range, not normally going below $35 \text{°C}$ or above $42 \text{°C}$. For infants or elderly individuals, the thermometer can be placed in the armpit, though this method usually yields a temperature reading about $0.5 \text{°C}$ to $1 \text{°C}$ lower than the actual body temperature.

Another scale for temperature is the Fahrenheit scale, where the unit is degree Fahrenheit, denoted by $\text{°F}$. A temperature of $37.0 \text{°C}$ on the Celsius scale is equivalent to $98.6 \text{°F}$ on the Fahrenheit scale. The Fahrenheit scale is less commonly used in scientific studies today.

In scientific work, the Kelvin scale is also used, with its unit being kelvin, denoted by $\text{K}$. The SI unit of temperature (International System of Units) is kelvin.

All three temperature scales—Celsius, Fahrenheit, and Kelvin—are named after the scientists who developed them. You can convert temperature from the Celsius scale to the Kelvin scale using the following formula: $\text{Temperature in Kelvin scale} = \text{Temperature in Celsius scale} + 273.15$

• The names of temperature scales (e.g., Celsius scale, Fahrenheit scale, Kelvin scale) begin with a capital letter.
• For the units, "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 lower-case letter.
• The symbols for all units ($\text{°C}$, $\text{°F}$, $\text{K}$) are capital letters.
• The degree sign ($\text{°}$) is not used with $\text{K}$.
• A space is left between the number and the unit when writing temperature (e.g., $37.0 \text{°C}$).
• For temperatures greater than one degree, the plural form "degrees" is used when writing the full unit (e.g., "37 degrees Celsius").

Laboratory Thermometer

A laboratory thermometer is used for various purposes in experiments and other scientific measurements. It typically consists of a long, narrow, uniform glass tube with a sealed end containing a bulb. This bulb holds a liquid, usually alcohol (often colored red for visibility) or mercury. A narrow column of this liquid can be seen inside the tube, which rises or falls as the temperature changes. The temperature reading is taken from the mark on the Celsius scale that aligns with the top level of the liquid column.

A common laboratory thermometer has a temperature range from $-10 \text{°C}$ to $110 \text{°C}$. The smallest value a thermometer can read is determined by observing the temperature difference between two bigger marks and dividing it by the number of smaller divisions between them. For example, if there are 10 divisions between $0 \text{°C}$ and $10 \text{°C}$, then one small division represents $10 \text{°C} / 10 = 1 \text{°C}$.

• Handle it with care, as it can break if it hits a hard object.
• Do not hold it by the bulb, as this can affect the reading.

Correct Way of Measuring Temperature Using a Laboratory Thermometer

To ensure accurate temperature measurement with a laboratory thermometer, follow these guidelines:

• Immersion: The thermometer's bulb must be fully immersed in the substance whose temperature is being measured. It should not touch the bottom or sides of the container (e.g., a beaker).
• Vertical Position: The thermometer should be held vertically and not tilted.
• Reading While Immersed: The temperature reading must be taken while the thermometer is still immersed in the substance. If the thermometer is removed from the liquid, the liquid column will begin to fall, leading to an incorrect reading.
• Eye Level: Your eye should be directly in line with the top level of the liquid column to avoid parallax errors.

When measuring the temperature of boiling water or melting ice, you might observe that the temperature remains constant even as heating or melting continues. This phenomenon occurs because the energy being supplied is used for a change of state (e.g., from solid to liquid or liquid to gas) rather than increasing the temperature.

Differences in temperature readings among students for the same substance (like boiling water) can occur if the correct procedure for reading the thermometer is not followed by everyone.

Air Temperature

Thermometers designed to measure air temperature are often seen hanging on walls in laboratories, clinics, or hospitals to give an approximate idea of room temperature.

Weather reports, found in newspapers, on TV news, or online, regularly provide information about the maximum and minimum air temperatures for a given day. These temperatures typically vary daily because weather is influenced by numerous factors. For example, temperatures generally rise during the summer season and fall during the winter season.

Know a Scientist

Anna Mani (1918-2001) was an influential Indian scientist, often called the 'Weather Woman of India'. She was instrumental in inventing and building numerous weather measurement instruments, which significantly reduced India's dependence on other countries for such equipment. Her work also extended to exploring the potential of wind and solar energy in India, contributing to India's emergence as a global leader in renewable energy.

Keywords

• Air Temperature
• Celsius Scale
• Clinical thermometer
• degree Celsius
• degree Fahrenheit
• Fahrenheit scale
• kelvin
• Kelvin scale
• Laboratory thermometer
• SI unit of temperature
• Temperature