1. What Are Thermometers?
A thermometer is a device used to measure temperature. It works by using some physical property of a material that changes predictably with temperature. Once the thermometer reaches thermal equilibrium with the object, its reading gives the temperature of that object.
Different thermometers use different principles like expansion of liquids, change in electrical resistance, or generation of voltage.
2. How Thermometers Work
Thermometers rely on a measurable physical change that happens when temperature changes. Each type uses a different property but follows the same idea: the thermometer and the object exchange heat until both reach the same temperature.
2.1. Liquid-in-Glass Thermometers
These thermometers use mercury or colored alcohol inside a thin glass tube. As the temperature increases, the liquid expands and rises; when the temperature falls, the liquid contracts and drops.
- Simple design
- Common for everyday temperature measurement
- Scale marked on the glass indicates temperature
2.2. Digital Thermometers
Digital thermometers use electronic sensors whose resistance changes with temperature. A small microchip converts this change into a temperature reading.
They respond quickly and are widely used in homes and medical applications.
2.3. Thermocouple Thermometers
These consist of two different metals joined at one end. When heated, a voltage is produced at the junction. This voltage is directly related to temperature.
They are used for high-temperature measurements like furnaces and engines.
3. Fixed Points in Temperature Measurement
To create a temperature scale, certain fixed reference temperatures are chosen. These are stable and repeatable points in nature.
3.1. Lower Fixed Point
The temperature at which pure ice melts (melting point of ice). In the Celsius scale, this is taken as 0°C.
3.2. Upper Fixed Point
The temperature at which pure water boils at standard atmospheric pressure. In the Celsius scale, this is 100°C.
4. Celsius Scale (°C)
The Celsius scale divides the temperature difference between the melting point of ice and the boiling point of water into 100 equal divisions. Each division is called one degree Celsius (°C).
- Melting point of ice: 0°C
- Boiling point of water: 100°C
4.1. Why Celsius Is Common
It is widely used in everyday life, weather reports, and scientific measurements. The scale is simple to understand because it is based on water, which is familiar in daily experience.
5. Fahrenheit Scale (°F)
The Fahrenheit scale is based on different reference points. The melting point of ice is taken as 32°F and the boiling point of water as 212°F. The interval between these two is divided into 180 equal parts.
- Freezing point: 32°F
- Boiling point: 212°F
5.1. Usage of Fahrenheit
The Fahrenheit scale is commonly used in some countries for weather forecasting and body temperature measurement because it provides finer subdivisions in the human comfort range.
6. Kelvin Scale (K)
The Kelvin scale is the absolute temperature scale used in scientific work. It starts from absolute zero, the temperature at which particle motion is at a minimum.
The Kelvin scale avoids negative values and directly relates to internal energy.
6.1. Relation Between Celsius and Kelvin
The relation between the two is:
\( K = ^\circ C + 273.15 \)
6.2. Absolute Zero
Absolute zero (0 K) corresponds to −273.15°C. This is the point where particles have the least possible energy. No substance can reach exactly 0 K, but experiments can approach very close to it.
7. Conversion Between Scales
Since different places use different scales, conversion formulas help compare temperatures easily.
7.1. Celsius–Fahrenheit Conversion
The relation between Celsius and Fahrenheit is:
\( F = \dfrac{9}{5}C + 32 \)
and
\( C = \dfrac{5}{9}(F - 32) \)
7.2. Kelvin–Celsius Conversion
\( K = C + 273.15 \)
This conversion is straightforward because both scales use equal divisions.
8. Real-Life Importance of Temperature Scales
- Weather reporting uses Celsius or Fahrenheit depending on the region.
- Kelvin is essential in science because it directly links to particle energy.
- Digital devices internally convert temperatures between scales for accuracy.
- Cooking, refrigeration, and heating systems rely on accurate temperature measurement.