One basic physical quantity that expresses how hot or cold a thing is its temperature. It is a vital component of daily life and is important for many industrial and scientific uses. In physics, engineering, medicine and meteorology, temperature measurement is essential. The concept of temperature, its units, several temperature scales, and measurement tools are all covered in this note.
Concept of Temperature
Temperature is a measure of the average kinetic energy of the molecules in a substance. When the temperature of a substance increases, the kinetic energy of its molecules similarly increases. The amount of heat that a thing can transmit or absorb from its environment is determined by this energy.
For definition of any standard scale, two fixed reference points are needed. The ice point and steam point of water are two convenient fixed points and known as the freezing and boiling points respectively. The two familiar temperature scales are the Fahrenheit temperature scale and the Celsius temperature scale. The ice and steam point have values 320 F and 2120 F, respectively , on the Fahrenheit scale and 00 C and 1000 C on the Celsius scale.
From the above graph, relation between Fahrenheit temperature and Celsius temperature is, tF – 32 / 180 = tc / 100.
Measurement of Temperature
Units of Temperature
The SI unit of temperature is the Kelvin (K). However, other commonly used units are:
Celsius (0C): Used in daily life and scientific applications.
Fahrenheit (0F): Commonly used in the United States.
Kelvin (K): Used in scientific and thermodynamic calculations.
The relationship between these temperature scales is given by:
Celsius to Kelvin: K = 0C + 273.15
Celsius to Fahrenheit: 0F = (0C × 9/5) + 32
Fahrenheit to Celsius: 0C = (0F – 32) × 5/9
Temperature Scales
Celsius Scale: The freezing point of water is 00C, and the boiling point is 1000C.
Fahrenheit Scale: The freezing point of water is 320F, and the boiling point is 2120F.
Kelvin Scale: The lowest possible temperature (absolute zero) is 0 K, equivalent to -273.150C.
Rankine Scale: Mostly used in thermodynamic calculations in engineering.
Instruments for Measuring Temperature
Various instruments are used to measure temperature based on different principles. Some common are given below:
Thermometers
Mercury Thermometer: Uses mercury expansion in a glass tube to indicate temperature.
Alcohol Thermometer: Uses colored alcohol instead of mercury and is suitable for very low temperatures.
Clinical Thermometer: Used in medical applications to measure body temperature.
Laboratory Thermometer: Measures temperature in scientific experiments.
Thermocouple
Consists of two different metals joined at one end, generating a voltage proportional to temperature.
Used in industrial applications due to its wide temperature range.
Resistance Temperature Detector (RTD)
Uses the change in electrical resistance of a metal (usually platinum) to measure temperature.
Highly accurate and stable.
Infrared Thermometer
Measuring temperature without contact by detecting infrared radiation.
It is used in medical and industrial applications.
Pyrometer
It measures very high temperatures, such as in furnaces and molten metals.
Thermal Expansion and Temperature Measurement
The concept of thermal expansion is frequently used in temperature measurement. It is quality of a substance on getting heat raises the volume. This expansion is working in instruments such as alcohol and mercury thermometers, where an increase in temperature is associated with a rise in liquid level.
Absolute Zero and Its Significance
At the lowest temperature, absolute zero (0 K or -273.150C), molecular motion stops. It is a theoretical limit that is vital to understand quantum physics and thermodynamics but is not achievable in practice.
Note
One essential component of science and technology is to measures the temperature. Accurate temperature measurement is aided by a variety of scales and tools in different applications.
The average kinetic energy of a substance’s particles is measured by its temperature. A thermometer is used to measure it, and it is expressed in Fahrenheit (0F), Kelvin (K), or Celsius (0C).
The three commonly used temperature scales are:
Celsius (0C)
Fahrenheit (0F)
Kelvin (K) (SI unit of temperature)
The conversions between scales are:
K = 0C+273.15K = 0C + 273.15K = 0C+273.15
0F = (9/5)0C + 320F = (9/5)0C + 320F = (9/5)0C+32
0C = (5/9)(0F−32)0C = (5/9)(0F – 32)0C=(5/9)(0F−32)
Some common thermometers are:
Mercury thermometer
Alcohol thermometer
Digital thermometer
Gas thermometer
Infrared thermometer