Temperature - Definition, Measurement, Temperature Units

Temperature refers to the hotness or coldness of a body. In specific terms, it is the way of determining the kinetic energy of particles within an object. Faster the movement of particles; more the temperature and vice versa. The temperature has a significant role in almost every area of our everyday lives as well as in all scientific disciplines, from physics to geology.

Hot Iron

To determine how hot an iron rod is, physicists measure it in temperature to be precise rather than mentioning how hot or cold the rod is.

Table of Contents

Temperature Definition

Temperature is a measure of the average kinetic energy of the particles in an object. When the temperature increases, the motion of these particles also increases.

Temperature is measured with a thermometer or a calorimeter. In other words, temperature determines the internal energy within a given system.

Relationship between Temperature and Kinetic Energy

Kinetic energy is the energy possessed by a body due to its motion. We see a range of kinetic energy in molecules because all molecules don’t move at the same speed. When a substance absorbs heat, the particles move faster, so the average kinetic energy and therefore the temperature increases.
As stated in the kinetic-molecular theory, the temperature of a substance is related to the average kinetic energy of the particles of that substance. When a substance is heated, some absorbed energy is stored within the particles, while some energy increases the motion of the particles. This is registered as an increase in the temperature of the substance.

Solids, liquids, and gases all have a temperature. The particles within a solid don’t move, but they have vibrational motion. The temperature increases when molecules vibrate faster. The melting point of a solid is the temperature at which the vibrational motion overcomes the forces of attraction holding the molecules in a solid formation.
Read More: Kinetic Theory of Gases

Heat vs Temperature

It is important to understand that heat and temperature are not the same. Although the two concepts are linked, they mean different things.
Heat describes the transfer of thermal energy between molecules within a system and it is measured in joules. An object can gain or lose heat, but it cannot have heat. Heat is not a property possessed by an object or system rather it is a measure of change.
Temperature describes the average kinetic energy of molecules within a material or system and is measured in Celsius (°C), Kelvin (K) and Fahrenheit (°F).
Concluding, we can say that heat is a transfer of thermal energy caused by a difference in temperature between molecules.

Temperature Measurement

As molecules are minuscule particles, we must use indirect methods to measure the kinetic energy of the molecules of a substance. As heat is added to the substance, the molecules move more rapidly. This increased motion causes a  small increase in the volume, or amount of space, taken up by most materials. There are devices that use the expansion of a substance to give an indirect measure of temperature. Such devices are called thermometers.

How does a Thermometer Measure Temperature?

Thermometers are the most common instrument to measure temperature. The simplest of thermometers is the liquid thermometer. They are a thin glass tube filled with a small amount of mercury.  Thermometers measure the temperature due to thermal expansion. An increase in the volume of substance because of the increase in the temperature is known as thermal expansion. A small change in the temperature causes changes in the volume of a liquid.  However, this form of effect is maximised when the liquid expands within the thin tube of the thermometer. When mercury gets hotter, it increases in size by an amount that’s directly related to the temperature. So if the temperature increases by 20 degrees, the mercury expands and moves up the scale by twice as much as if the temperature increase is only 10 degrees.

Making a Celsius scale is easier because it’s based on the temperatures of ice and boiling water. These are the two fixed points. When we dip the thermometer into ice, we observe that the mercury level marks the lowest point on our scale i.e, 0°C.  The mercury rises to the top of the thermometer and reads 100°C if we immerse it in boiling water. A thermometer’s scale is split into 100 equally spaced intervals between 0°C and 100°C.

What is the Temperature Sensor?

A temperature sensor is an RTD (Resistance Temperature Detector) or a thermocouple, that collects data about temperature from a particular source and converts the data into an understandable form for a device or an observer. Temperature sensors are used in many applications, such as food processing units and medical devices. The most common type of temperature sensor is the thermometer, which measures the temperature of solids, liquids, and gases. It is also a common type of temperature sensor, mostly used for non-scientific purposes because it is not so accurate. The following are a few temperature sensors besides the thermometer.

  • Thermocouples
  • Resistor temperature detectors
  • Thermistors
  • Infrared sensors
  • Semiconductors

Temperature Scales

Thermometers measure temperature according to well-defined scales of measurement. The three most common temperature scales are the Fahrenheit, Celsius, and Kelvin scales.

Thermometer Diagram

Celsius Scale & Fahrenheit Scale

The Celsius scale has a freezing point of water at 0ºC and the boiling point of water at 100ºC. On the Fahrenheit scale, the freezing point of water is at 32ºF and the boiling point is at 212ºF.  The temperature difference of one degree Celsius is greater than a temperature difference of one degree Fahrenheit. One degree on the Celsius scale is 1.8 times larger than one degree on the Fahrenheit scale.

  • 180/100 = 9/5

Kelvin Scale

Kelvin scale is the most commonly used temperature scale in science. It is an absolute temperature scale defined to have 0 K at the lowest possible temperature, called absolute zero. The freezing and boiling points of water on this scale are 273.15 K and 373.15 K, respectively. Unlike other temperature scales, the Kelvin scale is an absolute scale. It is extensively used in scientific work. The Kelvin temperature scale possesses a true zero with no negative temperatures. It is the lowest temperature theoretically achievable and is the temperature at which the particles in a perfect crystal would become motionless.

Relationship between Different Temperature Scales

The relationship between the three temperature scales is given in the table below:

Conversion  Equation 
Celsius to Fahrenheit
\(\begin{array}{l}T_{F^o} = \frac{9}{5}T_{c^o}+32\end{array} \)
Fahrenheit to Celsius
\(\begin{array}{l}T_{C} = \frac{5}{9}T_{F}-32\end{array} \)
Celsius to Kelvin
\(\begin{array}{l}T_{K} = T_{C^o}+273.15\end{array} \)
Kelvin to Celsius
\(\begin{array}{l}T_{C^o} = T_{K}-273.15\end{array} \)
Fahrenheit to Kelvin
\(\begin{array}{l}T_{K} = \frac{5}{9}(T(F^0)-32)+273.15\end{array} \)
Kelvin to Fahrenheit
\(\begin{array}{l}T_{F^0} = \frac{9}{5}(T(K)-273.15)+32\end{array} \)

Example 1: Converting different temperature scales
The room temperature is generally defined to be 25ºC. What is the room temperature in ºF? What is the room temperature in K?

Solution for 1: To convert from Celsius to Fahrenheit,

we use the equation:

\(\begin{array}{l}T_{F^o} = \frac{9}{5}T_{c^o}+32\end{array} \)

Substituting the values in the equation, we get

\(\begin{array}{l}T_{F^o} = \frac{9}{5}T_{25^o \,C}+32=77^0\,F\end{array} \)

Solution for 2: To convert from Celsius to Kelvin,

we use the equation:

\(\begin{array}{l}T_{K} = T_{C^o}+273.15\end{array} \)

Substituting the values in the equation, we get

\(\begin{array}{l}T_{K} = T_{25^o\,C}+273.15=298\,K\end{array} \)

Thermal Equilibrium

Did you know that thermometers take their own temperature, not the temperature of the object they are measuring?

Thermometers work on the fact that any two systems placed in thermal contact (meaning heat transfer can occur between them) will reach the same temperature. The heat will flow from the hotter object to the cooler object until they reach the same temperature. The objects are then said to be in thermal equilibrium, and no further changes will occur. Systems interact and change because of temperature differences, and these changes stop once their temperatures are the same. Thermal equilibrium is established when two bodies are in contact with each other and can freely exchange energy.

Absolute Temperature

Absolute temperature is the temperature of a body on a scale where 0 is considered as absolute zero. Kelvin is one of the most common absolute temperature scales. The temperature at which a body is in the lowest possible energy state is called absolute zero. When molecules reach this extreme temperature, their motion drops to zero value. In the case of molecules, their kinetic energy will reduce to zero or negligible. Absolute zero is also the lowest temperature that a gas thermometer can detect. At this temperature, no conventional electronic devices work, and no living organisms can survive.

Effects of Temperature

An increase or decrease in temperature causes various changes in the physical and chemical processes of life. Some of those are given below-

  • It affects the solubility, density, vapour pressure and physical properties of various materials along with the electrical conductivity
  • The rate of a chemical reaction is also affected by the temperature.
  • The thermal radiation from the surface of objects is also affected by temperature.

Watch the video and learn more about room temperature

Frequently Asked Questions – FAQs


Do substances expand or contract as they get hotter?

As the temperature rises, substances expand. Heat causes the molecules and atoms of a body to vibrate faster. As the atoms vibrate faster, the space between atoms increases. The motion and spacing of the particles determine the state of matter of the substance. The end result of increased molecular motion is that the object expands and takes up more space.


Which state of matter has the highest temperature?

Gas has the highest temperature.


Which temperature scale is the most common throughout the world?

Celsius is the most common temperature scale in the world.


What is the coldest temperature possible?

Absolute zero is the coldest temperature possible.


Which temperature scale is most commonly used by scientists?

Kelvin is the most commonly used temperature scale by scientists.

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