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Temperature Definition Physics

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In Physics, temperature refers to the physical properties of matter, which quantitatively expresses the hotness or coldness of a body. Before the evolution of the concept of temperature, we used to judge temperature on the basis of human perception - whether an object is hot or cold was mostly judged by human touch. However, this is not accurate. For example, during a cold morning, a wooden table might seem warmer than a metal cycle rod. However, both have an equal temperature based on the external atmosphere. Metal, being a better conductor of heat, draws heat out of your body faster, making it cooler than the wood (a poor conductor of heat). A simple temperature definition in science is the physical quantity measured using a thermometer. However, the temperature is related closely to the kinetic energies of the molecules and atoms inside an object.

Temperature and Kinetic Energy

The kinetic energy of an object, as per physics, is the energy that an object possesses due to its motion. Every molecule or atom has kinetic energy. Even in solids, when the molecules are tightly packed, all of them contain vibrational energy. Temperature is referred to as the average kinetic energy of all molecules. When a substance absorbs heat, the molecules start to move faster. This increases its kinetic energy. As a result, the temperature of the substance starts to increase. Eventually, the movement of molecules can result in the separation of the molecules as they start moving farther away from one another. For example, in solids, when the temperature increases, the molecules start moving faster, and this leads to the expansion of the solid. Eventually, if the temperature rises to the melting point of the solid, it can change its state as well. The sum of all kinetic energy in a body is called heat. Heat is a form of energy, and it can get transferred from one body to another.

Example of Temperature 

Many people often think of temperature as energy, which is not true. Temperature is just a measurement that shows the average kinetic energy of one atom or molecule. Hence, when we say something is hot or cold, we are generally using another reference point to define the hotness and coldness of a body. For example, if we say that the coffee is hotter than the ice-cream, we are using the ice-cream as a reference point to determine the temperature of the coffee. During weather reports, temperature and humidity are shown in two different columns. This is because hot and humid days are considered hotter than hot and dry days because of human perception rather than temperature changes. If the temperature of a day is the same, but the humidity is high, then a person might feel hotter since humid days do not evaporate sweat as fast as dry days do. Evaporation of sweat helps us stay cooler. 

What is the Absolute Zero?

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Absolute zero is the lowest possible temperature. It has been marked at 0 Kelvin or -273.15 degrees Celsius or -460 degrees Fahrenheit. At absolute zero temperature, there is no heat energy present in a substance. At this temperature, the particles present in a perfect crystal will remain motionless, that is all kinetic energy in the particle will become zero. Absolute zero is theoretically possible, but practically, we have not reached absolute zero temperatures yet. However, we have been able to reach close to absolute zero by using cryocoolers and dilution refrigerators.

Units of Temperature

Since temperature is a physical quantity, it has units that represent it. The SI unit of temperature is Kelvin. However, we also use two other units to measure temperature - Celsius and Fahrenheit. The units of temperature are related in this manner:

C = 5/9 (F - 32)

K = C + 273

How do Thermometers Measure Temperature? 

Thermometers are generally used to measure the temperature of a body. A thermometer has two important components - a temperature sensor like the bulb of a mercury-in-glass thermometer and a numerical scale. The mercury thermometer works on the principle of thermal expansion. Thermal expansion is the change in the volume of a substance due to the change in temperature. Since mercury is sensitive to temperature changes, it is used in thermometers. An increase in the temperature is quickly registered as the mercury inside the thermometer expands and rises upwards. The numeric scale in the thermometer gives the measurement of the temperature. There are three kinds of thermometers - Celsius Thermometers, Fahrenheit thermometers, and Kelvin Thermometer.

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Temperature Scales

Since thermometers have to be of a specific length, each thermometer has a fixed scale. Celsius thermometers are easy to manufacture as the fixed scale used for them is set at the freezing point and boiling point of water. Celsius thermometers have a lower fixed point set at 0ºC (freezing point of water) and 100ºC (boiling point of water). Fahrenheit thermometers have a lower fixed point at 32 ºF and an upper fixed point at 212 ºF. Kelvin thermometers have a lower fixed point at 273 ºK and an upper fixed point at 373 ºK. From these fixed points, we can derive the relationship between Celsius, Fahrenheit, and Kelvin too

\[\frac{C}{100}\] = \[\frac{F-32}{180}\] = \[\frac{K-273}{100}\]

FAQ (Frequently Asked Questions)

1. How Are Temperature And Hotness Of A Body Related?

The temperature and hotness of a body are related but not the same. Heat refers to the sum of the kinetic energy in the molecules of a body, while the temperature is the measurement of heat in a body. Hence, when the collective kinetic energy of a body increases, that is, when the heat increases, the temperature of the body also increases.

2. What Are Some Effects Of Temperature?

Some of the effects of temperature are:

  • Temperature affects the rate of chemical reactions.

  • Temperature helps in thermal expansion, solubility, density, etc.

  • Temperature affects thermal radiations.

  • Increasing temperature can result in higher collisions between particles.

  • Temperature increases the kinetic energy of molecules and, therefore, activates the changes in the state of substances.