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What is Temperature?

a clinical thermometer

Temperature is a measure of the internal energy within a substance, commonly referred to as heat. In simple terms, temperature is the measure of the hotness or the coldness of an object. Something that feels hotter generally has a higher temperature and vice versa. Thermodynamics, the branch of physics which deals with the conversion of different forms of energy is dependent on a principal factor which is temperature. There is a series of thermodynamic laws defined based on the movement of heat or the heat flow between objects. Two objects are said to have the same temperature if no heat flows between them. Otherwise heat flows from the hotter to the colder body. This is defined as the zeroeth law of thermodynamics. At an atomic or particle level temperature can be defined as the average energy in each degree of freedom in the particles in a system. The temperature of a solid is found by the vibrations of the atoms inside the body.

The SI unit of temperature is Celsius (named after Swedish astronomer Anders Celsius, and also known by the older name of degrees Centigrade. Thermodynamics uses the Kelvin scale of measurement which is the Celsius scale offset so that 0K is absolute zero or -273.15 Celsius. Absolute zero is a theoretical temperature at which matter has zero entropy. In addition the older Fahrenheit scale is also widely used. Originally the centigrade scale was defined in terms of the freezing and boiling points of water (0 and 100 degrees respectively). Current international standards now define the Celsius scale more accurately in terms of absolute zero and the triple point of water.

Measuring Temperature

Temperature is measured using thermometers that may consist of a simple bulb containing a liquid that expands into a capillary tube, and can be viewed against a graduated temperature scale. Other ways of measuring temperature use bimetallic strips that deflect and move an indicating pointer across a scale.

Electronic thermometers use either thermocouple, or a temperature sensitive resistor or semiconductor device. They have the advantage that the sensor may be located some distance from the electronics .

Because so many properties are dependent on temperature, there are many other ways of building thermometers. These may use, for example, variations in density, thermo-electric properties, measurement of emitted radiation (infrared thermometers, pyrometers) or liquid crystals.

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