Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot. When a heat transfer path between them is open, heat spontaneously flows from bodies of a higher temperature to bodies of lower temperature. The flow rate increases with the temperature difference, while no heat will be exchanged between bodies of the same temperature, which are then said to be in "thermal equilibrium".
In thermodynamics, in a system of which the entropy is considered as an independent externally controlled variable, absolute, or thermodynamic, temperature is defined as the derivative of the internal energy with respect to the entropy.
In an ideal gas, the constituent molecules do not show internal excitations. They move according to Newton's first law of motion, freely and independently of one another, except during collisions that last for negligibly short times. The temperature of an ideal gas is proportional to the mean translational kinetic energy of its molecules.
Quantitatively, temperature is measured with thermometers, which may be calibrated to a variety of temperature scales.
Temperature plays an important role in all fields of natural science, including physics, geology, chemistry, atmospheric sciences and biology.
Read more about Temperature: Use in Science, Temperature Scales, Thermodynamic Approach To Temperature, Statistical Mechanics Approach To Temperature, Basic Theory, Heat Capacity, Temperature Measurement, Theoretical Foundation, Examples of Temperature
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—Henry David Thoreau (18171862)