- 3461 - THERMODYNAMICS - definitions of temperature? The zeroth law of “thermodynamics” states that if two bodies are each in thermal equilibrium with some third body, then they are also in equilibrium with each other. “Thermal equilibrium” means that when two bodies are brought into contact with each other and separated by a barrier that is permeable to heat, there will be no transfer of heat from one to the other.
------------- 3461 - THERMODYNAMICS - definitions of temperature?
- In other words, the zeroth law means the three bodies are all the same temperature. James Clerk Maxwell put this perhaps more simply when he said, "All heat is of the same kind." (1875). What is most important is that the “zeroth law” establishes that temperature is a fundamental and measurable property of matter.
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- When the zeroth law was originally conceived in the 18th century, there were already two laws of thermodynamics. However, this new law, which presented a formal definition of temperature, actually superseded the existing laws.
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- The new law the "zeroth law” establishes that "the temperature of two systems is the only thing you need to know in order to determine which direction heat will flow between them."
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- The zeroth law of thermodynamics defines temperature and makes thermometers possible. For a thermometer to be useful, though, it must be first calibrated. All other basic units of measure, such as for length, mass, time, etc. are each defined according to a precise standard. In this case, scientists must not only define a unit of measure, but also the beginning point of the scale.
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- The most notable early efforts to standardize temperature measurement were those of German instrument maker Daniel Gabriel Fahrenheit. In the early 18th century, Fahrenheit invented the familiar glass-tube-type thermometers using both alcohol and mercury.
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- Fahrenheit also invented the Fahrenheit scale, which sets the freezing and boiling points of water at 32 degrees Fahrenheit and 212 degrees Fahrenheit, respectively, and is still used to this day, particularly in the United States.
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- Most of the rest of the world uses the Celsius scale, which assigns values of 0 degrees for the freezing point of water and 100 degrees for its boiling point at mean sea level.
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- All measuring scales used in science and engineering start at a value of zero. The concept of zero length, mass or time is relatively easy to grasp; however, zero temperature, or “absolute zero“, where there is absolutely no heat energy whatsoever, is more difficult to grasp.
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- This is because such a temperature has never been observed in nature or in the laboratory, and it is generally believed that it never will be; however, scientists have gotten very close.
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- The unit for thermodynamic temperature is the kelvin (K) and was originally defined according to the triple point of water. The triple point is defined as " the condition of temperature and pressure under which the gaseous, liquid, and solid phases of a substance can exist in equilibrium".
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- While this standard was convenient for ambient temperatures, it made measurements at very high and very low temperatures uncertain. In 2019, The International Bureau of Weights and Measures changed the standard to be based on the Boltzmann constant (k), which relates temperature to the average energy of the particles in a substance. That definition acknowledges that temperature is a measure of the average energy of molecular motion, according to the U.K.'s National Physical Laboratory.
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- Most thermometers contain liquid or metal that changes volume or shape depending on its temperature. When the liquid or metal reaches thermal equilibrium with the object or substance being measured, the temperature-sensitive property of the material in the thermometer can then be exploited to indicate its temperature.
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- Some types of thermometers use a liquid, typically alcohol or mercury, which expands or contracts with increasing or decreasing temperature. This small expansion is amplified by having a relatively large reservoir of liquid in a glass bulb connected to a long and very narrow glass tube.
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- In this way, a small change in the volume of the liquid in the bulb can cause a large change in the level of the liquid in the tube; the temperature can then be determined by reading the height of the liquid against a calibrated scale.
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- Another type of thermometer relies on the thermal expansion of metal. Again, the problem is how to amplify a very small change in size so it can be read on a scale. One method involves a coil with many loops, so that even a slight change in length gets multiplied by the number of loops and so is readable.
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- Another method exploits the fact that different metals expand at different rates when heated. Strips of two different metals with different coefficients of expansion, the degree to which the material expands for a certain amount of temperature change, can be laminated together so that the assembly will curl when heated. This "curling" can move a needle that can be read against a scale.
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- Another method for measuring temperature involves color changes in temperature-sensitive organic materials. These are typically useful only for measuring limited temperature ranges, such as indicating fever or monitoring room temperature.
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- Another device, called a “thermistor“, works based on changes in the electrical resistance of a semiconducting material due to its temperature. These devices can detect extremely small temperature changes and are used in bolometers, which measure radiation based on a change in temperature, and to monitor laboratory experiments.
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- However, no measurement would be possible without relying on the principle described in the “zeroth law“.
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February 13, 2022 THERMODYNAMICS - temperature? 3461
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