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The article on the vacuum cleaner is located elsewhere. In physics, a vacuum is the absence of matter in a volume of space. A partial vacuum is expressed in units of pressure. The SI unit of pressure is the pascal (abbreviated to Pa in usage). It can also be expressed using the torr, using the barometer scale, or as a percentage of atmospheric pressure using the bar. The antithesis of a vacuum, which is technically unachievable, is called a plenum. A perfect vacuum is not in the least obtainable in a laboratory; much of outer space is supposed to consist of an almost perfect vacuum, with a small number of molecules per cubic metre.
Degrees of vacuum
Creating a vacuumWhen creating a partial vacuum, the matter in the volume being evacuated flows differently at different pressures based on the laws of fluid dynamics. Initially a vacuum pump can be used to remove the material, as the molecules are interacting with each other and will push on their neighboring molecules in what is known as viscous flow. When the distance between the molecules increases, the molecules interact with the walls of the chamber more often than the other molecules, and compression pumping is no longer effective. At this stage, the system has entered a state called molecular flow, where the velocity of each molecule is approximately random. Methods to remove the remaining gas include:
At extremely low pressures, outgassing of the vacuum vessel occurs over time. Even if a high vacuum is generated in a hermetically sealed container, there is no guarantee that an adequately low pressure will continue unless outgassing has been accounted for. Outgassing is generally worse at higher temperatures. Even materials which are not naively considered absorbent will outgas. Water vapor is a primary outgas component, even in hard metal vessels (such as stainless steel or titanium). Outgassing can be reduced by desiccation prior to vacuum pumping. Vessels lined with a highly gas-permeable material such as palladium (which is a high-capacity hydrogen sponge) create special outgassing problems. To achieve ultra-high vacuum, the vessels are heated to a few hundred degrees celsius so the outgassing is faster. The gases coming out of the vessels are removed by pumps until the majority of the gas molecules have been removed, then the temperature can be lowered again. The process of outgassing would take unacceptably large amounts of time if perfomed at room temperature. The lowest pressures currently achievable in laboratory are about 10-13 Pa. The quantum-mechanical vacuumQuantum physics reveals that even an ideal vacuum, with a measured pressure of zero torr, isn't really empty. One reason is that the walls of the vacuum chamber emit light in the form of black-body radiation: visible light if they are at a temperature of thousands of degrees, infrared light if they are cooler. This soup of photons will be in thermodynamic equilibrium with the walls, and the vacuum can consequently be said to have a particular temperature. More fundamentally, there are quantum-mechanical fluctuations in the vacuum. This may be responsible for the observed value of the cosmological constant. Historical interpretationHistorically, there has been much dispute over whether such a thing as a vacuum can exist. Ancient Greek philosophers did not like to admit the existence of a vacuum, asking themselves "how can 'nothing' be something?". Plato found the idea of a vacuum inconievable: he believed that all physical things were instantiations of an abstract Platonic ideal and could not imagine an "ideal" form of a vacuum. Similarly, Aristotle considered that the creation of a vacuum was impossible - nothing could not be something. Later Greek philosophers thought that a vacuum could exist outside the cosmos, but not inside it. In the Medieval period, the idea of a vacuum was thought to be immoral or even heretical. The absence of anything implied the absence of God, and hearkened back to the void prior to the story of creqation in the book of Genesis. Medieval thought-experiments into the idea of a vacuum considered whether a vacuum was present, if only for an instant, between two flat plates when they were rapidly separated. There was much discussion of whether the air moved in quickly enough as the plates were separated, or, following William Burley whether a 'celestial agent' prevented the vacuum arising - i.e. whether nature abhorred a vacuum. After the Paris condemnations of Bishop Tempier, which required there to be no restrictions on teh powers of God, Following work by Galileo, Torricelli argued in 1643 that there was a vacuum at the top of a mercury barometer, and Robert Boyle later conducted experiments on the effects of a vacuum (for example, a canary would become unconscious, but revive when air was reintroduced). In 1654, Otto von Guericke conducted his famous Magdeburg hemispheresexperiment, showing that teams of horses could not separate two hemisphere from which the air had been evacuated. Concurrently, theories of the nature of light had concentrated on the idea of a luminiferous aether which would be the medium to convey waves of light (Newton's corpuscular theory having fallen out of favour). The experiment of Michelson and Morley in 1881, using an interferometer to attempt to detect the change in the speed of light caused by the Earth moving with respect to the aether, was a famous null result, showing that there really was no pervasive medium thoughout space. See alsoExternal links
ca:Buit da:Vakuum de:Vakuum et:Vaakum fr:Vide he:ריק it:Vuoto (fisica) nl:Vacuüm ja:真空 nds:Vakuum pl:Próżnia pt:Vácuo sv:Vakuum |
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