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Different gas laws relate the pressure of a gas to its volume, its temperature, or both. A rise in pressure affects both the melting point and the boiling point of a substance, raising the melting and boiling points of most substances. In the case of water, however, an increase in pressure lowers its melting point so that the pressure of a skate blade on an ice surface causes the ice below it to be converted to the liquid state (see states of matter; expansion). Bernoulli's principle relates the effect of the velocity of a fluid on the pressure within the fluid.
Buoyancy
A body immersed in a fluid experiences a larger upward pressure on its lower surface than a downward pressure on its upper surface because of the difference in height or depth between the two surfaces; this difference in pressure results in a buoyant force that pushes the body upward (see Archimedes' principle). If the weight of the body is less than the buoyant force, the body will rise; if the weight is greater, the body will sink. The buoyant effect of this pressure may be noted in the rise of balloons or other objects filled with gases, such as hydrogen or helium, that are less dense than air.
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