External Pressure on Fluids

Pressure Pressure is the amount of force applied over a given area on an object. When pressure is applied to matter, compression can result. Compression is a decrease in volume produced by a force. The tennis racket applies a force to the ball. The resulting pressure causes the ball to compress.

Gases Are Compressible A gas can easily be compressed because there is a large amount of space between its particles. Gas that is trapped in a container and heated will increase in pressure Heat causes the particles to move faster - these fast moving particles bounce off the sides of the container The increased pressure could cause the container to explode. Gas that is trapped in a container and cooled will decrease in pressure. The decreased pressure could cause the container to implode. This metal can has imploded as the gas inside is cooled.

Liquids and Solids Are Very Difficult to Compress The particles of liquids and solids are already so tightly packed together that squeezing them together is almost impossible. Solids and liquids are described as incompressible When force is applied to the bottle, the gas particles move closer together. The gas is compressed into a smaller volume. A bottle filled with liquid When force is applied to the bottle, the liquid does not compress. There is no room for the liquid particles to move closer together A bottle filled with gas

Compression and Deformation Solids can appear to be compressed if the “air spaces” in the material are compressed. An example would be squishing (compressing) a marshmallow. Solids can also appear to be compressed when they are deformed Deformation means to change shape without being forced into a smaller volume. Example: a ball hitting a solid surface As the ball deforms, it stores elastic energy which makes it bounce back upwards The player’s face and the ball are temporarily compressed and deformed.

Comparing Pressure Pressure depends on both the amount of force and also the area the force acts upon.

Fluids Under Pressure Fluids always move from high pressure to low pressure Fluids under pressure and compressed gases are used for a variety of everyday tasks Air molecules pulled by gravity = atmospheric pressure Air pressure increases as altitude decreases The more air there is above, the more it compresses the air molecules below Therefore, air pressure is lower (less dense) at high altitudes. When humans change altitude, our bodies try to equalize the pressure differences by having our ears “pop” (fewer air particles pressing from outside our body)

Pressure Differences Fluids will always attempt to move from high pressure to low pressure When we drink with a straw, we first remove air from the closed straw, which lowers the pressure inside. The atmosphere, having a higher pressure, then tries to get into the straw, and pushes the fluid up and out of the way to try to get into the straw! This same idea is used for many purposes, including hydraulics, water rockets and dental tools.

Changing Temperature Changing temperature of a fluid can affects its volume. In general, as a fluid gets colder, it contracts (its volume decreases.) As a fluid gets warmer, it expands (its volume increases.) Changing the temperature of a fluid can also affect pressure. As a fluid gets warmer, the particles move faster. In a sealed container (where the volume cannot change), the particles hit the container walls with more force, and pressure increases. When temperature drops, the particles move more slowly. The particles collide less with the container, and the pressure decreases.