Ch. 8.3 Pressure in Fluids

Pressure• pressure – the amount of force applied to a

given area– air, just like water, is a fluid (any substance that

flows)– air exerts pressure around you all the time– air pressure is the reason our ears “pop” when

changing altitude quickly• when outside air pressure changes, the air inside your

middle ear has to adjust, resulting in the popping sound

Changes in Pressure

• air pressure changes with altitude– the layers of the Earth’s atmosphere extend more

than 160 km above the surface of the Earth– all those air particles are being pulled towards the

surface of the Earth by the force of gravity– when we move to higher altitudes, there is less air

particles above us pushing down on us, and so, less air pressure

Pressure and AltitudeIn

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As the balloon rises in altitude (or elevation), the air pressure outside the balloon decreases, allowing the balloon to increase in size as the air pressure inside the balloon is countered by less force. As the balloon rises, it will continue to grow in size until it pops.

Increasing Air Pressure

Pressure and Depth

• similarly to air pressure changing with altitude, pressure in a fluid also changes with depth

• the greater the depth of water (or fluid), the greater the pressure at that point

• this is why we feel more pressure when swimming on the bottom of the pool than when we swim at the surface

Pressure and Fluid Flow• if a fluid is allowed to flow, it will always go

from an area of higher pressure to an area of lower pressure– we see this in weather systems as well

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Pressure and Temperature• when the temperature of a fluid inside a

container increases, the pressure increases– the pressure exerted by a fluid is the sum of all the

forces exerted by the individual particles in the fluid

– these particles move faster and strike the wall more often and with more energy when the temperature increases

• so, at constant volume, an increase in temperature results in an increase in pressure

Compression• compression – a decrease in volume caused

by a force– when you hit a tennis ball, the force of your

strings changes the shape of the ball– there is a large amount of space between the

particles in a gas like the air in a tennis ball– when a force is applied, the particles have space

to compress

Compressibility vs. Incompressibility• compressibility – the property of being able to

be compressed or made compact• incompressible – not capable of being

compressed (eg. materials in a liquid state)• gases are compressible while liquids are

incompressible, meaning that liquids have less space between particles to compress