4.5.5.1 Pressure in a fluid / atmosphere

what is a fluid

A fluid can be either a liquid or a gas.

The pressure in fluids causes a force normal (at right angles) to any surface.

particles in a gas

widely spaced

move rapidly

collide with each other and the walls of the container

when they collide with the walls they exert a force acting a right angles to the walls

equation for pressure

pressure = force normal to a surface/ area of that surface

*p* = *F/A*

pressure, *p*, in pascals, Pa

force, *F*, in newtons, N \n area, *A*, in metres squared, m2

what is atmosphere

thin layer of air round the earth

at the earths surface atmosphere is most dense- greater number of air molecules in a given volume

less dense as altitude increases

what is the pressure of the atmosphere caused by

air molecules colliding with a surface

relationship between height and atmospheric pressure

atmospheric pressure decreases with an increase of height

Air molecules colliding with a surface create atmospheric pressure.

The number of air molecules (and so the weight of air) above a surface decreases as the height of the surface above ground level increases.

So as height increases there is always less air above a surface than there is at a lower height. So atmospheric pressure decreases with an increase in height.

==the higher the elevation there are fewer air molecules above the unit of area than the same area at lower heights so there is a smaller weight so less pressure==

idealised assumptions, for a simple model of the atmosphere

isothermal, so it is all at the same temperature

Transparent to solar radiation

opaque terrestrial radiation

why do packets swell up in aeroplanes

at ground level the air pressure inside the bag is the same as the air pressure outside the bag

when aeroplanes are flying, the air pressure in the cabin is kept at a slightly Lower level than the air pressure on the ground

this means that the air pressure inside the bag is now grater than the air pressure in the cabin

causing the bag to inflate

why do mountain climbers take oxygen with them

air pressure on a mountain may be too low to supply all the oxygen the need

equation for pressure of a column of liquid

pressure = height of the column × density of the liquid × gravitational field strength

\[*p* = *h ρ g* \] \n pressure, *p*, in pascals, Pa \n height of the column, *h*, in metres, m \n density, *ρ*, in kilograms per metre cubed, kg/m3

gravitational field strength, g, in newtons per kilogram, N/kg (In any calculation the value of the gravitational field strength (*g*) will be given.)

when does pressure of a liquid increase

pressure of liquid increases with depth (height of column) because as the depth increases there is a greater weight of liquid acting downwards

pressure of liquid increases with density of liquid because liquids with a greater density have a greater weight acting downwards

factors determining pressure

* the weight of the water above abject felt as a downward force, as pressure due too weight increases as there will be more water above the object


* denser the liquid, the larger the mass per unit of volume the larger its weight will be
* gfs- bigger gfs, larger weight

upthrust

a partially or totally submerged object experiences a greater pressure on the bottom surface than on top of the surface. this creates a resultant force upwards which is known as upthrust

something sinks if

weight is larger

something floats if

upthrust is larger

how to calculate upthrust

if density of object:

if more dense, the object will sink

if less dense it will float