P = pressure, Pascal (Pa) F = forces, N A = area, m 2 ( Unit: N m -2 ) State relationship between pressure and area.
Pressure depends on 2 factors:
a) forces acting
b) area of surface
Pressure increases as surface area decreases.
APPLICATION INVOLVING HIGH PRESSURE A sharp knife has a very small surface area on its cutting edge so that high pressure can be exerted to cut the meat. Nails, needles and pins have very sharp ends with very small surface areas . When a force is applied to the head of a nail, the pressure will drive its sharp end into a piece of wood easily. The studs on a football boot have only a small area of contact with the ground . The pressure under the studs is high enough for them to sink into the ground , which gives extra grip.
APPLICATION INVOLVING LOW PRESSURE Skis have a large area to reduce the pressure on the snow so that they do not sink in too far. A tractor moving on soft ground has wide tires to reduce the pressure on the ground so that they will not sink into the ground. A wide shoulder pad of a heavy bag will reduce the pressure exerted on the shoulder of the person carrying the bag.
Atmosphere is a thick layer of air surrounds the Earth.
The air has mass and weight.
The weight of atmosphere acts on surface of objects and atmospheric pressure is produced .
Characteristics of Atmospheric Pressure
1. Varies with the height of the object above sea level .
At higher altitude, the temperature and density of air are lower .
As a result, the frequency of collision of molecules is lower .
Thus, atmospheric pressure is lower .
2. Atmospheric pressure acts in all direction .
At sea level , the atmospheric pressure is about 100 000 Pa (1×10 5 Pa) 1 atmosphere (1 atm) = 10 5 Pa / 10 5 Nm -2 At 5600 m above sea level, the atmospheric pressure is about 50 000 Pa (5×10 4 Pa) The pressure in the space around satellite is almost zero . An aircraft travelling at an altitude of 11000 m will experience an atmospheric pressure of 25 000 Pa (2.5×10 4 Pa) Atmospheric pressure varies with height
Activities to show existence of atmospheric pressure
The water remains in the glass.
This is due to the atmospheric pressure acts on the cardboard pushing it up.
When sucked, air inside straw is removed , leaving a low pressure area .
The higher atmospheric pressure acts on the surface of water and pushed drinks into the straw.
b) Sucker hook
When the sucker hook is press, the air inside is forced out and leaving a low pressure area .
The higher external atmospheric pressure outside pressed down on the hook and makes its stick firmly to the wall.
1) The fan sucks out the air when it is switched on. 2) Space in x become partially vacuum. 3) The higher atmospheric pressure forces the air and dust particles into the dust bag 4) Dust particles are trapped but the air still can flow through the exit. x c) Vacuum cleaner
Instrument for measuring Gas Pressure a) Bourdon Gauge
Mercury barometer contains mercury enclosed in a tube.
Glass tube that has been filled with mercury then inverted into a bowl of mercury.
At sea level or 1 atmospheric pressure, mercury level is at 76 cm Hg or 760 mm Hg.
The level does not drop further because the mercury column is held up by atmospheric pressure .
Atmospheric pressure and its units a) At a sea level, the atmospheric pressure : 1 atm = 760 mm Hg = 76 cm Hg = 0.76 m Hg b) Atmospheric pressure in unit Nm -2 or Pa : = h g =0.76 × 13600 × 9.8 =10 5 Nm -2 / 10 5 Pa
h = height of mercury column
= density of mercury (13600 kgm -3 )
g = gravitational field strength, Nkg -1
c) Atmospheric pressure in unit metre water: P atm = 10 m water