Atmospheric pressure is the force per unit area exerted by an atmospheric column of air above a particular point.
Atmospheric pressure at sea level (1 Patm) is:
- 76cm Hg or 760mm Hg, or
- 101.3kPa or approx. 105 Pa
Category: Pressure
Define pressure
Pressure is force acting perpendicularly per unit area of a surface.
Describe how a barometer work.
A barometer consist of a long glass tube and a liquid reservoir, usually mecury.
The tube is first submerged in the reservoir, and completely filled with the liquid in the reservoir.
Then, the tube is held inverted to a vertical position, with the mouth of the tube still in the liquid reservoir, and a pocket of vacuum above the liquid column inside the tube.
The atmospheric pressure exerts a force on the surface of the liquid reservoir, pushing the liquid up the tube. The pressure of the liquid column in the tube will be equal to the atmospheric pressure.
Describe how a hydraulic press work.
A hydraulic press consists of two pistons of different sizes.
The space between the two pistons is filled completely with a liquid, usually hydraulic oil.
When a force is applied on the smaller piston, the pressure exerted on the liquid at the smaller piston is transmitted equally throughout the liquid.
Hence, the same pressure is also exerted at the bigger piston.
Since P = F/A, the F/A ratio will be the same at both pistons.
Since A1 < A2, hence F1 < F2.
So a smaller effort on the smaller piston (F1) will be able to lift a bigger load at the bigger piston (F2).
Pressure in a syringe
The work done at the barrel is equal to the work done at the nozzle.
Word Done = Force x Distance.
The force at the barrel > the force at the nozzle, so for equal work done,
distance moved by water leaving barrel per second < distance moved by water leaving the nozzle per second.
This results in a greater speed of the water leaving the nozzle.
Alternative Answer:
Volume of water leaving the barrel per second = Volume of water leaving the nozzle per second.
The cylindrical volume = Base area x distance moved by the water.
Since base area of barrel > base area of nozzle, so for equal volumes,
distance moved by water leaving barrel per second < distance moved by water leaving nozzle per second.
This results in a greater speed of the water leaving the nozzle.