Chapter 5 - Pressure Flashcards
Define “pressure”
The force acting normally per unit area
Formula to calculate pressure
P = F/A
[ pressure = force / area ]
What is the SI unit for pressure?
Pascal (Pa)
1 Pa = 1 ____
N m^-2
Factors affecting pressure
- Area on which force acts
- Magnitude of the force acting on the surface
- Direction of the force acting on the surface
Formula to calculate pressure due to a liquid column
P = hρg
[ pressure due to a liquid column = height of column x density of liquid x gravitational acceleration ]
The pressure in a liquid is ________ along the same horizontal level of the liquid.
the same
The pressure in a liquid is proportional to the ________ and ________ of the liquid.
depth
density
The pressure in a liquid is ________ in all directions
equal
What is the density of water?
1.0g cm^-3 = 1000kg m^-3
What is the density of mercury?
13.6g cm^-3 = 13600kg m^-3
What makes hydraulic systems work?
Liquids are almost incompressible and they transmit pressure equally in all directions (Pascal’s Law)
What is the advantage of using a hydraulic system?
It can multiply the force applied
Formula relating F1, A1, F2, A2
How is this derived?
F1/A1 = F2/A2
- Effort=F1 acts on an Area=A1 to supply an input pressure=P1
- Input pressure is transmitted onto Area=A2
- Since P1 = P2 (output pressure=P2), F1/A1 = F2/A2
Since A2 > A1, it follows that F2 ____ F1
>
Formula relating A1, d1, A2, d2
How is this derived?
d2/d1 = A1/A2
- When F1 is applied, small piston needs to be pushed down a longer distance=d1 for big piston the move up a shorter distance=d2
- Volume of liquid pushed by small piston=V1 is equal to volume of liquid that goes below large piston=V2
- Since V2 = V1, A2d2 = A1d1
Since A2 > A1, it follows that d2 ____ d1
Atmospheric pressure is due to…
the force per unit area exerted against a surface by the weight of air above that surface
In which direction does atmospheric pressure act?
In all directions
What surfaces experience atmospheric pressure?
All surfaces exposed to the atmosphere
What are common non-SI units for atmospheric pressure?
Atmosphere (atm)
Centimetres of mercury (cm Hg)
What is the atmospheric pressure at sea level?
1 atm
76 cm Hg
What happens to the mercury column when more mercury is poured into the reservoir? Why?
As reservoir level rises, the mercury column level will rise by the same amount to maintain the column height h since pressure, density and gravitational acceleration remains constant.
What happens to the mercury column when the glass tube is tilted? Why?
Perpendicular height of mercury column to the reservoir is unchanged as pressure is dependent on the vertical height and not on the length of the column.