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.
What happens to the mercury column when the barometer is taken to the top of a mountain? Why?
Mercury column height decreases. Lower atmospheric pressure at higher altitudes.
What happens to the mercury column when water is introduced into the space above the mercury column? Why?
Mercury column height decreases. Water vaporises and sets up vapour pressure in the space above the mercury column and presses downwards on the mercury column. Column indicates the pressure difference between atmosphere and water vapour pressure.
What happens to the mercury column when there is a crack in the glass tube along the mercury column above the mercury reservoir? Why?
Height of mercury decreases to the same level as the reservoir. Air outside the tube at the crack is at atmospheric pressure while the pressure inside the tube is lower than atmospheric pressure. The air will move from a higher pressure region to a lower pressure and so enters the column. The air only stops when pressure difference inside and outside the column is zero, hence height difference will be zero.
Concept of pressure in a drinking straw
- When you suck, your lungs expand and pressure in the mouth decreases
- The greater atmospheric pressure pushes down on the surface of the liquid and so forces the liquid up to your mouth
Concept of pressure in a rubber sucker
- When the rubber sucker is pressed into the smooth surface, air is removed and a near vacuum is created behind the sucker
- Sucker stays in place because atmospheric pressure > pressure of vacuum
What is excess pressure?
The difference in the gas pressure and the atmospheric pressure
Calculating gas pressure from manometer (if gas pressure > atmospheric pressure)
PG = PA + h
[ gas pressure = atmospheric pressure + height difference ]
Calculating gas pressure from manometer (if gas pressure
PG = PA - h
[ gas pressure = atmospheric pressure - height difference ]
State Boyle’s Law
For a fixed mass of gas at a constant temperature, the volume V of the gas is inversely proportional to its pressure P.
Formula (Boyle’s Law)
PiVi = PfVf
where the subscripts i and f indicate the initial and final conditions respectively
When a capillary tube with a column of air trapped by a column of mercury is placed horizontally, what is the pressure acting on the column of air?
Pressure = PA
When a capillary tube with a column of air trapped by a column of mercury is placed vertically, what is the pressure acting on the column of air?
Pressure = PA + h
When a capillary tube with a column of air trapped by a column of mercury is placed vertically upside down, what is the pressure acting on the column of air?
Pressure = PA - h
