P1.3 Pressure Flashcards
Earth’s atmosphere
700 km deep
100,000 Pa = 100kPa
Protects the Earth, thickness of atmosphere very thin compared to earths diameter
So if you pump air out of a sealed can the can collapses because lower pressure in the can.
When the can is not sealed, equal forces act on it so it does not collapse
Why gases exert pressure
Particles collide with walls of their container
When they collide they each exert outward force on the walls
If material cannot stretch then pressure increases
How temperature affects gas pressure
When temperature increases, average speed of gas particles increases
More frequent collisions = larger force over a certain area meaning pressure increases
When temperature decreases, gas pressure decreases
Pressure
Pascals (Pa)
1Pa = 1N/m²
1kPa = 1000Pa
Extrapolation
Dotted line on a graph continuing from a straight line that is an estimate on what would happen if you continue to change the independent variable
How is pressure and volume related
(Inversely proportional)
As volume doubles, pressure halves
As volume halves, pressure doubles
Inversely proportional because collisions of gas particles produce force that is a right angles to surface
Halving volume means you double number of collisions per second between gas particles and container
How doing work on a gas affects temperature
Internal energy of a gas increased by:
Heating
Doing work on it
If you apply a force to a pump and move it in you do work on the gas and it gets hotter
Average speed of particles will increase because their kinetic energy increases when they collide with moving piston, so temperature is higher
What happens to atmospheric pressure as you go up
Decreases - less air particles pushing down exerting force
NOTE: A pressure gauge will always read atmospheric pressure
Thermometers will always read room temperature
Liquid pressure
Water molecules are very close together
Collide with each other, container, and anything in the water.
Liquid pressure acts in all directions
As you go deeper, water pressure increases: can be shown with holes in a bottle, water from holes at bottom travels further horizontally
Pressure at particular depth will be greater in denser liquid, as greater weight of liquid pushing down
The pressure due to a column of liquid is proportional to the depth or height
Gas pressure equations
Initial pressure x Initial volume = final pressure x final volume
Pressure = density x 10N/kg x height
Liquid pressure equation
Pressure (Pa) = height of column(m) x density of liquid (kg/m ³) x 10N/kg
P = h x d x 10
Difference in pressure between 2 depths = larger pressure - smaller pressure
Total pressure is atmospheric pressure + liquid pressure
Why there is upward force on a floating object (upthrust)
To balance its weight
Water exerts a bigger upwards force on the body than the downwards force exerted by the air
Pressure at top of object is less than pressure at bottom of object
Weight of floating object = pressure at bottom x area at bottom - pressure at top x area at top
Why some objects float and others sink
Submarines float at different depths in the ocean
If pressure difference together with the area is big enough then net force will be enough to balance the weight
When submarines are on the surface, tank are full of air making water go into the tanks increases the weight and there is not enough upthrust to balance it so it sinks
To reach surface again, tanks are filled with compressed air to remove water and weight so that upthrust balances it
A ship has a mass of 20 million kg
The area in contact with the water is 20000m²
Estimate the depth at which the ship floats
The density of water is 1000 kg/m³ and gravitational field strength is 10 N/kg
For the ship to float the upthrust = pressure difference x area = weight
Step 1: Write down what you know.
mass of ship = 20 million kg = 20 000 000 kg
area = 20000 m ²
density of water = 1000 kg/m
gravitational field strength = 10 N/kg
Step 2: Calculate the weight of the ship.
weight of ship = 20000 000 kg x 10 N/kg
= 200 000 000 N
Step 3: Equate the equations for pressure, make depth the subject,
and calculate it.
pressure in water
= depth × density × gravitational field strength force exerted
= pressure x area
= depth × density × gravitational field strength × area
So, weight of ship
= depth × density × gravitational field strength x area
and depth = weight / (density x gravitational field strength x area)
Depth = 200 000 000N / (1000kg/m³ x 10N/kg x 20 000 m²)
Depth = 1m (1sf)
Convert mass (kg) to weight (N)
Mass x 10N/kg = Weight
