Pressure (separate science only) Flashcards
Explain what pressure is
how do you calculate pressure?
Example calculation: what is the pressure when a force of 25 N acts over an area of 0.05m2?
Pressure is the average effect of the forces exerted by the particles in a fluid on the wall of their container over a surface area
Pressure can be calculated by this formula:
Pressure = force/area
Pressure is measured in pascals (Pa)
Force is measured in Newtons (N)
area is measured in meters squared (m2)
Example:
Pressure = force/area
= 25/0.05
= 500 Pa
What happens to the pressure an object exerts on the ground it is standing on if
- The mass is increased
- The surface area of the base is increased
- If the mass is increased then the pressure will become greater. This is becuase thewe sill be a greater weight force acting over the same area
- If the area is increased then the pressure will go down. This is becuase the force is now spread out over a larger area, so the average force in any one place is lower.
How do you convert a measurement in cm2 into m2 for use in a pressure calculation?
as 1 cm is 0.01m 1 cm2 is 0.01m x 0.01 m = 0.0001 m2
This means that you have to divide the measurement in cm2 by 10,000 to turn it into m<span>2</span>
Explain why the pressure of a liquid increases with depth, for example in a lake or a sea
The pressure in a liquid is caused by the weight of the liquid above it. The deeper you go the greater the mass of the water on top of you. This means that there is a greater weight force pushing down on the water you are in, and that force is transferred to you.
As the surface area that the force is acting over (the area of the bottom of the lake/sea you are in) remains the same, this causes the pressure to increase with depth
What is the equation that links the depth of any point in a liquid to the pressure felt at this point
eg: water has a density of 1000kg/m3, what is the depth at which you will feel a pressure of 35000 pa
The density of a column of fluid can be found from the equation:
pressure = height x density x gravitational field strength
pressure is measured in pascals (Pa)
height is measured in meters (m)
density is measured in kilograms per meter cubed (kg/m3)
gravitational field strength is measued in newtons per kilogram (N/kg)
Example:
P = h x p x g
h = P/(p x g)
= 35000/(1000 x 10)
= 3.5m
Explain how to derive the equation for the pressure at any point in a liquid
- Pressure can be found from this equation:
Pressure = force/area
- The force is the weight force of the liquid on top of the area:
W = m x g
- the mass of the water can be found from the volume and density of the liquid
m = p x V
- the volume can be found from the heigh multplied nu the area of the base
V = h x A
- substitute equation 4 into equation 3 to find the mass
m = p x h x A
- Substitude this into the equation for weight
W = p x h x A x g
- Substiute this into the pressure equation
P = (p x h x A x g)/A
- Cancel off the areas
P = p x g x h
Explain why a dam has to have thicker walls at the bottom compared to the top
The deeper the fluid gets, the greater the pressure pushing on the walls. This means that there needs to be thicker walls deeper down to cope with the higher pressure of the fluid at that depth.
Describe how atmospheric pressure changes with height from the surface of the earth
The higher you go, the lower the atmospheric pressure. This is becuase at greater altitudes there is less air pushing down from above on the ground below. This means that there is a lower force and therefore a lower pressure.
Explain why the density of air is greater at sea level than is is at the top of a mountain
The density of the air is greater when there is more air on top pushing it down. This is becuase the weight of the air on top pushes the particles closer together.
As you go up a mountain you are getting higher so there is less air above you pushing down. This means that the particles are not pushed as close together so the pressure is less
Explain how to calculate the force of upthrust acting on an object.
Example calculation: water has a density of 1000 kg/m3. Calculate the force of upthust on an object of volume 2m3
The force of upthrust is equal to the weight of the fluid displaced. It can be found from this equation:
Upthrust = mass of fluid displaced x gravitational field strength
= density of fluid x volume x gravitational field strength
Example:
upthrust = p x V x g
= 1000 x 2 x 10
= 20,000 N
How does the increase in the density of the air in the atmoshphere closer to the surface of the earth cause the increase in the pressure?
Pressure is caused by collisions of the particles in the fluid with the sides of the container, in this case the floor and objects on it. At greater depths there is a greater density, which means that there are more particles hitting the same area.
As more particles hit the same area, and each one exerts a force, the combined force exerted over the same area is greater. This means that the pressure increases
Explain how pressure differences cause the lift force that keeps an aeroplane in the air
The shape of the wing means that there is a lower pressure at the bottom of the wing as there is at the top of the wing. This means that there is a greater force pushing upwards from underneath than there is acting on the top of the wing pushing down.
The result is an upward force that balances the downwards force of the weight of the airplane.
Explain why the equation pressure = h x p x g cannot be used to calculate changes in atmospheric pressure over a vely large change of height.
This is becuase the strength of gravity changes with height from the surface of the earth. This has no effect over small distances but has a significant effect over large distances.
As the value of g is different over different heights, the change of height cannot be used to calculate the change of pressure accurately.
Describe the sizes of the forces that are acting when an object is floating.
a) completely submerged
b) on the surface
a) When something is totally submerged and floating, the upthrust force is equal to the weight force of the object
b) When something is floating on the surface, the force of upthrust balances the weight force, but is lower than the maximum upthrust. When is it is submerged the object floats upwards, after it breaks the surface, it is displacing less water as some of the volume is out the water. this reuces the upthrust force until it is floating high enough that the weight force balances the upthrust.
How does the density of the object and the fluid relate to whether or not something will float or sink?
An object will float if the density of the object is lower than the density of the fluid
An object will sink if the density of the object is greater than the density of the fluid
Explain how to experimentally determine the force of upthrust on an object
- Hang the object from a Newton meter and measure the weight force
- Submerge the object in the liquid and measure the weight force with it submerged
- Calculate the difference in the weight forces in and out of the liquid. This difference is equal to the upthrust force provided by the liquid
