Pressure Chapter Notes | Physics for GCSE/IGCSE - Year 11 PDF Download

 Page 1


Forces and Matter
? Hooke’s Law
? Load is proportional to extension of a stretched spring
? ?x (extension) = stretched length - original length
? F = k?x
? F = force
? k = spring constant
? ?x = extension
? Spring constant
? Force/ extension
? S.I unit is N/m
? ? Parallel springs  k = k1 + k2
? ? Joined springs  1/k = 1/k1 + 1/k2
? Force is directly proportional to extension
? Once spring loses elasticity and deforms, it doesn’t go back to original form
? Force applied per unit area is known as stress
? The extent of stretching/ compression produced as a material responds to stress is known as
strain
? ? Extension  F= kx
? ? Compression  F = -kx (just a notation, not actually applied)
? Types of deformation
? Elastic deformation
¦ When the stress is removed the material returns to the dimension it had before
the load was applied. Deformation is reversible
? Plastic deformation
¦ When large stress is applied to a material, when removed, the material does not
spring back to the previous dimension. Permanent and irreversible deformation
? Minimal value of stress which produces plastic deformation is known as of the elastic limit
material
Page 2


Forces and Matter
? Hooke’s Law
? Load is proportional to extension of a stretched spring
? ?x (extension) = stretched length - original length
? F = k?x
? F = force
? k = spring constant
? ?x = extension
? Spring constant
? Force/ extension
? S.I unit is N/m
? ? Parallel springs  k = k1 + k2
? ? Joined springs  1/k = 1/k1 + 1/k2
? Force is directly proportional to extension
? Once spring loses elasticity and deforms, it doesn’t go back to original form
? Force applied per unit area is known as stress
? The extent of stretching/ compression produced as a material responds to stress is known as
strain
? ? Extension  F= kx
? ? Compression  F = -kx (just a notation, not actually applied)
? Types of deformation
? Elastic deformation
¦ When the stress is removed the material returns to the dimension it had before
the load was applied. Deformation is reversible
? Plastic deformation
¦ When large stress is applied to a material, when removed, the material does not
spring back to the previous dimension. Permanent and irreversible deformation
? Minimal value of stress which produces plastic deformation is known as of the elastic limit
material
Pressure
? Pressure = Force/ area
? S.I unit is N/m2 or Pascal(Pa)
? 1 N/m2 = 1 pascal
? Solids
? Pressure = Force/Area
? ? Less area  More pressure
? Remember to always turn cm to m when finding pressure
? Liquids
? P = hdg
¦ h = height(metres)
¦ d = density(kg/m3)
¦ g = gravity(10N/kg on earth)
? Atmospheric pressure is the pressure exerted by air/ atmosphere
? Pressure caused by liquid = hdg
? Pressure in a liquid = pressure caused by liquid + Atmospheric pressure
? Relations
? Density is directly proportional to pressure
? Height is directly proportional to pressure
? Gravity is directly proportional to pressure
Manometer
? An instrument used to measure the pressure between 2 gases or liquids.
Its a U-shaped tube holding a small amount of liquid
? When both ends are open, the level of the liquid on either side is equal.
? If one end is connected to a gas supply, the gas pushes down on the
liquid and forces it round the bend
? The levels are now unequal, showing that there is a pressure difference
? The greater the pressure, the further it is pushed
? When atmospheric pressure is greater than the gas supply, then it is
known as suction
? Suction = atmospheric pressure - difference in height
? Gas supply = atmospheric pressure + difference in height
Page 3


Forces and Matter
? Hooke’s Law
? Load is proportional to extension of a stretched spring
? ?x (extension) = stretched length - original length
? F = k?x
? F = force
? k = spring constant
? ?x = extension
? Spring constant
? Force/ extension
? S.I unit is N/m
? ? Parallel springs  k = k1 + k2
? ? Joined springs  1/k = 1/k1 + 1/k2
? Force is directly proportional to extension
? Once spring loses elasticity and deforms, it doesn’t go back to original form
? Force applied per unit area is known as stress
? The extent of stretching/ compression produced as a material responds to stress is known as
strain
? ? Extension  F= kx
? ? Compression  F = -kx (just a notation, not actually applied)
? Types of deformation
? Elastic deformation
¦ When the stress is removed the material returns to the dimension it had before
the load was applied. Deformation is reversible
? Plastic deformation
¦ When large stress is applied to a material, when removed, the material does not
spring back to the previous dimension. Permanent and irreversible deformation
? Minimal value of stress which produces plastic deformation is known as of the elastic limit
material
Pressure
? Pressure = Force/ area
? S.I unit is N/m2 or Pascal(Pa)
? 1 N/m2 = 1 pascal
? Solids
? Pressure = Force/Area
? ? Less area  More pressure
? Remember to always turn cm to m when finding pressure
? Liquids
? P = hdg
¦ h = height(metres)
¦ d = density(kg/m3)
¦ g = gravity(10N/kg on earth)
? Atmospheric pressure is the pressure exerted by air/ atmosphere
? Pressure caused by liquid = hdg
? Pressure in a liquid = pressure caused by liquid + Atmospheric pressure
? Relations
? Density is directly proportional to pressure
? Height is directly proportional to pressure
? Gravity is directly proportional to pressure
Manometer
? An instrument used to measure the pressure between 2 gases or liquids.
Its a U-shaped tube holding a small amount of liquid
? When both ends are open, the level of the liquid on either side is equal.
? If one end is connected to a gas supply, the gas pushes down on the
liquid and forces it round the bend
? The levels are now unequal, showing that there is a pressure difference
? The greater the pressure, the further it is pushed
? When atmospheric pressure is greater than the gas supply, then it is
known as suction
? Suction = atmospheric pressure - difference in height
? Gas supply = atmospheric pressure + difference in height
Barometer
? An instrument used to measure atmospheric pressure
? Consists of glass tube at least 80cm in length
? The tube is filled with mercury(Hg) and then carefully inverted
into a trough containing mercury
? Once the tube is safely inverted, the level of the Hg in the
glass tube drops
? Length of the Hg column is measured from the surface of the
Hg in the trough is about 76cm
? The space above the Hg column is a vacuum
? As atmospheric pressure decreases, length BD decreases
Both point J and K exert the same pressure