Ch 6 Pressure

• Surface area vs contact area

  Contact area: area touching

  Surface area: areas that can be seen
• What is the relationship between force and pressure?

  -the greater the magnitude of force, the greater the pressure
• If the weight of a man is 700N, each foot area is 0.50m^2, he is standing on 2 feet. Calculate pressure exerted by 1 foot.

  M1: 700 N / 2 = 350 N (Weight of each foot)

  350 N / 0.50 m^2 = 700 N/m^2

  M2: Pressure = total weight / total contact area

  = 700 N / 0.50 m^2 * 2

  = 700 N/m^2
• How does hydraulic system work?

  -Force 1 applied on piston 1, enclosed liquid experiences a pressure 1 of applied force 1 / contact area 1 (of piston 1)

  -Pressure 1 is transmitted equally to other parts of the enclosed liquid and to piston 2

  -Pressure 2 exerted on piston 2 is the same as pressure 1

  -Force exerted by piston 2 is pressure 2 * contact area 2
• How do you find force exerted on the second piston?

  Force 2 = Force 1 * Area 2 / Area 1
• What is the force needed to lift the car of mass 800kg?

  Radius of piston 1 = 10cm

  Radius of piston 2 = 100cm

  800kg = 8000 N

  Contact area of piston 1 = pi * (10)^2 = 100 pi cm^2

  Contact area of piston 2 = pi * (100)^2 = 10 000 pi cm^2

  Pressure at 1 = Pressure at 2

  Force 1 / 100 pi = 8000 / 10 000 pi

  Force 1 / 100 = 8 / 10

  Force 1 = 80 N
• How do you determine the density of a liquid?

  1.Weigh empty measuring cylinder with calibrated electronic balance

  2.Pour liquid into measuring cylinder and weigh both measuring cylinder and liquid

  3.Find mass of liquid by subtraction of readings

  4.Measure volume of liquid using measuring cylinder

  5.Calculate density
• When is a displacement can used?

  When mouth of measuring cylinder is too small
• How do you level a liquid in a displacement can accurately?

  -Pour liquid into can until it overflows

  -Wait until liquid drains from can and levels
• What is the application of density?

  -Identify substances

  -Identify purity of substances
• Water pressure is exerted in all directions
• What is the relationship between depth and pressure? Why?

  -The lower the depth, the greater the pressure

  -pressure underwater comes from weight of water above

  -Deeper-> weight of water is greater, thus an increase in water pressure
• Where do you find height when calculating pressure

  from surface of liquid to point given in the question/bottom of liquid

  => depth
• What is the water pressure at air-liquid surface?

  -Zero -> height = 0

  -Only atmospheric pressure is exerted

  -air-liquid surface does not experience water pressure
• When is there no atmospheric pressure?

  -in space

  -in enclosed spaces (vacuum)
• What is the purpose of a mercury barometer?


  -measure any gas pressure
• What is in the space above mercury in a barometer?

  a vacuum
• What is the density of mercury?

  13600 kg/m^3
• What happens when the pressure inside mercury is equal to atmospheric pressure?

  mercury does not move upwards anymore
• What happens to the length of mercury when the tube is lowered?

  unchanged
• What happens to the length of mercury when the tube is tilted?

  unchanged
• What happens to the length of mercury when the tube is narrower?

  unchanged
• What happens to the length of mercury when the tube is lifted?

  unchanged
• What is the total pressure made up of at this point?

  Water pressure + atmospheric pressure

  
• What is the pressure

  1. at A?
  2. at B?

  1.Atmospheric pressure

  -> 0 mmHg

  2. No atmospheric pressure

  -> typically 76 mmHg (DO NOT MEMORISE)

  
• What does it mean when the liquid is

  a)at point A?

  b)At point B?

  a)Pressure of gas is higher than atmospheric pressure

  b)Pressure of gas is lower than atmospheric pressure

  
• What happens when pressure at A is equal to gas pressure?

  liquid in manometer stops rising

  
• How do you find pressure at point A?

  Pressure at A - atmospheric pressure = height * density * gravitational field strength

  
• How do you find the pressure of gas in a manometer?

  Pressure of gas - atmospheric pressure = height * density * gravitational field strength

  Pressure of gas = height * density * gravitational field strength + atmospheric pressure
• Where is the height of the manometer?

  B

  
• Which of the 2 barometers is faulty of atmospheric pressure is 76mmHg

  A

  
• What is the pressure of the trapped air in the tube?

  Pressure of trapped air = atmospheric pressure

  
• Why is the liquid not falling down?

  Pressure due to trapped air and liquid = atmospheric pressure

  
• Which of the 2 barometers is faulty if atmospheric pressure is 76mmHg? Why?


  B (barometer with lower mercury column is ALWAYS the faulty one)

  -> there is gas in the space above mercury that is exerting pressure on the mercury

  -> causing its height to be lower