P1.3 - Pressure

• Particle theory

  • Matter is made up of very small, constantly moving particles
  • The warmer something is the more these particles are moving
• Gas particles


  • They are free to move around in completely random directions
  • They randomly bang into each other and whatever else gets in the way, like the walls of their container
  • They are very small, but they still have a mass
  • When they collide with something, they exert a force on it
• Pressure of gases

  1. Collisions cause a net force on the inside surface of the container

  2. The force acting per unit area is the pressure
• The more particles there are in a given volume


  The more often they'll collide with the walls of the container, and with each other, so the higher the pressure will be
• Changing the Temperature changes the pressure
• Gas pressure

  • Depends on how fast the particles are going
  • Depends on how often the particles hit the walls
• If you heat a gas in a sealed container

  • Energy is transferred to the kinetic energy stores of the gas particles
  • The particles move faster
  • The particles hit the container walls harder and more often
  • The pressure increases
• If the gas is cooled

  • The particles have less energy
  • The particles move less quickly
  • The particles hit the walls with less force and less often
  • The pressure is reduced
• Constant temperature:

  1) If you put the same amount of gas in a bigger container (i.e.. increase the volume), the pressure will decrease, as there'll be fewer collisions between the gas particles and the container's walls. 

  2) If you reduce the volume, the particles get more squashed up and so they hit the walls more often, hence the pressure increases. 

  3) For a fixed amount of gas at a constant temperature, this leads to the equation: Pressure (Pa) x volume (m²) = constant 
• A change in pressure can cause a change in volume:


  1) A gas exerts a force on its container due to collisions between the particles and the walls of the container. These collisions happen in random directions, but add together to produce a net force at right angles to the wall of the container: 

  2) If the pressure of a gas is increased (e.g. by heating it) this force increases. If the pressure of a gas is decreased (e.g. by cooling it) this force decreases  

  3) If a gas is in a container that can change volume e.g. a balloon, this can change the volume of the container, and so the gas.
• Unless it's in a vacuum, the outside of a gas container is also under pressure from whatever's around it e.g. atmospheric pressure from the air
• If a balloon isn't expanding or contracting then the pressure (and force) of the gas pushing outwards is equal to the pressure (and force) of the air outside the balloon pushing inwards
• If you increase the pressure of the gas inside the balloon then the force pushing outwards will be higher than the force pushing inwards – there's a net outwards force on the walls of the balloon, this causes the balloon to expand
• As the balloon expands

  1. The gas particles inside it hit the walls less often

  2. This causes the pressure inside the balloon to decrease
• Once the pressure inside the balloon has fallen back to the same level as atmospheric pressure, the balloon will stop expanding
• If you reduce the pressure inside the balloon then the pressure on the outside of the balloon will be bigger than the pressure on the inside, so there'll be a net inward force and the balloon will shrink
• As the balloon shrinks

  1. The pressure inside the container rises

  2. As the particles of the gas hit the walls of the container more often

  3. Until pressure inside = pressure outside again
• What happens when you change the pressure acting on a gas from outside its container?

  -You can compress a syringe filled with air by pushing hard on the plunger.

  -The volume of the gas in the syringe will decrease until its pressure is equal to the pressure you're exerting on the plunger. 
• Pressure of a balloon:

  A) inward

  B) outside

  C) outward

  D) inside

  
• Doing work on a gas can increases its internal energy which increases its temperature
• Doing work on a gas

  • Can be done mechanically, e.g. with a bike pump
• Doing work on a gas with a bike pump

  1. Gas exerts pressure on the plunger so exerts a force on it

  2. Work must be done against this force to push down the plunger

  3. This transfers energy to the kinetic energy stores of the gas particles so increases the internal energy and therefore the temperature.
• Pumping up a tyre

  Transfers energy from the gas to the thermal energy store of the tyre, making the tyre warmer
• The Earth's atmosphere stretches to roughly 100 km above the Earth's surface
• Atmospheric pressure


  • The weight of the air high in the atmosphere pushes down on the air around us
  • Atmospheric pressure acts in all directions
  • We can't feel atmospheric pressure because it's always there and equal in every direction
• • The lower you are, the more atmosphere there is above you, so the higher the atmospheric pressure is.
  • If you gain height, there's less atmosphere above you, so the atmospheric pressure lowers
• Atmospheric pressure is affected by the density of the atmosphere, which also varies with height
• The density of the atmosphere gets higher the closer you are to sea level
• The density of the atmosphere gets higher the closer you are to sea level. This is because the weight of the air above pushes down on the air below it, compressing it
• Other factors like temperature also affect the density of air, but you can assume the density of the atmosphere is uniform at a given height
• Atmospheric pressure:

  A) sea level

  B) high up

  
• Liquid Pressure Causes Upthrust and Makes Things Float: 


  • If you submerge (or partially submerge) an object in a liquid, it experiences liquid pressure from all directions due to the particles of the liquid. 
  • This pressure increases with depth, due to the weight of the 'column' of liquid directly above the object. 
• Liquids can't be compressed, so their density is the same everywhere (unlike gases). 
• The pressure at a given depth equation:

  Pressure due to a column of liquid (Pa) = height of column (m) × density of liquid (kg/m³) x g (N/kg) 

  • Here g is the gravitational field strength. It's equal to about 10 N/kg  
  • You can also use this equation to find the difference in pressure between two depths - difference in pressure = difference in depth x density x g. 
• 1. As pressure in a liquid increases with depth, the force pushing upwards on the bottom of an object due to the liquid pressure is greater than the force pushing down at the top of the object. 
  2. This causes an overall upwards force, called upthrust.  
  3. The upthrust acting on an object is equal to the weight of fluid it has displaced. If this upthrust is equal to the object's weight, then the object will float. If the upthrust is less than the object's weight, it will sink.
• To make an object float, you need to make it less dense than the liquid you're trying to float it on - so it will have displaced a volume of water with a weight equal to its own weight before it's completely submerged.