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					Gas Pressure




               D. Crowley, 2007
                                         Monday, September 17, 2012


Gas Pressure
To understand what causes gas pressure
   Diffusion
    Look at the diagram which shows bromine gas and air contacting
     each other.
    The models show the direction the particles take - which one is
     correct?
             Air                             The cover was removed,
                                             and after about an hour the
                                             glass jar looked like this

             Cover




             Bromine




Model 1                  Model 2                     Model 3
Diffusion
 Diffusion is the net movement of particles from an area of high
  concentration, to an area of low concentration
 So the particles gradually move from where there are lots of
  them, to areas where there are less (net just means the overall)
 It’s a bit like lots of football fans leaving a stadium after a game.
  At the stadium they are in a high concentration, but as they
  leave, they ‘diffuse’ into the streets and travel home, to where
  there are less football fans (low concentration)




                       Diffusion - net movement from a high
                       concentration to a low concentration
Tyres
 When you ride your bike its important to have your tyres
  pumped up - but why do you do this?
 Has anyone ever ridden over bumpy ground with flat tyres?
  What was it like?
 Pumping up your bike tyres cushions you against bumps when
  you ride, and this is all because of gas pressure…
Balloon
 Draw an outline of a balloon - can you draw the particles, and explain
  what is going on?
 Why does the balloon keep its shape when you blow it up? Why does it
  loose its shape after a few days? Why can it pop if you blow it up too
  much?
Balloon
 Draw an outline of a balloon - can you draw the particles, and explain what is
  going on?
 Why does the balloon keep its shape when you blow it up? Why does it loose its
  shape after a few days? Why can it pop if you blow it up too much?


                                                        Air is a gas. When you blow a balloon
                                                        up, many gas particles get trapped
                                                        inside.

                                                        The particles move around quickly and
                                                        randomly, but they often collide with
                                                        other gas particles, as well as the side
                                                        of the balloon.

                                                        When these gas particles hit the side
                                                        of the balloon they cause pressure.
                                                        The more these particles hit the side,
                                                        the greater the pressure becomes.

                                                        This is why when you pump up a tyre
                                                        too much, or blow a balloon up too
                                                        much, it can burst - you’ve caused too
                                                        many particles to crash into the side of
                                                        the tyre / balloon, causing the pressure
                                                        to get too high!

                                                        Eventually the balloon will shrink - this
                                                        is because they are not completely air
                                                        tight. As air escape, the pressure
                                                        decreases as less particles are
                                                        colliding with the sides (and it shrinks)
Gas pressure
 Gas pressure causes a balloon / tyre to keep its shape


 The pressure is caused by the trapped gas particles colliding into the
   sides of the container they are in

 The more particles there are in there, the greater the pressure becomes
   (until you try to put too many particles in, when often the balloon pops)!




                                   Gas particles hit the wall of their
                                   container, causing pressure
Heat
What about heat?

   Two balloons, of equal size, will experience different temperatures - one will be
    placed in some warm water, the other in cold water…

 What do you think will happen to the particles inside the balloon if it is heated?
  (hint: think about what the gas particles do when they get hot)
 What do you think will happen to the particles inside if the balloon is cooled?


   If a gas is heated up, its particles move around more quickly. They will hit the
    sides of the balloon harder, and more often. This will then increase the pressure
    (and the balloon expands)

   The opposite happens when you cool it - the particles move slower, crash into
    the sides with less force, and less often, decreasing the pressure (so the balloon
    shrinks)
Custard bomb…
   So why do balloons and tyres pop if you put too much air in? Why do aerosol
    cans explode when they get hot?

   Blow up a balloon too much and you’ll have so many particles hitting the side of
    it that the pressure will be too great - causing the balloon to pop.

   If a gas is heated up, its particles move around more quickly. They will hit the
    sides of the container harder, and more often. This will then increase the pressure
    - heat it up too much, and you’ll create too much pressure inside, causing the
    container to explode.
Gas Pressure
 Complete gas pressure 1 - stick this into you book…
Gas Pressure
    Get your neighbours book, and mark
     the answer in - 1 mark per correct
     answer…

1)   The particles in a gas are moving
     around all the time. The particles bump
     into the sides of their container. The
     force of the particles hitting the sides
     causes pressure. If you put more
     particles into a container, there will be
     more particles to collide with the
     walls, and the pressure will increase

2a) The air pressure inside will decrease
    (less particles colliding into the
    container)

2b) The bottle will collapse inwards
    because the pressure on the outside
    (pushing the bottle in) is now much
    greater than the pressure on the inside
    (pushing the bottle out).

				
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