4.5.5.2 Atmospheric Pressure

Question 1

What is the atmosphere?

Answer 1

• the atmosphere is a thin layer (relative to the size of the Earth) of air around the Earth

Question 2

Where is the atmosphere the most and least dense?

Answer 2

• atmosphere is most dense at the surface of the Earth
• at the Earth’s surface there are the greatest number of air molecules in a given volume
• the atmosphere gets less dense with the increase in altitude (as we move away from the surface of the Earth)

Question 3

What is atmospheric pressure caused by?

Answer 3

air molecules colliding with a surface creates atmospheric pressure

Question 4

Why does atmospheric pressure decrease in altitude?

Answer 4

• air molecules colliding with surface creates pressure
• at increasing altitude distance between molecules increases as as altitude increases there are fewer molecules
• so number of collisions with a surface decreases / so weight of air above less than weight of air below

Question 5

Gas pressure:

Answer 5

• when gas particles collide with the walls of their container they exert a force on the wall
• the total force exerted by the particles per unit area is gas pressure

Question 6

What is the motion of gases like?

Answer 6

in constant random motion

Question 7

What is the speed of gases?

Answer 7

• high speed (range of speeds)
  
  • but high speed on average

Question 8

Why can’t you apply p = hpg to atmosphere?

Answer 8

can’t apply it to atmosphere as density isn’t uniform unlike in sea

Question 9

Why do mountain climbers often take a supply of oxygen with them?

Answer 9

• mountain climbers often take a supply of oxygen with them
• as the air pressure on a mountain may be too low to supply all the oxygen needed

Question 10

Why does a packet of crisps on board an aeroplane enlarge?

Answer 10

• at ground level the air pressure of the packet inside the bag is equal to the air pressure outside the bag
• however, when aeroplanes are flying, the air pressure in the cabin is kept at a slightly lower level than the air pressure on the ground
• this means that the air pressure inside the bag is greater than the air pressure inside the cabin causing the bag to inflate and increase its volume to have an equal air pressure to the cabin

Question 11

Why can you boil water at a low temperature on Mt Everest?

Answer 11

• as there is a very low atmospheric pressure on Mt Everest due to the number of air molecules (and so weight of the air) above a surface decreasing as the height of the surface above the ground level increases meaning there are fewer molecules to collide with any surface (therefore pressure decreases)
• meaning the water does not need to increase in temperature very much to be equal in pressure to the atmospheric pressure and boil

Question 12

What happens to a helium balloon when you release it?

Answer 12

• the balloon rises due to upthrust which is greater than the weight of the balloon causing there to be a resultant upwards force due to helium having a lower density than air
• the atmospheric pressure decreases with altitude
• the pressure inside the balloon remains the same - the pressure inside the balloon is caused by particles colliding with the inside surface of the balloon
• as the balloon rises it will expand and have a larger volume allowing it to have a lower pressure and become equal to the atmospheric pressure outside the balloon
• this causes the tension in the balloon to increase and eventually burst