Hi. It’s Mr. Andersen and this is AP Physics essentials video 129. It is on gas pressure.
This clock to the left is measuring time. But on the bottom it is measuring air pressure
which would be gas pressure in the atmosphere itself. What is interesting is as it decreases
we get stormy weather, as it increases we get dry weather. Why is that? Well as the
air in the atmosphere is all pushing out, if it is a high pressure it is descending
are we are going to have clear weather. But if it is a low pressure that means the air
is moving up, it will hit a dew point and we are generally going to have cloudy and
rainy weather. Now you could build a barometer if you wanted to. All you do is take some
gas, trap it in a container. We could use a balloon with a rubber band over the surface
of it. So we have a set amount of gas inside the jar. We could then tape a straw to the
top, mark some high and low points on a piece of paper behind the barometer and then as
the air pressure outside the jar increases it is going to be pushing down. And so that
straw is going to go up. This is a high pressure. As it relaxes there is not as much pressure
on the surface. If it is a low air pressure then it is going to let that pressure on the
inside exert a force and it is going to move up, now we are going to measure low pressure.
And if you were to correlate to what the weather is like, when it is low pressure, storm and
cloudy. When it is high pressure it is going to be a clear day. And so gas exerts a force
on the container in which it is in. It is bouncing around and so that force over the
given area is equal to the pressure. How much force is that gas exerting on the surface
of the container. Another way to measure that pressure is it is the change in momentum.
As those gas molecules collide with the container, how much does their momentum change? What
is neat about gas is since it is randomly moving around that gas pressure is going to
be uniform throughout. So if we have one pressure here it is the same as the pressure on the
left side or the pressure on the inside of the container itself. So what is pressure?
Pressure is a force exerted per unit area. So to give you sense of what pressure is,
measured in pascals, imagine I take a box that is a 10 newton box. So this is going
to be a really large box. It is 4 meters on one side, 1 meter on the other and 2 on the
third side. And so how much pressure is exerted on the table? Well we have to calculate the
force, which I have already done. It is 10 newtons. And then we have to figure out what
is the area on which it is pressing. Well if you look at this box and we assume that
the underside is blue as well it is just the blue side that is impacting the table itself.
And so what is our force? It is going to be 10 newtons. What is the area? Well we calculate
the area by taking 4 by 1, length times width, and that is going to be 4 meters squared.
So what is going to be my pressure? It is going to be 2.5 pascals, which is a really
low number. If we were to take that same exact box however and turn it on its side, like
this, would the pressure change? Well for sure. That is because the 10 newton box again,
we are pressing down on a smaller area. And so our force is still going to be 10 newtons.
What is the area? It is the green side now pressing down, 2 times 1. And so our area
is 2 meters squared so our pressure is going to 5.0 pascals. And so if it is a smaller
area we are going to have a greater pressure. Now what is interesting about gas is it is
what is inside the container or it could be the air pressure on the outside of the container
itself. It is not an object. It is actually the molecules colliding that are creating
the pressure. So if we look at this PHET simulation, this measures pressure in atmospheres instead
of pascals. One atmosphere is going to be 101 kilopascals and that gives you a sense
of how much pressure is inside the gas. So if I add some molecules to the container,
what are we getting? Well we are starting to read pressure on the inside. It is colliding
with the sides of the container. As I increase the number of molecules, watch what happens
to the pressure. Again they are colliding with each other in the container and we are
getting an increase in the pressure. How else could we increase the pressure? If we increase
temperature we could increase the pressure. If we decrease the size of the container we
could increase pressure as well. And so what is happening, the molecules are colliding
with the sides of the container and that is what is creating this pressure. What is interesting
is if we take a gauge that measures that, no matter where I put that gauge, even inside
the container, it is going to be uniform throughout. And that is important when you are solving
problems like this. Calculate the force exerted on a piston if we know what the pressure is,
435 pascals and we know the area of the piston itself. So the first thing you have to realize
is that since the pressure is uniform throughout this container, what is going to be the pressure
pushing on the piston? It is 435 pascals. So I set up my formula, pressure equals force
over area. I am solving for force. I know pressure and area and so this is a pretty
easy problem. So I will multiply both sides by area, so I can isolate force on the left
side. And then I simply calculate it by multiplying those two values. So it would a 78 newton
force exerted on that piston. And so did you learn to make claims about how the pressure
is tied the force exerted by the molecules on the area of the container? And then finally,
pressure is force over area and so if you know two of those values you can calculate
the other one. And I hope that was helpful.