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Current time:0:00Total duration:7:51

Pressure and the simple mercury barometer

Video transcript

we typically consider matter to exist in three distinct states we've got solids we've got liquids and we've got gases and of those three probably the least often considered are gases and maybe that's because we can't often see or hold gases too well but it doesn't mean that gases aren't important in fact the very air that you're breathing right now is a gas so at a macroscopic kind of big picture level the gas phase refers to a system in which the molecules don't have a definite shape or volume so I've got kind of a cloud of gas here and it it doesn't have a shape or a volume to it and at the nitty-gritty kind of microscopic level we're talking about a substance with with molecules that are freely moving through space so these particles are moving kind of freely through space and this means that the energy of their motion which is their kinetic energy is greater than the potential energy of the intermolecular forces that might hold these molecules together as a solid now one of the more intuitive properties of a gas is that it exerts pressure on its surroundings so a gas exerts pressure on the surroundings and for example when you inflate a balloon the gas inside exerts a pressure on the elastic sides making them kind of grow taut and then eventually causing them to expand and so what is this pressure that's causing the the gas in the balloon to expand well pressure is actually a measure of force per unit area so what's actually going on here is that the little particles in here are moving around the container so they have a velocity they're moving around the container and when they collide against the container they exert a force on the area of collision and while one little particle collision might not have a huge amount of pressure when you think about the huge amount of particles and the huge number of collisions we pretty easily get enough pressure to fill the balloon and the neat thing about balloons is we typically don't fill them up a little bit we can fill them up a lot and the more we fill them up the taller the sides get indicating that that they have more pressure so what might cause the pressure to increase well according to our little pressure formula here pressure is directly proportional to the force so let's think about what would cause the force to increase well we know that force is equal to the mass times acceleration so if we increase the magnitude of the acceleration we can increase the force because force and force and acceleration are also directly proportional that means if we have a greater change in velocity because that's what acceleration is it's a change in velocity so if we have a greater change in velocity during the collision we can increase the force and that means that the faster our little particles are moving the greater the force and thus the greater the pressure and if you remember from our last video we said that temperature was a measure of the average kinetic energy of the particles so really what we're saying is that when we increase the temperature we're increasing the pressure that the gas exerts so what else might cause the pressure to increase well if the total pressure is the sum of those at little individual collisions more collisions would mean more pressure but how do we increase the number of collisions one method might be to add more particles because more particles means more collisions so more moles of gas remember that that moles is simply referring to the number of particles more moles of gas means more pressure now we could also increase the frequency of these collisions by making the container smaller because the particles would have less space to move around and would therefore hit the sides of the container more frequently so if we decrease the volume if we decrease the space of the container we're going to increase the pressure so we can change the pressure of a gas but how do we measure those changes well a long time ago in 1643 a former student of Galileo named Evangelista Torricelli so tor tor ocelli asked same question while he was trying to measure the changing pressure of the gas in our atmosphere and he solved the problem by inventing the torch Elian barometer and barometer is is a device that measures pressure and so he took a glass tube and he filled it up with mercury so he took a glass tube then he filled it up with mercury and he quickly flipped it flipped it over he flipped the tube over and he stuck the open end into an open dish of mercury so he stuck the tube into an open dish of mercury open and down and interestingly enough most of the mercury stayed in the tube and the mercury stayed in the tube even though it was trying to flow out because as it tries to flow out it exerts a pressure on the mercury in the dish which which then causes the mercury in the dish to push upward against the air and when the pressure of the rising mercury meets the pressure of the atmosphere pushing down on the liquid surface the mercury that's that's in the tube it can't flow anymore so the pressure in the atmosphere traps some of the mercury inside the tube and we see that at sea level the height of the mercury that's left in the column is about 760 millimeters now if we were to measure the height of the column on top of a giant mountain the column would be shorter because there isn't as much air pushing down on the open liquid so more mercury from from the column could escape in fact on the ski slopes of a Breckenridge Colorado the height of the column would only be about 520 millimeters so with less weight of the atmosphere pushing down on the surface of the mercury in the dish more can escape so the amount in the column is shorter in order to get a baseline for comparison we say that the pressure of the atmosphere at sea level is one standard atmosphere or 180 N and because a lot of manometers which are kind of generic devices that measure pressure use mercury pressures often measured in millimeters of mercury which we know that one atmosphere because it's at sea level would equal 760 mm of mercury and in honor of Torah chellie this millimeters of mercury unit is often called a torso one atmosphere equals 760 millimeters of mercury which equals 760 Torr now because pressure is a measure of force per area and that and the SI unit for force is the Newton and the SI unit for area is the square meter we have another unit of pressure which is the Newton per square meter and we call that the Pascal but if we measure the pressure of the atmosphere at sea level in Pascal we get one atmosphere is equal to 101,325 pascals and so it's worth quickly mentioning Pascal's because it might be a unit of pressure that allows us to translate into other standard units but mostly because this number is annoyingly large we typically measure pressure in atmosphere or tor especially when we're dealing with the pressure of gases