I can’t seem to wrap my head around the term pressure.
Vehicle tyres use air pressure, toilets faucets etc use pressure (presumably water pressure),
pressing onto something applies pressure, our blood has pressure, temperature is also affected by “pressure”.
I know there are various types of pressure, and I can’t think of any more examples at the moment, but my point is “pressure” sounds like a very arbitrary or vague umbrella term to me.
Help me make sense of it?
In: 3
So the pressure of inclosed gases are derived from the kinetic gas theory.
We model a gass closed into a container as a buch of balls with some velocity for each. We model it as an ideal gas which means that these little balls don’t interact with each other. (Or there are only elastic collisions between the balls, these are not quantitatively different. There are more realistic gas models that do consider interaction but it’s not a huge difference.)
Another thing about an ideal gas is that the velocity distribution over the particles are dependent on temperature. We’d expect a normal distribution (bell/Gauss curve) since very high/low velocities are unlikely. (This is the Maxwell-Boltzmann distribution.)
So we have some pretty solid average velocity for the tremendous amounts of particles. These particles collide with the walls of the container and exert a force. We can use conservation of energy/momentum to calculate the average force on the walls of the container. So we have some average force over some area so we norm for the the area to give us force over unit area, which is by definition what pressure is.
Of course as with many things in thermodynamics we make a bunch of approximations and take averages but since we usually consider a lot of particles, because of the law of large numbers, the deviations tend to cancel all together so averages fit reality really well. This microscopic model is pretty good, but for the aforementioned reasons macroscopic equations work accurately. These macroscopic equations relate pressure, volume, temperature, etc. These things. These are called equations of state, considering that a thermodinamical system is in equilibrium these quantities are related in some specific way. But of course all of these quantities come from the microscopic behaviour of particles, mostly their motion and mass.
Pressure is a force over an area.
1 Pa = 1 N/m^2
A knife can cut through things because it can transfer a small amount of force to such a small area that it creates incredible pressure.
You can also think about it as a potential energy per unit volume.
1 Pa = 1 J/m^3
Water in a pipe is under pressure. If I want that water to go uphill, I need to give it energy. That the energy to move a certain volume of water is the pressure.
Gas molecules oppose each-other like magnets with the poles facing together. The gravity of the earth pulls downward on gas molecules which results in atmospheric pressure. A balance is found between the force of gravity and the opposition between the molecules.
If you force these molecules together even more, there is pressure that’s even greater than atmospheric pressure, like inside of tires. This causes the compressed gas to exert outward force from the inside of the tire, which helps it retain it’s shape while holding the weight of a car.
Pressure is basically any force that interacts with an object, that “presses” on it perpendicularly. The formula for pressure is P = F/A, where P is pressure, F is Force and A is area. Pressure will increase as the area decreases or the force increases.
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