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
Fluid (liquid and gas) particles are always in constant and random motion. When they bounce off of each other and hit the walls of their container, they exert a certain amount of force on it. The average amount of force exerted by these particles per unit area is defined as pressure. When something’s temperature increases, it gains kinetic energy, also known as ‘movement’ energy. When something’s kinetic energy increases, it moves faster. As you can guess, when particles begin to move faster, they bounce off of each other more often and collide with their surroundings at faster speeds, exerting a higher force. Thus, pressure increases with temperature.
Pressure is the total force in an area divided by the area itself. So you can think of it as the force intensity over a certain area.
If the pressure is 10N/m^2, then that pressure existing on an object with the area of 1m^2 results in that object experiencing 10N of force. Pressure is a useful measure because it normalizes the calculation by area and thus the outcome can be scaled easily.
The formula for pressure is force per unit of area. There is pressure whenever there is a force applied to an area. What force and what area is arbitrary, so the definition is kinda vague.
Usually the force is a mechanical one. Air pressure in a tyre is related to the force air has inside the tyre. Blood pressure is related to the force blood is pumped in your veins and arteries. And so on.
You actually got it in your response: pressure is basically how hard something is pressing against something else. It’s the “press” in pressure.
Air pressure in your tires is how hard the air is pushing against your tire walls, water pressure in a faucet is how hard the water is pressing against the air as it exits, blood pressure measures how hard your blood is pushing against your blood vessel walls, etc.
Intuitively, pressure is a measure of how much the particles in a liquid or gas (or even a solid sometimes) are pushing on their neighbors because of crowdedness. Particles can be crowded because there’s a lot of them packed into a tight space, or because they’re hot so they want to take up more space, or both.
Numerically, pressure is measured by asking how many units of force it pushes with, per unit of surface area. One common pressure unit is PSI, or “pounds per square inch.” If you inflate your bike tire to 50 psi, that means that each square-inch of rubber in the tire, is receiving 50 pounds of outward force from the air inside it.
When you get small enough, everything has a force that attracts things and a force that repels them. The attraction force keeps the parts of things like atoms from flying apart. The repel force keeps them all from crunching together. You can think of those forces like tiny springs.
Imagine you have a ball made of springs. If it’s out in the open it’s in equilibrium – it’s not flying away or collapsing in. Now imagine you take that ball in your hands and start squeezing it down. The more you crush it the more the repelling force pushes against you. The tighter you squeeze, the harder it gets to push even more.
What you feel when the springs are trying to push outward is pressure. The more you can squeeze it the harder those springs will try to get back to equilibrium, so the more pressure you will feel
A good way to start thinking about it intuitively is to consider laying a brick on the back of your hand. No big deal. Now consider putting a pebble on the back of your hand before you put the brick on. Not very comfortable. Now consider standing up a needle and trying to balance the brick on that. Ouchie.
So each time it got worse, the brick weighed the same. It got worse because that weight was being applied to a smaller and smaller area on your hand. We effectively increased the pressure each time we chose a smaller area of contact. That’s why pressure is always measuring force and area (pounds per square inch)
It’s why snow shoes work. Knives. Submarines. Hydraulics. The list is very long, but the idea is always about changing either the force being applied, or the area over which it is applied.
Pressure is the application of force.
When air is compressed, it applies outward pressure on the container. (Air tank, Tire, Balloon) Depending on the design and materials used, the container may expand (balloon).
Fluid can also be pressurized. This is how water flows through a pipe, and it is also how blood moves through your veins and arteries. In a public water system, water towers use gravity to help maintain the water pressure in the pipes below the tank.
Pressure is affected by temperature as when items are warmed, they expand. When they cool, they contract. Heating water in a pressure cooker causes the water to boil, turning some of the water into steam. The heat causes water to transition from a liquid to a gas (steam) and this builds pressure because steam is compressible.
Pressure can also be sensed as stress. Stress is a reaction to pressure. In a person, feeling pressure can make you upset or tense. In an object, like a balloon, the stress is in the tension of the rubber. If the stress is too high, the balloon breaks.
Most have already captured what pressure is mathematically. Your other question in how it’s related to temperature is fairly straightforward too if you think about it through the ideal gas law. PV=nRT
P, pressure
V, volume
n, number of moles of the compound
R, constant fixed number
T, temperature
For the purposes of this think of both n and R as constant.
As you increase temperature, if you are in a fixed space (the volume is constant), then the pressure shares a linear correlation with the temperature. And inversely, if the space has room to expand/contract the pressure will remain constant as temperature fluctuates
This is why tires need to be filled up more frequently in the winter. The outside air lowers the PSI because of the fixed volume. And it’s also why you can visibly see a flat tire, once the pressure is low enough, it actually occupies less volume.
imagine you get hit by a big ball with some speed on your back
you experience a push
what if instead of one ball you get hit by multiple smaller balls? if the smaller balls together where the same weight as the ball before and they hit you at the same time the push feels somewhat the same.
but if the hits are a bit spaced apart its just smaller balls hitting you repeatedly so easier to not fall back.
If you now start decreasing the ballsize/weight but also the amount of balls, while they are still flying with the same speed against your back you stop feeling the individual hits but just a general force.
You might also notice that if the balls where to keep hitting the same spot on your back instead of your entire back it feels more intense.
This motivates us to define this intensity now called pressure as some force over an area, which is of interest for gases and liquids as they are just “small balls” (atoms/molecules) flying around, colliding with each other and also its surrounding.
The reason why pressure, volume, temperature and particle number are related is following :
Temperature gets first defined as some unchanging property of a very big system. Then you find a condition when a given system has the same temperature as this very big system with set temperature. And then you find out that temperature is related with the average kinetic energy of the particles in your system of interest.
so pressure is what you experience as a wall of the container which is related to the speed of the balls (it gets more complicated as the balls dont have an uniform speed but a distribution with an average speed) while Temperature is related to the square of the average speed.
The ideal gas law pV ~ NT (proportional) that the energy needed to create the volume by pushing against the pressure p (energy is if the force is constant F* distance, where force is equal to pressure times area -> E= F*d=(p*A) * d = p * V) is proportional to the entire kinetic energy (temperature related to average kinetic energy times particle number)
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