Pressure inside a car cabing lower than atm. ?

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  • #1
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Main Question or Discussion Point

Say we have a highly idealized though experiment where a container of ideal gas is moving at constant velocity through an atmosphere (of the same gas, same temp. etc). Now if this container is sleek enough to not disturb the air and there is no turbulence, then will the pressure inside the container be lower than the pressure of the atmosphere?
 

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  • #2
rcgldr
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Movement (constant velocity) of the container has no effect on the pressure of the gas inside the container.
 
  • #3
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No. Are you thinking of Bernoulli's principle? That doesn't apply here since you are not comparing pressures along a streamline.
 
  • #4
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This may or may not be relevant to what you are trying to figure out but your title mentions cars which I happen to be somewhat of an expert on. The pressure inside a moving car is higher then the surrounding air. This is done by positioning the ventilation intake in a high pressure area of the cars exterior, usually at the base of the windshield. The reason it is engineered that way is so that any water which may find a point of entry to the vehicle will tend to be pushed back out instead of sucked in.
 
  • #5
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From the perspective of the box it looks like air flowing past it. Think about it a little.
Bernoulli won't help with this.
 
  • #6
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So the container is sealed?

Aside from friction heating causing the internal temperature to rise and therefore pressure to increase (which you've already written off), why would there be any effect on internal pressure?
 
  • #7
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poke a hole in the container (on the face perpendicular to the velocity)
 
  • #8
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poke a hole in the container (on the face perpendicular to the velocity)
Are you confusing dynamic pressure with static pressure?
 
  • #9
rcgldr
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poke a hole in the container (on the face perpendicular to the velocity)
That would create the equivalent of a static port, which would "hide" inside the boundary layer at the surface of the car, and the pressure would be the same as the ambient pressure of the outside air, regardless of the speed of the car, as long as the speed isn't enough to invoke effects related to the speed of sound.
 
  • #10
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Then why is the chemical potential higher for particles inside the box?
 
  • #11
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Are you asking a question or posing a riddle?

If it's a question, why don't you put everything your thinking on the table instead of putting silly cryptic one liners.

All they serve to do is boost your post count.
 
  • #12
rcgldr
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Then why is the chemical potential higher for particles inside the box?
The chemical potential energy isn't higher, but in a frame of reference based on the ambient air outside, the kinetic energy of the air inside the box is higher because of it's speed, since kinetic energy is relative to a frame of reference.

If this is leading up to a rocket and it's onboard fuel as an example, ingnoring losses to heat, chemical potential energy is converted into kinetic energy of the rocket and the expelled fuel. Assuming no external forces, it's a closed system, the center of mass of the rocket and expelled fuel does not accelerate, and the total energy (chemical potential energy and kinetic energy of rocket and expelled fuel) of the system remains constant, as observered from any inertial (non-accelerating) frame of reference.
 
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  • #13
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It's an honest question that I've been trying to figure it out for a while. The reason I can't figure it out on my own is because it seems like a little bit of a paradox since the 2 possibilities seem to contradict each other.

And I don't lay out everything I have in my head because I don't want people looking over points and/or going apeshyt and talking past each other, I'd rather have linear logic here.

When I look at chemical potential, it says that the particles in the moving box have a higher value. Boltzmann factor for the particles in the box is also higher.
But if it is true that particles from the box tend to move outside, then how can you explain this physically; i.e. there aren't more frequent collisions with the box boundary than if it was still, so I don't see what's going on.

Honestly I think the problem is pretty simple and I've explained it as best I could.
 
  • #14
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The chemical potential energy isn't higher, but in a frame of reference based on the ambient air outside, the kinetic energy of the air inside the box is higher because of it's speed.
I'm not talking about chemical potential energy, I'm talking about chemical potential, a concept from thermodynamics/stat mech.
 

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