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Electroboom: How Right IS Veritasium?! Don't Electrons Push Each Other??

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electrodacus:

--- Quote from: Naej on July 13, 2022, 08:58:14 am ---Clearly you never learned physics.
It's ok but stop believing you're the best, and that what you say must be true just because you wrote it, or you'll never learn anything with this kind of attitude.

Alright so here's how it works.
I replaced the 1t balls by walls, and this is how the car is propelled: a ball (elastically) bounces on the wall, then on the car, then it is accelerated back towards the wall.

Speeds in ground reference frame:
- wall : 10m/s
- car : 20m/s
- ball : -10m/s

Using 50J for the ball, you get a mass of 1kg.

Because 1kg<<1t, you get a collision like on a wall.
So in the reference frame of the ball, the speed of the bat is (approximately) its opposite.
It is -10-10=-20 m/s and turns into 20m/s, which in the ground reference frame is 30 m/s.

Because 1kg<< the mass of the car, you also get a collision like on a wall.
In the reference frame of the car, the speed of the ball is 30-20=10 m/s, so the reflected ball is at -10 m/s, which in the ground reference frame is 10 m/s.
You need to remove 20 m/s to restart the cycle, so you take the energy for it at the wheel.
In the reference frame of the car, the speed must go from -10 m/s to -30m/s so you need 1/2*1kg*(30²-10²)=400J to accelerate the ball.

The momentum gained is m*delta v=m*(30-(-30))=60 kgm/s, which means you can add delta p*car speed=1200 J to the car kinetic energy in the ground reference frame.
You remove the 400J taken at the wheel and you just added +800J to the car.
Repeat until reaching the desired speed.

Now if you don't believe this:
1) make sure you really understand momentum, reference frames, energy (there are high school physics books out there)
2) tell me where I'm wrong
3) don't talk about supercapacitors

--- End quote ---

You are getting very close to understanding pressure differential energy storage.

There is no wall but there are other ball's that that this ball interacting will vehicle will bounce on to similar to your wall idea.  It looks like a wall a little bit because there are so many small balls in real air compared to 1.2kg balls spaced 1m apart.
The air density is 1.2kg per m3 thus the 1.2kg ball analogi witch is spaced 1m apart.

Now with a non compressible gas there will be no bounce (elastic type collisions) is like balls are already all in contact with each other and can flow around another but will not bounce (there is no compression to be had).

As you explained with the wall analogy this is what can make a higher concentration of balls in the back of the propeller compared to the front or bat if you prefer that.


With a sail vehicle that is stationary you still have a pressure differential between back side and front side of the sail.
Since the balls (air molecules) bounce back of the sail and other balls come from the back there is a higher concentration of balls on the back side of the sail while there are very few balls on the back just what comes in from the side due to low pressure.
The difference is that as the sail vehicle starts to move pushed by all this bouncing balls on the back of the sail the pressure difference drops and by the time vehicle is at wind speed there is no pressure differential at all so same numbers of particles hit the back and front of the sail at random just from thermal motion.

With the propeller connected to wheels vehicle the initial conditions are exactly the same and the blades on the propeller act the same way as the sail.
but as soon as vehicle starts to move pushed by wind part of the kinetic energy the air molecule / ball provided the vehicle with will be put in the propeller so subtracted from what could have been vehicle kinetic energy thus vehicle takes longer to accelerate than the sail vehicle but the fact that the sail hits back (with less energy else vehicle could not move)  means part of the energy is used to maintain this pressure differential (it still drops) but much slower  meaning that by the time the vehicle is at wind speed there is still significant amount of stored energy and as far as vehicle is concerned is like the wind still blows relative to vehicle.


Maybe the treadmill model is even simpler to understand as there is no wind indoors but during the initial phase when you need to keep the vehicle on the treadmill this pressure differential is created  using energy from the treadmill.
As soon as you release the vehicle vehicle can no longer take any energy from treadmill but it has the stored pressure differential that pushes the vehicle against the treadmill direction.
What you called the wall has no 0m/s relative to ground (ground not treadmill surface) but there are particles bouncing between that stationary wall and propeller blades.
But this pressure differential while partly recycled as the vehicle moves forward using some of the energy to supply friction and returning the rest the pressure differential drops and so is the acceleration rate that depends on that.


Sorry for the very long explanation. I just got a bit excited as you start to understand the pressure differential energy storage in compressible fluids.
In a non compressible fluid the balls touch each other and there is no elastic energy to be stored they will just be pushed by the so called wall and so same vehicle with propeller/similar construction in a incompressible fluid could never exceed wind/flow speed.             

PlainName:

--- Quote from: electrodacus on July 13, 2022, 03:58:02 pm ---
--- Quote from: dunkemhigh on July 13, 2022, 08:49:24 am ---Really? Seriously? The propeller has knowledge of its power source?

Either you're using the wrong words (to match your wrong equation) or their real meaning has escaped you.

--- End quote ---

... there is no time where propeller has propulsion role ...


--- End quote ---

That wasn't the question. You've effectively said that if a propeller is turning, it isn't having any interaction with the air. That's like saying air going through a turbine doesn't exert a force on the blades under any circumstances.

Oh, but you allow that if the propeller knows it's powered by a particular source, it can interact with the air.

How can your other stuff make sense if something this basic is misunderstood?

electrodacus:

--- Quote from: dunkemhigh on July 13, 2022, 04:36:46 pm ---
That wasn't the question. You've effectively said that if a propeller is turning, it isn't having any interaction with the air. That's like saying air going through a turbine doesn't exert a force on the blades under any circumstances.

Oh, but you allow that if the propeller knows it's powered by a particular source, it can interact with the air.

How can your other stuff make sense if something this basic is misunderstood?

--- End quote ---

No that is not what I'm saying. Obviously propeller will have interaction with air is just not directly accelerating the vehicle.
You get that wind (moving air particles) is the only energy input in this system.
The only way for air to transfer the energy to vehicle is through collisions with vehicle body and that includes the propeller blades.
As propeller blade start to rotate powered with part of the wind power they will hit back the particles that hit the blades.
If wind / moving air particles provide 1000W based on gear ratio selection a percentage of this will be taken at the wheels say 400W and put in a say 70% efficient propeller.
So remaining 600W is used to increase vehicle kinetic energy while the 400W * 0.7 = 280W will push back against those wind particles mostly increasing the pressure differential (or more like not allowing the pressure differential to decrease that fast).

If this vehicle will have been in a non compressible fluid so that those 280W could be used to increase pressure differential then this design will have been incredibly useless as you will have gotten 600W + 280W = 880W of increase in vehicle kinetic energy instead of just 600W in increased vehicle kinetic energy + 280W put in to storage and so not able to exceed wind / flow speed thus you were better off with an equivalent size sail to get all 1000W instead of just 880W of acceleration.       

Naej:

--- Quote from: electrodacus on July 13, 2022, 04:36:42 pm ---You are getting very close to understanding pressure differential energy storage.

--- End quote ---
Yeah I understand dams and pneumatic systems.
But here there is no fluid, so you can't make up some nonsense about fluid energy storage.
There's only kinetic energy storage.

electrodacus:

--- Quote from: Naej on July 13, 2022, 05:27:34 pm ---
--- Quote from: electrodacus on July 13, 2022, 04:36:42 pm ---You are getting very close to understanding pressure differential energy storage.

--- End quote ---
Yeah I understand dams and pneumatic systems.
But here there is no fluid, so you can't make up some nonsense about fluid energy storage.
There's only kinetic energy storage.

--- End quote ---

You may have seen the graph below. Air is a compressible fluid (gas). Link to source https://en.wikipedia.org/wiki/Axial_fan_design
Do you think that below graph is wrong / incorrect ?
The red line is the propeller as seen from the side and this is how the air(compressible fluid) will look around the blackbird if motion was from right to left.
 If you agree with the fact that there is a pressure difference between the two sides of the propeller swept area (a disk) then you will also agree that will require energy to create and also this pressure differential pulls and pushes the propeller witch is attached to the vehicle so it pulls and pushes the vehicle.

That swept area of the propeller on blackbird is about 20m2 and if you take just 1m on each side and the that volume will be 20m3 on each side of the propeller and for just a 2.5% above and below ambient pressure so ambient is about 100kPa then say 102.5kPa for P2 and 98.5kPa for P1 the total energy stored there is about 28Wh and that is about 4x more than needed to accelerate the 300kg blackbird to 13m/s (their speed record).

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