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# Please can someone explain the classic helium balloon in a car? Watch

1. (Original post by Puddles the Monkey)
I normally get pushed backwards into the seat when a car accelerates. Heavy breaking throws you forwards.

The balloon goes in the opposite direction!
Oops!
I think other people have posted with the correct idea - please ignore mine, I posted without considering it properly!

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2. (Original post by lerjj)
Have you ever stirred a glass of water with ice in it? The ice goes the wrong way, because the surrounding fluid is denser and is pushed more.

What is surrounding the helium balloon? Why woudn't they have used a different type of balloon? What differs between you and helium?
Not paid close attention to my ice stirring experiences, but I shall have a look at the next available opportunity.
3. (Original post by anosmianAcrimony)
You're sort of half right. The balloon rises in air and moves forward in an accelerating car for the same reason - the air around the balloon is heavier than it, and therefore is more affected by the acceleration of the car and acceleration due to gravity. In both scenarios the acceleration causes a larger force to be put on the air than on the balloon. A force is being put on the balloon to move backwards or down, but more​ force is being put on the air, so the air pushes the balloon out of the way and forwards or upwards.

which is what I said.
4. (Original post by cole-slaw)
which is what I said.
which is really what I said in post 3, but I couldn't be asked to write the full explanation up back then.
5. You're going to love the concept of holes in semiconductor physics OP.
6. (Original post by cole-slaw)
You're going to love the concept of holes in semiconductor physics OP.
Pray do tell!
7. (Original post by cole-slaw)
You're going to love the concept of holes in semiconductor physics OP.

The thing is, i understand the theory of semiconductors with conduction band valence bands etc etc.

This balloon thing is just weird physics at play. semiconductors is average degree level chemistry.
8. (Original post by Puddles the Monkey)
Pray do tell!
So a hole is an energy state where there should be an electron, but isn't.

As electrons move into these empty energy states, they leave other energy states empty behind them.

So its as if the hole itself were moving. So physicists are able to model it as if it is a positive particle in its own right.
9. (Original post by Motorbiker)
The thing is, i understand the theory of semiconductors with conduction band valence bands etc etc.

This balloon thing is just weird physics at play. semiconductors is average degree level chemistry.

Well, just think of the balloon moving through the air as a hole moving through the electron sea.
10. (Original post by cole-slaw)
Well, just think of the balloon moving through the air as a hole moving through the electron sea.
That actually makes sense.
11. (Original post by Motorbiker)
That actually makes sense.
A more elucidating example might be if you replaced the helium balloon in the example with some kind of solid shelled vacuum balloon. Then you can literally envisage it as being a "hole" in the air.
12. (Original post by cole-slaw)
Unfortunately its *******s.
If the air was "sloshing around" as the car moved, you would feel it, wouldn't you. It would be like being outside on a windy day..
No you wouldn't.
(Original post by cole-slaw)
Gravity and acceleration are equivalent. Hence anything that rises under the force of gravity is correspondingly going to go in the opposite direction under acceleration.
Rises under the force of gravity? What sort of tosh is that?

The movement of the balloon is down to changing air density in the vehicle caused by the movement of the vehicle..
I
13. (Original post by mphysical)
No you wouldn't.
Yes you would. Humans are capable of detecting moving air flows. At least, all the ones I know are.

Rises under the force of gravity? What sort of tosh is that?
Go outside and let a helium balloon move freely under gravity. Does it rise or fall?

The movement of the balloon is down to changing air density in the vehicle caused by the movement of the vehicle..
Not really, no.
14. (Original post by cole-slaw)
Yes you would. Humans are capable of detecting moving air flows. At least, all the ones I know are.
The air is moving imperceptibly due to acceleration of vehicle. This in no way compares to 'wind'
(Original post by cole-slaw)
Go outside and let a helium balloon move freely under gravity. Does it rise or fall?
Impossible to test on the planet I am on because of air. The helium balloon rises because helium is less dense than air. On the moon the balloon would fall like a brick.
(Original post by cole-slaw)
The movement of the balloon is down to changing air density in the vehicle caused by the movement of the vehicle..
Not really, no.
Your analogy with semi-conductors is nice but over complicates my simple explanation
15. (Original post by mphysical)
The air is moving imperceptibly due to acceleration of vehicle. This in no way compares to 'wind'Impossible to test on the planet I am on because of air. The helium balloon rises because helium is less dense than air. On the moon the balloon would fall like a brick.
Your analogy with semi-conductors is nice but over complicates my simple explanation
Your explanation is wrong. The air in an accelerating car does not "slosh around", it barely moves at all.

The balloon acts like a hole does in a valence band, it provides somewhere for the air to go.

No holes, no conduction through VB. No balloon, no air movement.
16. (Original post by cole-slaw)
Your explanation is wrong. The air in an accelerating car does not "slosh around", it barely moves at all.
If you travel at 70mph the air in the car also travels at 70mph. Brake hard and the air still continues to travel at 70 until it hits something. (Newtons First Law). That is why we wear seat belts.
All the air actually does not smash into the windscreen. Because of the hydraulic properties of air, it compresses more to one end of the vehicle becoming more dense to the front and less dense to the back.
This more dense air forces the balloon away towards the less dense air.
Exactly the same reason a balloon rises. Nothing to do with gravity as you state.
(Original post by cole-slaw)
The balloon acts like a hole does in a valence band, it provides somewhere for the air to go. No holes, no conduction through VB. No balloon, no air movement.
I fail to see how the balloon relates to the hole left by a displaced electron.
17. (Original post by mphysical)
If you travel at 70mph the air in the car also travels at 70mph. Brake hard and the air still continues to travel at 70 until it hits something. (Newtons First Law). That is why we wear seat belts.
All the air actually does not smash into the windscreen. Because of the hydraulic properties of air, it compresses more to one end of the vehicle becoming more dense to the front and less dense to the back.
This more dense air forces the balloon away towards the less dense air.
Exactly the same reason a balloon rises. Nothing to do with gravity as you state.
I fail to see how the balloon relates to the hole left by a displaced electron.
Just to clarify: tou think that a helium balloon rising is NOTHING to do with gravity?
18. (Original post by cole-slaw)
Just to clarify: tou think that a helium balloon rising is NOTHING to do with gravity?
A helium balloon rises because it is less dense than the surrounding air.
It will continue to rise until it meets an atmospheric pressure equal to the balloon/helium combination.

I am not sure why you believe gravity forces the balloon up.
Newton asked a similar question 400 years ago.

Granted, the air is more dense at lower altitudes because of the force of gravity.
But the only gravitational force on a free balloon is pulling it down.

Yes, in a car the force produced when turning, braking accelerating all mimic gravity and will act on the balloon until the much greater effect of air pressure takes over.
19. (Original post by mphysical)
A helium balloon rises because it is less dense than the surrounding air.
It will continue to rise until it meets an atmospheric pressure equal to the balloon/helium combination.

I am not sure why you believe gravity forces the balloon up.
Newton asked a similar question 400 years ago.

Granted, the air is more dense at lower altitudes because of the force of gravity.
But the only gravitational force on a free balloon is pulling it down.

Yes, in a car the force produced when turning, braking accelerating all mimic gravity and will act on the balloon until the much greater effect of air pressure takes over.

Oh dear oh dear.
20. Every molecule of air has mass and therefore exhibits both properties of inertia and momentum. (Newton's laws - an object continues in its line of motion or rest unless acted upon by an external force).

As the car accelerates forward, the air inside the car becomes compressed towards the rear of the car because of INERTIA. This creates a higher pressure region of air towards the rear of the cabin and a lower pressure region towards the front. That pressure differential causes the balloon to move towards the front of the cabin under acceleration.

Once the acceleration stops and the car moves at a constant velocity, the air pressure will equalise and the balloon will revert to the upright after some oscillation.

Reverse the process for deceleration and the MOMENTUM of the air inside the car, causes a higher pressure region to develop to the front of the cabin with a consequential force pushing the balloon to the rear.

Turning the car causes the air inside to compress against the outside radius of the turning circle and hence the pressure now pushes the balloon towards the centre of rotation and will continue to dos so for as long as the car keeps turning.

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