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Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Christopher Mitchell on 14/09/2008 10:59:59

Title: What is the Bernoulli Principle?
Post by: Christopher Mitchell on 14/09/2008 10:59:59
Christopher Mitchell  asked the Naked Scientists:

My name is Chris and I frequently listen to your podcasts (http://www.thenakedscientists.com/HTML/podcasts/) from the States. I'm currently studying to be a high school science teacher and had a question about Bernoulli's principle.  

We use it to explain why an aeroplane gets lift and why your shower curtain seems to suck in on your while showering. My professor stated that we don't actually know "how" an aeroplane gets lift using this principle, my question is why do we not know and what exactly do we know about it?  Thank you

Chris Mitchell

What do you think?
Title: What is the Bernoulli Principle?
Post by: DoctorBeaver on 14/09/2008 11:57:07
As far as I'm aware, we do indeed know why an aeroplane wing gives lift. It's to do with pressure differential. Or are you inferring that there is something more fundamental involved?
Title: What is the Bernoulli Principle?
Post by: lyner on 14/09/2008 13:41:10
The Bernouli effect is not actually needed for a plane to get lift. Many wings have an aerofoil shape of cross section. This is a sort of stretched, curved tear drop. The distance across the top surface is greater than the distance across the bottom surface. As the wing goes through the air, the air which is deflected to the top surface has to go further than the air on the bottom surface in order that the two air streams come together at the trailing edge. This means that it has to go a bit faster. The Bernouli effect / principle tells us that the pressure drops as a fluid increases in speed so the pressure on top of the wing is a bit less than the pressure underneath - giving you lift.
But you don't need an aerofoil to obtain lift - a flat sheet, tilted upwards at the front will experience a vertical force as it is driven forwards through air because of the deflection of air downwards and the 'conservation of momentum'.
The only difference between the two types of wing is, I think, the drag factor. The aerofoil is particularly good at low speeds, I believe.
You can also get a plane with an aerofoil wing to fly upside down by pointing the leading edge upwards - 'beating' the aerofoil's effect.
A similar aerofoil effect happens on the sails of a yacht - their shape naturally takes up the shape when they are 'pointing' in towards the wind and will let them sail within about 45degs to the actual direction of the wind due to the Bernouli effect.
Title: What is the Bernoulli Principle?
Post by: DoctorBeaver on 14/09/2008 14:06:12
I know there was a Bernoulli hard drive system a few years ago.
Title: What is the Bernoulli Principle?
Post by: lyner on 14/09/2008 19:38:47
I think that was based on a low friction air bearing. (Or possibly just someone's name.)
Title: What is the Bernoulli Principle?
Post by: DoctorBeaver on 14/09/2008 19:58:47
Sophie - from http://www.iomega.com/25years/index.html (http://www.iomega.com/25years/index.html)

After two years of development, the Iomega Alpha-10 Bernoulli drive launched in 1982 as the first magnetic storage product to offer built-in protection from disk crashes. Iomega’s founders employed an aerodynamic principle known as the “Bernoulli effect” to pull the Alpha-10’s flexible disk up to the drive’s read-write heads, where a cushion of air protected the heads from hitting the magnetic storage platter. The design was essentially crash-proof, and although the eight-inch diameter, 10-megabyte removable disks were massive by today’s standards, it was an impressive technological leap.
Title: What is the Bernoulli Principle?
Post by: lyner on 14/09/2008 21:30:57
Well, you learn sunnink every day.

Didja also know that Apple's latest laptop hard drive uses an accelerometer to tell  when it is being dropped and takes the heads away quickly to avoid damage when it hits the ground.

I also heard a story about M&S(?)'s early point of sale terminals which used microcomputers with hard drives which all died very quickly. The explanation was that the till drawers used to close so often with a bang and they jarred the drives to death after a very short time.

Thank God for flash memory.
Title: Re: What is the Bernoulli's Principle?
Post by: Mitko Gorgiev on 23/11/2019 10:10:38
When a body moves through space filled with air, then higher pressure is created in front of it, while lower pressure/depressure behind it. The higher pressure is plus, the lower pressure is minus. I use to call this a ‘principle of an arrow’ (− >—> +).

* arrow1.JPG (2.67 kB . 260x95 - viewed 2205 times)
The greater the velocity of the body is, the stronger is the plus in front of it as well as the minus behind it. This principle can be found in many things, among others also in the so-called “Bernoulli’s principle”.
What is “Bernoulli’s principle” in its basic form? The picture below shows it. The water flows through a wider pipe and then through a narrower pipe. The velocity of the water increases in the narrower pipe. As a result, the water column over it is lower than over the wider pipe.

* Bernoulli.JPG (17.81 kB . 880x300 - viewed 2276 times)
Why is that so? The water columns over the pipes could be imagined as side-tails of the water-body. Since the velocity of the water is greater in the narrower pipe, a stronger minus (i.e. stronger depressure) appears in its tail than in the tail of the wider pipe, thus the air-pressure from above lowers the water column over the narrower pipe more.
At the same time a stronger plus appears at the front part of the narrower pipe. Everyone knows that the water-jet which comes out of a pipe reaches farther if we narrow the pipe. That happens because higher pressure occurs in the front part.
So, higher pressure at the plus-side (the front-side), lower pressure at the minus- side (the back-side).
The principle of an arrow can also be seen with light. The archetype of this pattern (≈ principle) is the flame of a candle or a cigarette lighter. A violet-blue color appears on the back and a yellow-red color on the front of this fiery arrow. (The violet/blue/cyan are minus-colors, the yellow/orange/red are plus-colors. In the past they have been called cold and warm colors, but now I additionally call them minus and plus-colors. )

* sveka1.JPG (2.89 kB . 110x110 - viewed 2215 times)
And this pattern can be found wherever colors appear. Look at, for example, these colors obtained from a light source (it is in the middle) with the help of a diffraction grating (the arrows are added by me).

* diffraction1.JPG (10.76 kB . 287x198 - viewed 2224 times)
A manifestation of the Bernoulli’s principle can be seen also with another fluid, and that is electricity. If we make a series connection of two wires of same metal, but of different cross-sectional areas, place them on a table in straight south-north direction, under each piece place a compass (the compasses are identical) and then connect this assembly to a battery, we will notice that the deflections of the needles are different. The magnetic needle under the thinner wire makes greater deflection than the needle under the thicker wire.

* magn deflection.JPG (25.39 kB . 520x450 - viewed 2297 times)
Title: Re: What is the Bernoulli Principle?
Post by: alancalverd on 23/11/2019 12:56:48
Alas no. The current is the same in both wires, so the magnetic field is identical outside the wire. The current density is greater in the thinner wire but you won't notice the difference unless the thicker wire is much wider than the length of the compass needle.
Title: Re: What is the Bernoulli Principle?
Post by: syhprum on 23/11/2019 14:24:27
Take care to have some current limiting arrangement in the circuit or you may well destroy both wires and battery
Title: Re: What is the Bernoulli Principle?
Post by: Mitko Gorgiev on 23/11/2019 19:30:53
Alas no. The current is the same in both wires, so the magnetic field is identical outside the wire. The current density is greater in the thinner wire but you won't notice the difference unless the thicker wire is much wider than the length of the compass needle.
I speak on basis of experiments. What experimental evidence I have for my assertion about the deflection of the magnetic needles you can read in my book, which is downloadable free of charge.
Title: Re: What is the Bernoulli Principle?
Post by: Mitko Gorgiev on 23/11/2019 22:20:27
Take care to have some current limiting arrangement in the circuit or you may well destroy both wires and battery
With several carbon-zinc 1.5V AA batteries (which I usually use) there is no need for current limiting resistors. Neither the wires nor the batteries will be destroyed if this short circuit is not kept as such too long.
Title: Re: What is the Bernoulli Principle?
Post by: hamdani yusuf on 25/11/2019 04:10:11
Alas no. The current is the same in both wires, so the magnetic field is identical outside the wire. The current density is greater in the thinner wire but you won't notice the difference unless the thicker wire is much wider than the length of the compass needle.
I speak on basis of experiments. What experimental evidence I have for my assertion about the deflection of the magnetic needles you can read in my book, which is downloadable free of charge.
Interesting. My experience using clamp meter never show difference in Ampere reading due to cable size. Where can I download your book? Which part of your book contains this particular experiment?
Title: Re: What is the Bernoulli Principle?
Post by: Mitko Gorgiev on 25/11/2019 07:29:03
Alas no. The current is the same in both wires, so the magnetic field is identical outside the wire. The current density is greater in the thinner wire but you won't notice the difference unless the thicker wire is much wider than the length of the compass needle.
I speak on basis of experiments. What experimental evidence I have for my assertion about the deflection of the magnetic needles you can read in my book, which is downloadable free of charge.
Interesting. My experience using clamp meter never show difference in Ampere reading due to cable size. Where can I download your book? Which part of your book contains this particular experiment?
Since here it is not allowed to post external links, I can tell only to search for my full name and you will find my book on academia edu (pages 23-26).
Title: Re: What is the Bernoulli Principle?
Post by: alancalverd on 25/11/2019 09:11:20
If the wires were in parallel, you would indeed have a larger current flowing through the thicker wire, but in series (as drawn here) , what goes in must come out, so the current is the same in both.

Read up on Ohm's Law and Kirchoff's Law for a formal explanation.

And for what it's worth, the electron flow (i.e. in most wires, including Cu) is in the opposite  direction to that shown. p-type conductors such as Al have hole flow in the "conventional current" direction (see "Hall Effect"). It's all very confusing to students, and nothing to do with Bernouilli.
Title: Re: What is the Bernoulli Principle?
Post by: Mitko Gorgiev on 25/11/2019 09:29:52
If the wires were in parallel, you would indeed have a larger current flowing through the thicker wire, but in series (as drawn here) , what goes in must come out, so the current is the same in both.

Read up on Ohm's Law and Kirchoff's Law for a formal explanation.

And for what it's worth, the electron flow (i.e. in most wires, including Cu) is in the opposite  direction to that shown. p-type conductors such as Al have hole flow in the "conventional current" direction (see "Hall Effect"). It's all very confusing to students, and nothing to do with Bernouilli.
The wires are connected in series. The current strength is the same through the both wires, and it applies also for the strength of the magnetic field around them. Nevertheless, the two magnetic needles show different deflections. You can see the experimental evidence and the explanation in my book (pages 23-26). You can find my book on academia edu if you search my full name.
Title: Re: What is the Bernoulli Principle?
Post by: hamdani yusuf on 26/11/2019 12:01:25
Since here it is not allowed to post external links, I can tell only to search for my full name and you will find my book on academia edu (pages 23-26).
I think you should create a new topic in New Theory section due to policy of this forum. You can explain your theory in more details. The topic of diffraction and magnet deflection can be discussed in separate threads. I've made extensive experiments about light diffraction and share them here https://www.thenakedscientists.com/forum/index.php?topic=66301.0
Let's see if you can explain the results using your theory.

I've browse your website and find an interesting experiment shown below
Quote
I have posted originally there this interesting experiment I came across recently in the book Physik, Band 2 from the author Hermann von Baravalle (pages 83–84). Look at the setup below.
When high DC voltage is connected to it, then sparks start to jump across the upper part of the circuit.
(https://qph.fs.quoracdn.net/main-qimg-b080b4ee81ae6fd0fe96f9f2267a36d3)
As soon as the poles of the voltage source are reversed, then sparks start to jump across the lower part of the circuit (drawing below).
(https://qph.fs.quoracdn.net/main-qimg-40331f2ded4a266a80ef5e3f2253fb50)
The two different ends where the sparks jump are a thick sharpened wire and a metal plate.
I consider this as yet another great evidence that the electric current flows de facto from the Plus to the Minus-pole of the battery.

I think I can try to repeat and verify the experiment and then upload it to my Youtube channel.
Title: Re: What is the Bernoulli Principle?
Post by: Bored chemist on 26/11/2019 13:14:06
I consider this as yet another great evidence that the electric current flows de facto from the Plus to the Minus-pole of the battery.
It doesn't indicate anything like that.
Title: Re: What is the Bernoulli Principle?
Post by: Mitko Gorgiev on 26/11/2019 19:12:33
I think you should create a new topic in New Theory section due to policy of this forum.
Dear Hamdani, thank you for your advice. I have done that. Here is the link:

https://www.thenakedscientists.com/forum/index.php?topic=78153

I will see your experiments on light diffraction and post a comment if I have something interesting to tell.
Best regards, M

Title: Re: What is the Bernoulli Principle?
Post by: hamdani yusuf on 27/11/2019 01:04:55
I consider this as yet another great evidence that the electric current flows de facto from the Plus to the Minus-pole of the battery.
It doesn't indicate anything like that.
It wasn't me who wrote that. I have the same experience as this not so long ago. I checked and found that it occurs when you select someone's statements but then click someone else's action button.
Title: Re: What is the Bernoulli Principle?
Post by: Bored chemist on 27/11/2019 12:42:25
It wasn't me who wrote that.
Sorry for the confusion.
As you say, the quote function sometimes makes a mess of attribution.
Title: Re: What is the Bernoulli Principle?
Post by: Mitko Gorgiev on 16/12/2019 08:19:15
I consider this as yet another great evidence that the electric current flows de facto from the Plus to the Minus-pole of the battery.
It doesn't indicate anything like that.
What does it indicate according to you?
Title: Re: What is the Bernoulli Principle?
Post by: alancalverd on 16/12/2019 10:57:58
The spark experiment is indeed counterintuitive, and contrary to what happens in vacuo. We use field emission of electrons from a sharp spike, in several applications such as cold-cathode x-ray tubes and electron microscopes, and the source electrode is always negative.