1

General Science / Re: How to calculate fan power - formula?

« on: 05/05/2018 11:00:03 »

The equation P = ½ρAv³ is correct. ρ is the density of air, A the area of the fanduct, and v the exhaust speed.

However this only calculates the power transferred to the air. To calculate the fan speed you need to know its efficiency, which is a  complicated and nonlinear function of the blade geometry and v. That said, if you assume 100% transfer at the optimum speed for a given fan, then the swept volume per second is Av and each blade sweeps  an area A ω times per second where ω is the rotation speed, so you can calculate ω for a fan with n blades.

2

Physics, Astronomy & Cosmology / Re: How is magnet size and shape calculated to provide a given magnetic field?

« on: 27/09/2017 14:03:20 »

Can you answer, to my questions on this forum.

I very much doubt that he has the skills you seek.

This is the key to possible solutions that I want.
link: https://www.thenakedscientists.com/forum/index.php?topic=71465.0

Looking at that link I see a comment:

I am admittedly curious as to what would happen if you constructed a shielded magnet like the type shown in post number 6. Would the magnetic field stop working altogether?

I think Kryptid is right. The magnetic field lines will flow through the shielding material from the enclosed pole and at the open end the field lines will try to take the shortest route to the open pole - the surrounding shielding will act as the opposite pole. You will have made something similar to a horseshoe magnet where one pole (the outer pole) surrounds the inner one, this means the field will not extend out as far as an unshielded pole. However, I think you will need to leave an air gap between the poles otherwise the field extent will be minimal.

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Physics, Astronomy & Cosmology / Re: Is it possible to make a shield to block a magnetic field?

« on: 25/09/2017 01:14:41 »

If you are trying to make a "magnet" with just one pole then you are wasting your time.
https://en.wikipedia.org/wiki/Magnetic_monopole

I am admittedly curious as to what would happen if you constructed a shielded magnet like the type shown in post number 6. Would the magnetic field stop working altogether?

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Physics, Astronomy & Cosmology / Re: Is it possible to make a shield to block a magnetic field?

« on: 24/09/2017 22:34:21 »

If you are trying to make a "magnet" with just one pole then you are wasting your time.
https://en.wikipedia.org/wiki/Magnetic_monopole

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Physics, Astronomy & Cosmology / Re: Is it possible to make a shield to block a magnetic field?

« on: 24/09/2017 09:38:37 »

Mu metal has been used for years as a magnetic screen.
It's a little more effective than iron, but a lot more expensive.
As far as I can tell giron is similar.

What are you hoping to achieve?
If you just want to put a box round a magnet so you can send it by post without annoying the carrier, then a biscuit tin would do the job fairly well.

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Physics, Astronomy & Cosmology / Re: Is it possible to make a shield to block a magnetic field?

« on: 24/09/2017 07:19:55 »

As @Bored chemist  says an iron plate will do the job as will sheet steel. You have to make sure that the metal does not saturate, so it will work as long as you aren't using extremely powerful magnets. You also need the shield to extend beyond the length and width of the magnet or you will get leakage around the sides. Best to experiment.

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Just Chat! / Re: Sociable? discussion

« on: 23/09/2017 16:22:33 »

What's wrong with Gravatron? It's a metal(probably not) that floats because it's the only metal that does not change weight due to it's given temperature? You're find it one day...

Nickle may work but probably will only bend the field in to a different shape. Think some coins are made of nickle. I gave  up, I know there's the sea and all that but were supposed to be here forever.

I came up with the perpetual motion car, guess called  the gravity motion car.
If it can charge for a day may be able to post the Patent application,
To be honest, you're only about 3 years behind me.

I have 2 different types of propeller wind type devices and better motor/generator designs along with my 30 other inventions. And possibly using these propellers on a wire for high altitude wind or sun power generation. Things may be OK.

I have an invention that may surprise you. It moves and can't be moved back. Not built it or have the design fully still in my mind... yea, something like that but different. Came from an idea for earthquake building protection.

What does this post have to do with the original question?

Trying to design a perpetual motion machine is like trying to design a machine made of pumps, tubes and containers that will give more water out than you put into it. It just doesn't work.

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Physics, Astronomy & Cosmology / Re: Is it possible to make a shield to block a magnetic field?

« on: 23/09/2017 14:09:01 »

An iron plate would do the job.

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Physics, Astronomy & Cosmology / Re: How is magnet size and shape calculated to provide a given magnetic field?

« on: 23/09/2017 09:35:08 »

You could use a datasheet like this one to calculate some of these factors.

Density= 7.5g/cc
Use the dimensions to work out the mass of each magnet

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Physics, Astronomy & Cosmology / Re: How is magnet size and shape calculated to provide a given magnetic field?

« on: 22/09/2017 20:35:43 »

You get the greatest force of attraction when they are in direct contact.
But it also requires the greatest amount of energy to separate them again (in fact, slightly more energy than was released when they first came into contact).

To some extent, you can concentrate magnetic fields where you want them by using high-permeability ferromagnetic materials like iron or nickel which "conduct" magnetic fields more easily than air.

See: https://simple.wikipedia.org/wiki/Permeability_(electromagnetism)

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Physics, Astronomy & Cosmology / Re: How is magnet size and shape calculated to provide a given magnetic field?

« on: 20/09/2017 21:47:16 »

This web calculator might be of some help to you: https://www.kjmagnetics.com/calculator.asp

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Physics, Astronomy & Cosmology / Re: What arrangement of magnets produces the greatest magnetic field strength?

« on: 19/09/2017 15:47:33 »

Can someone with experience give me an answer to post #4 and #5? Thanks in advance.

Both magnets have the same height, but magnet A has more total volume (and presumably material) than magnet B, 13,320 cubic millimeters vs. 10,123 cubic millimeters. All else being equal, magnet A should be the stronger of the two.

As to your other question, area 3b appears to be closer to the magnet north pole than area 3a, so the field should be stronger there.

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Physics, Astronomy & Cosmology / Re: How does making holes in a magnet affect the magnetic field?

« on: 18/09/2017 05:36:30 »

There is a maximum magnetic flux which a particular ferromagnetic material can stand before it enters "magnetic saturation".
If you require more magnetic flux, you have to increase the area.

Reducing the area (by drilling holes in it) will reduce the maximum magnetic flux.
So I agree with Kryptid's conclusion.

See: https://en.wikipedia.org/wiki/Saturation_(magnetic)

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Physics, Astronomy & Cosmology / Re: How does making holes in a magnet affect the magnetic field?

« on: 17/09/2017 23:12:52 »

Although my knowledge of magnetism is ultimately limited (especially when it comes to the way that shape affects magnetic fields), I would say that any magnet with a hole in it would have a weaker field than one of identical shape without the hole. The reason being that the magnet without the hole would contain more total magnetized material.

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Physics, Astronomy & Cosmology / Re: What arrangement of magnets produces the greatest magnetic field strength?

« on: 16/09/2017 10:42:42 »

Helo Kryptid
Thanks for your feedback and suggestions. I'll take this answer seriously.
Whether it is just your opinion or the result of your experience. I do not have a measuring instrument.

Still, I will wait for other members of the forum to say their opinion.

You should definitely take Kryptid's feedback seriously. Based on the quality of previous answers  the reply is based on both knowledge and experience rather than opinion.

In addition I would say:
Practical magnets are rarely perfectly smooth or have coatings, this reduces the quality of contact between the magnets and reduces the overall strength of a stack.

The stack should not be higher than the diameter of the magnets, beyond this the strengths do not add together, but start to decrease towards a limiting value. As an example with measurements of 10mm dia x 1mm thick magnets
Number of magnets.             Force
1.                                                 0.5
2.                                                 1.0
3.                                                  1.5
4.                                                  1.9
5.                                                  2.1

It's also worth noting that in a stack of magnets the attraction between the 2 end magnets is often less than that between the end of the stack and the metal object being attracted.


Hope those points help.

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Physics, Astronomy & Cosmology / Re: What arrangement of magnets produces the greatest magnetic field strength?

« on: 14/09/2017 22:59:41 »

As you add more and more magnets of decreasing size in a north-to-south configuration, you eventually reach a point where the net magnetization approaches zero (i.e. the distribution of north and south poles becomes increasingly homogeneous inside of this compound magnet). For this reason, I say the single magnet approach has the strongest field.