magnet talk

At risk of stating the obvious, it wouldn't be twice as strong if it didn't pull with

2x the force. But then, this might be a trick question....

How big is the ball?

there are rare earth ball magnets that might help

Nope, not a trick question...the rating on a magnet is based on the weight it will hold isn't it? I am not interested in how much weight the magnet WILL hold but how the weight of the item to be held affects the attraction between the two. Is it just a simple "double the magnet... double the force" or is there somekind of tricky physics at work here?

LLB

A rare earth magnet works well on a 1 inch ball but we need to go down to a

1/2 inch ball.... would that reduce the attraction by one half?

LB

>

Take a look at these

Tell me if reversing the chicken and the egg might work for you

Instead of a ferrous ball being attracted to a magnet would a magnetic ball on ferrous material help?

The problem is not in the magnet, it is in the magnetic circuit. All other things being equal, a magnet twice as strong is usually larger and work on a larger surface. You will notice that the attractive force between parallel surfaces gets stronger as they get closer. The reason is that the magnetic flux is controlled by the length of the gap, air being a poor magnetic conductor.

The ideal magnet shape is a short legged U shape, well approximated by the classic horseshoe.

Since you are attracting a sphere, your U shape is not ideal because the sphere surface can not be all close to the magnet poles, being round.

You need to modify the shape of the magnetic circuit to "cradle" the sphere once it is close to the magnet.

You can machine the magnet OR you can add two "polar expansions" machined in such a way to fill the air between a flat magnet surface and the sphere. The polar expansion can be a mild steel.

The U shape is not always used or visible. Think of a 1x2x3 block, if you have the North on one of the 1x2 faces and the South on the other. The magnetic flux runs outside from N to S but it has to cross about 3" of air gap and therefore will be weak.

If you position a piece of iron parallel to the 2x3 surface, the magnetic circuit length, in air, will be reduced to twice the gap from block to Iron.

The flux (strength) will now be much more intense.

On the other hand if you were to place the iron parallel to 1x2 face, the flux will be practically unaltered because this configuration does not substantially reduce the air path length, consequently the attraction will be weaker.

To answer your question more directly, a larger/stronger magnet will increase the attraction as long as its dimensions are comparable with the sphere 1" The flux of a 1/4" cube magnet will have to spread out to 1" dia to affect the sphere and the total path in air is roughly 1.75".

The flux of a 1" cube magnet will be "potentially" 16 time more intense and will pass almost entirely through the sphere, the air path is now roughly

3". What you lost in air gap more than made up in flux intensity.

If you take a 12" cube magnet, the flux is 144 times stronger that 1" magnet. But the sphere is bathed by roughly 1/144 of it because of the sphere small cross-section you have not gained much on the flux. But the air path is now roughly 11" long, you lost big.

The example are not numerically super accurate, I have treated the sphere as a cube and made other glaring simplifications, but it gives the idea of the phenomenon at work.

You may post this question on the alt.electrical.engineering or alt.sci.physic and get a more scientific answer.

Mauro

Thanks for your answer.. it sort of cleared my mind. LLB

While physics (especially when reduced to formula) is not my area of expertise, I think there need to be considered the distance from the magnet that the object is located.

As an example let us assume that you have a magnet that is capable of holding 1 pound of iron when in direct contact with the pole of the magnet.

At one inch from the same piece of iron, the force exerted will be significantly weaker. As MG most elegantly pointed out, the geometry of the item is also going to be a factor in the equation.

In the way I think and understand the physics involved, you are going to need one big mother of a magnet to lift *up* a one pound iron ball, and a somewhat less (although still very powerful) of a magnet to get in motion to roll on a level plane.

From practical experiments (play) with nifty and strong rare earth magnets, once you are able to apply enough force to get a chunk of iron in motion with the magnet, be sure your finger or any other body part is no way in between, or it will, if you are lucky, only hurt lie a mother and make you bleed a bit, if unlucky, you will forgo the use of that part. (As well as hurt like a mother and bleed profusely.)

If I recall correctly, the force of attraction increases as the inverse cube of the separation. That's why magnets seem "grabby" as you move towards them. That's also why magnetic levitation is tricky. You need an active non-linear control loop to stabilize things if you lift an object with a magnet. On the otherhand, if you hold stuff up with magnetic repulsion, it takes LOT of force to push things together.

Doug White

Other things being equal, yes -- regardless of the weight of the ball.

Other things being equal, yes.

The force on the ball depends on the total flux in the airgap between ball and magnetic pole piece in the direction of the desired force. With a permanent magnet, this will depend on the length and cross-sectional area of the airgap, which in turn depends a lot on the geometry of the system of magnet, other ferromagnetic material (iron or steel) and the ball itself. It will also depend on the "strength" of the magnet which is a function of its geometry, magnetization level, and material. "Strength" might be defined as the flux density it is able to produce in a given airgap between ball and polepiece, assuming that the flux return path is normal to the direction of the desired force.

A flux density of 1 tesla (below saturation for most steels) would produce a force of about 90 lb on a 1" dia ball. Devising a magnetic circuit to accomplish this could be a challenge.

You are concerned with field strength. Before you try a new magnet, (I am assuming the magnet's bigger than the ball), try to concentrate the flux. Make a steel cone, (or other convenient shape), The size of the magnet pole at the magnet end, and the size of the ball at the ball end. Interpose this between the magnet and the ball and see what happens. MadDog

I don't think there will be ANY force on a NONmagnetic ball..

I had one of those big super magnets that would suck the picture off a TV screen. If I waived this in front of a bunch of brass keys on the hook I could get them to wiggle. Some force is applied but it is not the same as the attraction to iron.

Or was that due to eddy currents?

Dave

Eddy currents!

Of course, what I ment was a ball that was not a magnet.

LLB