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Forces of Magnets Attraction>Repulsion?

by AFSstudent
Tags: forces, magnets
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AFSstudent
#1
Feb13-12, 01:35 AM
P: 5
Hello I have come across a video that interested me,

(Attraction vs Repulsion in Magnetic Fields 2)
http://www.youtube.com/watch?v=8sUpF...eature=related

In this demonstration it shows the attractive magnetism is stronger than repulsive.
1) From this is it safe to say that the force of attraction is greater than repulsion?
2) What explanations are there that support/go against the outcome of this demonstration?

This really interest me to the point where I want to experiment and calculate just how much stronger attraction/repulsion is than the opposite.
3) Any idea how I can do this?
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Simon Bridge
#2
Feb13-12, 04:06 AM
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1. no
2. none - the "identical magnet" inserted was not in fact identical. Needs to repeat the experiment, swapping the roles of the magnets. Then there may be something worth talking about.

3. get some magnets and some springs (for measuring force) and try them out
AFSstudent
#3
Feb16-12, 05:09 PM
P: 5
Quote Quote by Simon Bridge View Post
1. no
2. none - the "identical magnet" inserted was not in fact identical. Needs to repeat the experiment, swapping the roles of the magnets. Then there may be something worth talking about.

3. get some magnets and some springs (for measuring force) and try them out
Thanks for the reply,
I'm just unsure about a few things.
2. How could you tell that the magnets were not identical other than the chips on the magnet itself (from what I made of it this is what I thought).
3. Do I calculate the spring tension force when the poles come into contact? And how would I be able to use springs to calculate repulsive force?

Simon Bridge
#4
Feb16-12, 10:59 PM
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Forces of Magnets Attraction>Repulsion?

2. because their attraction and repulsion are not the same - ergo, the magnets do not have the same field properties. One of the characteristics of scientific method is it's ability to establish general rules or "laws" for nature and this is one of them. He [the experimenter in the vid] got his result, most likely, because he did not use a proper control. Had he swapped the roles of the magnets, he would have got the opposite result.

There are all sorts of problems in the vid - the lack of a proper control is just the most glaring one. This needs to be addressed before we can even tell that anything interesting is happening.

Do you know how bar magnets get their magnetism in the first place?

3. no. you need to use the spring tension to balance the magnetic force without contact.

You want to set up the magnets on a track, fix one in place, and attach the other to a spring. Any retarding force will do - you can tilt the track and use gravity. In fact, carefully done, gravity will be better since you don't have to worry about details of material properties.

Fix a magnet to one end of the track and have the other magnet free to move at the other end. Test repulsion first - tilt the track slightly so the free magnet slides gently towards the fixed one until is stops. Measure the position of the magnet for different tilt angles and plot the graph.

to test attraction, turn the free magnet the other way round, and tilt the track so gravity pulls it the other way. This will be the fiddly one. Plot the equilibrium position against angle for this too.

repeat both cases with the roles of the magnet reversed.

repeat again using an unmagnetised iron bar in place of the fixed magnet ... do this for both magnets.

There are lots of variations like using solenoids and varying the current, making a pendulum, and so on. You should realize that this is one of those things that has been tested to death over centuries.

However - this sort of thing is very useful for learning about scientific method.
Focus on setting up the experiment to isolate the particular property you want to investigate - this involves critical thinking: you have to debunk your own work.
A.T.
#5
Jun28-12, 08:19 AM
P: 4,194
What about this explanation?
http://www.supermagnete.de/eng/faq/repulsion
Simon Bridge
#6
Jun28-12, 10:39 AM
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That's pretty good and well done for finding it.
[in case it vanishes later - the magnetic fields of the magnets affect the magnetic zones inside them - aligning the magnets improves the alignment of the individual zones making the magnets stronger in this configuration ... the other way disrupts the alignment weakening the magnetism.]

The effect mentioned there is very small indeed while the effect shown in the video is quite dramatic. I stand by my original statement - the problems with the experiment as shown need to be addressed before we can tell if anything significant is happening.

The experimenter could separate his magnets by hand while the one in the faq would have resisted 78kg! The effect was about 5-6% in the faq while the experiment was too dodgy to estimate.

Very cool magnets for 30 euros... and nice to confirm that the effect exists.
Again, well done.
A.T.
#7
Jun28-12, 02:15 PM
P: 4,194
Quote Quote by Simon Bridge View Post
The effect mentioned there is very small indeed while the effect shown in the video is quite dramatic.
I'm no expert on magnets. But couldn't the difference depend on the quality of magnets? In the high quality magnets tested on the linked site the molecular magnets are mostly fixed in their orientation, so the difference is small. In magnets with a smaller fraction of fixed molecular magnets there would more deorientation under a repulsive field.
Quote Quote by Simon Bridge View Post
Again, well done.
I googled for an explantions after seeing this video posted here on PF:



The mods seem to have deleted the thread. Probably due to the cranky claim that this falsifies Columbs Law. This claim makes no sense, because the law applies to point charges (monpoles), which have no orientation. I think it is better to offer a correct explanation and clear things up.
mfb
#8
Jun28-12, 05:17 PM
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While the video in the first post cannot be used to measure anything, it could have symmetric properties and therefore really show the difference of some percent. The acceleration with a single repulsing magnet is quite large, while the acceleration with both magnets looks smaller.

And the last video is just pointless. The effect he "discovered" is known, and his experimental device is unfeasible for showing the effect.
Simon Bridge
#9
Jun28-12, 07:24 PM
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Well that's a youtube video so, hopefully, the facts about it's content have been posted several times on it's comments by now. It's probably a good idea to spread true ideas in the paths of those searching in relation to these false ones though.
paulparis
#10
Jul2-12, 09:15 AM
P: 1
Hi need someone to help me calculate the strength of a C core stator (round in shape) that is made form .14 mm electric silica steel. The C core has a 27.4 mm gap and opposite the gap is a coil. Can I connect with someone privately on this issue ? Paul @ giallof1@gmail.com
sharkbait
#11
Jul2-12, 08:10 PM
P: 1
I am new here, I would like to pose this question for input.

If I wanted to force a iron cylinder object back and forth within a encasing repeatedly just for fun. I would have similar magnetic poles on either end of the iron object to cause attraction to successfully create a repetitive motion? Or would I use magnetic pole opposites to cause stronger repetitive motions back and forth?

Thank you.
Simon Bridge
#12
Jul3-12, 12:23 AM
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Hi paulparis & sharkbait, welcome to PF.
Your questions will get better attention is you start new threads.

@paulparis: if you want to have a private consult about your question, you should expect to pay someone. This forum pays for the support by making the answers public so everyone benefits.

@sharkbait: you have to drive the cylinder to get a sustained repeated motion.

Make a new thread, each of you, and say when you've done it.
bhat.vaibhav
#13
Jul3-12, 02:31 AM
P: 2
according to me the answer of this is the mass of the 2 objects.
if the mass of one of the magnet is less than that of the other one then it will atteacted towards it with a great force and will repel with a smaller force
Simon Bridge
#14
Jul3-12, 02:38 AM
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Hi bhat, welcome to PF;
Have you considered how Newton's 3rd Law would affect your suggestion?
CWatters
#15
Jul3-12, 02:41 AM
P: 3,216
sharkbait..

Some electric door bells use an electro magnet as a solenoid to move a bit of iron back an forth. Press the button and the solenoid causes the iron to move and strike a bell. When released a spring or gravity moves it back and causes it to strike another bell, sometimes with a different tone.
CWatters
#16
Jul3-12, 02:51 AM
P: 3,216
Quote Quote by AFSstudent View Post
1) From this is it safe to say that the force of attraction is greater than repulsion?
Possibly.

I believe some people have missed a vital fact. When you bring two magnets together the field between them changes so it is very hard to compare like with like. The way it changes is likely to be different for N-N and N-S. So at any given distance apart the forces may well be different because the field conditions are different.

There is no way to use this effect to make any kind of energy gain (aka "free energy").
Simon Bridge
#17
Jul3-12, 03:01 AM
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@CWatters: people who missed that should revisit post #5 :)


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