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Author Topic: Does gravity affect a magnetic field ?  (Read 20268 times)

Offline Hadrian

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Does gravity affect a magnetic field ?
« on: 07/12/2009 16:28:26 »
Magnetic force v gravity

So I get a small magnet and hold it over a metal pin. At some point the pin appears to defy gravity and rises up to the magnet. 

If there was less gravity would it move sooner? 

What happens in zero gravity?


[MOD EDIT - HADRIAN, PLEASE PHRASE YOUR THREAD TITLES AS QUESTIONS. THANKS. CHRIS]
« Last Edit: 26/12/2009 10:54:35 by chris »


 

Offline PhysBang

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Re: Does gravity affect a magnetic field ?
« Reply #1 on: 07/12/2009 16:55:03 »
It would move sooner. There might be some small electromagnetic interaction between the pin and the table, but it wouldn't be significant next to the magnet you have. Since the pull of the magnet gets weaker exponentially, it wouldn't have an obvious effect on the needle until you moved it closer that a foot, I would guess.
 

Offline Hadrian

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Re: Does gravity affect a magnetic field ?
« Reply #2 on: 08/12/2009 10:58:17 »
It would move sooner. There might be some small electromagnetic interaction between the pin and the table, but it wouldn't be significant next to the magnet you have. Since the pull of the magnet gets weaker exponentially, it wouldn't have an obvious effect on the needle until you moved it closer that a foot, I would guess.

Interesting thanks.  I often imagined what you could do with a big amount Magnetic Force in outer space.  Perhaps pull the space station. LOL
 

Offline lightarrow

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Re: Does gravity affect a magnetic field ?
« Reply #3 on: 08/12/2009 13:00:55 »
It would move sooner. There might be some small electromagnetic interaction between the pin and the table, but it wouldn't be significant next to the magnet you have. Since the pull of the magnet gets weaker exponentially, it wouldn't have an obvious effect on the needle until you moved it closer that a foot, I would guess.

Interesting thanks.  I often imagined what you could do with a big amount Magnetic Force in outer space.  Perhaps pull the space station. LOL
Yes, but the magnet would be pulled down as well... ;)
 

Offline graham.d

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Re: Does gravity affect a magnetic field ?
« Reply #4 on: 08/12/2009 14:56:35 »
I don't wish to be picky but the fall off of a magnetic field with distance is inverse square (not exponential). As magnets only occur as dipoles the fall off with distance would be faster (inverse cube if I remember correcly) but still not exponential. And it would never be zero so that in zero gravity the pin would move towards the magnet no matter how far away it was (allbeit very slowly if the magnet was a long way off). And, as lightarrow says, the magnet will start accelerating towards the pin assuming both are in free space although, assuming the magnet is of greater mass than the pin, by much less than the pin's acceleration.
 

Offline PhysBang

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Re: Does gravity affect a magnetic field ?
« Reply #5 on: 08/12/2009 15:23:00 »
I am assuming that the magnet is about the size of a refrigerator magnet. At even a couple of feet, I'm estimating that the pull on the pin would not be significant. This would be an interesting exercise to work out, though.
 

Offline graham.d

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Re: Does gravity affect a magnetic field ?
« Reply #6 on: 08/12/2009 17:26:29 »
It's difficult to calculate with a magnet without knowing how far apart the poles are and the strength etc. The question of zero gravity implies being not earth-bound and away from other fields. The question would be what is "significant" as the force between the pin and the magnet would never be zero. What you could probably say (if my memory of inverse cube law for a dipole is right) is that the force at 1 metre away will be 1 millionth of what it would be 1cm away. This probably isn't quite right because the inverse cube law only applies when the distance is much larger than the pole spacing, but you get the picture.
 

Offline Hadrian

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Re: Does gravity affect a magnetic field ?
« Reply #7 on: 09/12/2009 12:42:01 »
I wonder if there has ever been any experiment done on this in outer space?

and thanks everyone for sharing my interest in this topic.
« Last Edit: 09/12/2009 12:53:05 by Hadrian »
 

Offline Mr. Scientist

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Re: Does gravity affect a magnetic field ?
« Reply #8 on: 09/12/2009 13:21:33 »
Magnetic force v gravity

So I get a small magnet and hold it over a metal pin. At some point the pin appears to defy gravity and rises up to the magnet. 

If there was less gravity would it move sooner? 

What happens in zero gravity?


In zero-gravity weight equals nothing. What is the equation for weight in conjugate respect to the mass in zero-gravity?

A little homework question i would like you to solve - it's not too hard. :)
 

Offline vwilmot

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Re: Does gravity affect a magnetic field ?
« Reply #9 on: 10/12/2009 10:03:15 »
 [:(!]
Magnetic attraction is not simple inverse square, and is not simple inverse cube.
If it was simple then that would certainly have been established before 1600 by William Gilbert.

Isaac Newton noted in Principia Book 3 Proposition 6 Cor.5 that magnetic attraction was not inverse square and that it often decreases with distance 'not in the duplicate but almost in the triplicate proportion of the distance'.

Magnetic attraction varies with differences in the shape and structure of magnets, as well as with their magnetic strength. And unlike other simpler forces, magnetism also involves other motion effects besides attraction like in compass motions - as see newbielink:http://www.new-science-theory.com/william-gilbert.html [nonactive].
 [:(!]
 

Offline graham.d

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Re: Does gravity affect a magnetic field ?
« Reply #10 on: 10/12/2009 16:54:08 »
Well I said it was from memory that, at distances from a dipole much greater than the distance between the poles, the force is inverse cube. So given the comment above, I checked and I was right. There are plenty of calculations on the web. Here is one:

http://teacher.pas.rochester.edu/phy122/Lecture_Notes/Chapter30/chapter30.html

Look at section 30.3

Magnetsim was not really well understood in the 16th century so referencing work from that time is probably not going to be as good as looking at what is known today. I agree that if close to the dipole the force is much more complex, hence the caveat.
 

Offline Geezer

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Re: Does gravity affect a magnetic field ?
« Reply #11 on: 10/12/2009 18:40:29 »
One potential "fly in the ointment". Where do we find this "zero gravity" thingy?
 

Offline Hadrian

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Re: Does gravity affect a magnetic field ?
« Reply #12 on: 11/12/2009 11:31:43 »
One potential "fly in the ointment". Where do we find this "zero gravity" thingy?

indeed
 

Offline Mr. Scientist

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Re: Does gravity affect a magnetic field ?
« Reply #13 on: 11/12/2009 13:41:19 »
W=Mg

If weight is dependant on the gravitational acceleration, then inspace the acceleration is zero compared to the force we experience on earth. If you fit the correct values into the equation, you will find your weight to equal zero.
 

Offline vwilmot

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Re: Does gravity affect a magnetic field ?
« Reply #14 on: 11/12/2009 16:17:26 »
 [:(!]
Graham.d you are basically right about a case far from the case being considered - 'I get a small magnet and hold it over a metal pin'.

And like almost everybody you seem to have not studied early physics much, it is of course widely assumed to be worthless these days though I am not so sure.
 

Offline Mr. Scientist

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Re: Does gravity affect a magnetic field ?
« Reply #15 on: 11/12/2009 16:18:44 »
[:(!]
Graham.d you are basically right about a case far from the case being considered - 'I get a small magnet and hold it over a metal pin'.

And like almost everybody you seem to have not studied early physics much, it is of course widely assumed to be worthless these days though I am not so sure.

How unfortunate then.
 

Offline Geezer

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Re: Does gravity affect a magnetic field ?
« Reply #16 on: 11/12/2009 17:41:18 »
W=Mg

If weight is dependant on the gravitational acceleration, then inspace the acceleration is zero compared to the force we experience on earth. If you fit the correct values into the equation, you will find your weight to equal zero.

Indeed W=mg, but are you sure g is zero?
 

Offline lightarrow

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Re: Does gravity affect a magnetic field ?
« Reply #17 on: 11/12/2009 19:45:06 »
[:(!]
Graham.d you are basically right about a case far from the case being considered - 'I get a small magnet and hold it over a metal pin'.

And like almost everybody you seem to have not studied early physics much, it is of course widely assumed to be worthless these days though I am not so sure.
Arbitrary assumptions.
You don't seem to have studied basic logic much...  :)
 

Offline syhprum

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Re: Does gravity affect a magnetic field ?
« Reply #18 on: 11/12/2009 21:52:01 »
If I use a long magnet so that only one pole has any effect on the pin does the inverse cube law still apply or does it revert to the inverse square law ?
 

Offline lightarrow

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Re: Does gravity affect a magnetic field ?
« Reply #19 on: 13/12/2009 13:09:14 »
If I use a long magnet so that only one pole has any effect on the pin does the inverse cube law still apply or does it revert to the inverse square law ?
~ the second one.
 

Offline Mr. Scientist

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Re: Does gravity affect a magnetic field ?
« Reply #20 on: 13/12/2009 13:11:24 »
W=Mg

If weight is dependant on the gravitational acceleration, then inspace the acceleration is zero compared to the force we experience on earth. If you fit the correct values into the equation, you will find your weight to equal zero.

Indeed W=mg, but are you sure g is zero?

Yes - this is why we are weightless in space.
 

Offline Geezer

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Re: Does gravity affect a magnetic field ?
« Reply #21 on: 13/12/2009 20:25:44 »
W=Mg

If weight is dependant on the gravitational acceleration, then inspace the acceleration is zero compared to the force we experience on earth. If you fit the correct values into the equation, you will find your weight to equal zero.

Indeed W=mg, but are you sure g is zero?

Yes - this is why we are weightless in space.

That's not true. Let's say we are in the space shuttle orbiting the Earth. We are apparently "weightless", but g is not zero. Earth's gravity continues to attract us and maintains us in orbit. If g suddenly became zero, we would start moving in a straight line tangential to our previous orbit and we would leave Earth behind.

OK, but what happens when we are far enough away from the Earth or the Sun for them to exert a meaningful gravitational force on us. Now we are moving in a straight line, so g is zero, right? Nope. That's not right either. Assuming we are still in our galaxy, we are still under the influence of its gravitational field, so g is still nonzero. Mind you, the radius of our orbit in this situation is rather large, so our path will approximate a straight line, but it is not straight, therefore g is nonzero.
« Last Edit: 13/12/2009 20:31:26 by Geezer »
 

Offline Hadrian

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Re: Does gravity affect a magnetic field ?
« Reply #22 on: 14/12/2009 13:38:25 »
James Oberg explains the phenomenon this way:

"The myth that satellites remain in orbit because they have "escaped Earth's gravity" is perpetuated further (and falsely) by almost universal use of the zingy but physically nonsensical phrase "zero gravity" (and its techweenie cousin, "microgravity") to describe the free-falling conditions aboard orbiting space vehicles. Of course, this isn't true; gravity still exists in space. It keeps satellites from flying straight off into interstellar emptiness. What's missing is "weight", the resistance of gravitational attraction by an anchored structure or a counterforce. Satellites stay in space because of their tremendous horizontal speed, which allows them — while being unavoidably pulled toward Earth by gravity — to fall "over the horizon." The ground's curved withdrawal along the Earth's round surface offsets the satellites' fall toward the ground. Speed, not position or lack of gravity, keeps satellites up, and the failure to understand this fundamental concept means that many other things people "know" just ain't so."
 

Offline Geezer

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Re: Does gravity affect a magnetic field ?
« Reply #23 on: 14/12/2009 17:13:21 »
Thanks Hadrian! I was beginning to think I might be going off my rocker. (Jibes, no doubt, will follow  ;D)
 

Offline Mr. Scientist

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Re: Does gravity affect a magnetic field ?
« Reply #24 on: 14/12/2009 17:36:39 »
No - he is wrong. There is no gravitational accceleration due to the planet earth in accordance to weight. If that where true, then we would be able to stand inside our space shuttles.

This is pretty much basic physics.
 

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Re: Does gravity affect a magnetic field ?
« Reply #24 on: 14/12/2009 17:36:39 »

 

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