Magnetism

[billwald]

Say a steel ball rolling down a ramp is deflected by a magnet. Does this decrease the magnetic energy of the magnet?

 

[arunma]

Say a steel ball rolling down a ramp is deflected by a magnet. Does this decrease the magnetic energy of the magnet?

Nope. The energy contained in a magnetic field depends only on the strength of the field itself (actually, it depends on the square of the field strength).

But my guess is that you're wondering where the energy comes from to deflect the ball. If so, then the answer is that it comes from the potential energy of the ball itself. When setting up the experiment, it will be necessary to hold the ball in place while setting up the magnet, and the magnet will act on the ball with some attractive force. In setting up the experiment, you are doing work on the magnet (or the ball, depending on what you put in place first). That work goes into the potential energy of the system, which is then released when you conduct the actual experiment.

Well, hope that helps!

 

[us38]

Say a steel ball rolling down a ramp is deflected by a magnet. Does this decrease the magnetic energy of the magnet?

Yes. The magnetic field creates an electrical current in the ball, which creates a counter magnetic field. This permanently decreases the field strength of the original magnet.

 

[arunma]

Yes. The magnetic field creates an electrical current in the ball, which creates a counter magnetic field. This permanently decreases the field strength of the original magnet.

Hmm, I'm not sure about this (not meaning that you're wrong, but rather that I've never heard about this effect before). While such effects as temperature and radiation can reduce the magnetization of a magnet, I'm not aware of any means by which induced currents can knock ferromagnetic domains out of alignment. Where have you heard about this effect? I would be interested to investigate it, and see if electromagnetic induction really does reduce a magnet's field strength.

 

[us38]

While such effects as temperature and radiation can reduce the magnetization of a magnet, I'm not aware of any means by which induced currents can knock ferromagnetic domains out of alignment.

Via Lenz's law, as the ball rolls by the magnet it, the induced current (caused by the magnetic field not being uniform everywhere in the ball) creates a magnetic field opposed to the original field. Since the mangetic domains are not perfectly stable, they turn slightly to allign in the second field. This creates an overall reduction in the original field's strength.

 

[billwald]

Thanks for the discussion.

 

[arunma]

Via Lenz's law, as the ball rolls by the magnet it, the induced current (caused by the magnetic field not being uniform everywhere in the ball) creates a magnetic field opposed to the original field. Since the mangetic domains are not perfectly stable, they turn slightly to allign in the second field. This creates an overall reduction in the original field's strength.

I've been thinking about this for the past few days, and I think you're probably right. I don't suppose this effect has a name? If so, I'd like to study it in some more detail.

If the usage of a magnet really does reduce its field strength, it does so by a very small amount, since permanent magnets' lifespans don't appreciably decrease with usage. Furthermore, this effect must steal energy from somewhere, since there is an intrinsic energy in a magnetic field. The energy would certainly not come from the dynamic effect that the magnet has on whatever it is influencing (since that energy is accounted for by potential, which I described earlier). It seems to me that the energy lost by a permanent magnet's field must go into the small magnetization imparted to the object being moved by the magnet. I suppose this makes sense, since magnetizable objects do indeed acquire a small net magnetization when exposed to a magnetic field.

Sound about right to you?

 

[pgp_protector]

About magnetic memory I believe the term you're looking for is hysteresis.
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/hyst.html

 

[arunma]

About magnetic memory I believe the term you're looking for is hysteresis.
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/hyst.html

Well, hysteresis is certainly a related issue. Hysteresis is usually discussed in the context of applying a net external magnetic field to a permanent magnet in order to alter its magnetization. Here, an external magnetic field is (probably) being applied to the permanent magnet. But the effect is small enough that we would not need to analyze an entire hysteresis loop.

 

[Deamiter]

Via Lenz's law, as the ball rolls by the magnet it, the induced current (caused by the magnetic field not being uniform everywhere in the ball) creates a magnetic field opposed to the original field. Since the mangetic domains are not perfectly stable, they turn slightly to allign in the second field. This creates an overall reduction in the original field's strength.

I've got a physics degree (working on a second) and I have to disagree quite strongly with this. It is true that the overall strength of a permenant magnet can be lessened by disrupting some of the magnetic dipoles within the magnet, but this is a TINY effect and in no way responsible for the deflection of the ball.

Arunma was right in the beginning except that he didn't mention gravity (though I'm sure he was thinking it). There are two forces acting on the ball -- gravity and magnetic attraction (I'll assume attraction). Initially you have a potential energy associated with both -- it would take energy to pull the ball up the ramp and away from the magnet. As the ball rolls down the ramp, some of the potential energy (due to gravity) is converted into kinetic energy (motion). As the ball gets closer to the magnet, potential energy due to the magentic field is also converted into kinetic energy and the ball changes direction.

As arunma pointed out, a magnetic field decreases with the square of distance whereas gravity decreases directly with distance. That's why you don't notice the effect of the magnetic field until the ball is much closer to the magnet. Well that and the relative field strengths of gravity and the magnetic field, but that's not really important.

The POINT is that the magnetic field does not emit or otherwise lose any energy to move the ball.

To confuse things further, when the ball is nearer the magnet, the field strength is indeed decreased as the ball effectively shorts some of the field lines. The magnetic field emitted by the magnet is not affected, but the magnetic field detected far from the magnet would look smaller.

 

[arunma]

I've got a physics degree (working on a second) and I have to disagree quite strongly with this.

Ah, a fellow...person with a physics degree. Pleased to meet you!

It is true that the overall strength of a permenant magnet can be lessened by disrupting some of the magnetic dipoles within the magnet, but this is a TINY effect and in no way responsible for the deflection of the ball.

Arunma was right in the beginning except that he didn't mention gravity (though I'm sure he was thinking it). There are two forces acting on the ball -- gravity and magnetic attraction (I'll assume attraction). Initially you have a potential energy associated with both -- it would take energy to pull the ball up the ramp and away from the magnet. As the ball rolls down the ramp, some of the potential energy (due to gravity) is converted into kinetic energy (motion). As the ball gets closer to the magnet, potential energy due to the magentic field is also converted into kinetic energy and the ball changes direction.

As arunma pointed out, a magnetic field decreases with the square of distance whereas gravity decreases directly with distance. That's why you don't notice the effect of the magnetic field until the ball is much closer to the magnet. Well that and the relative field strengths of gravity and the magnetic field, but that's not really important.

The POINT is that the magnetic field does not emit or otherwise lose any energy to move the ball.

To confuse things further, when the ball is nearer the magnet, the field strength is indeed decreased as the ball effectively shorts some of the field lines. The magnetic field emitted by the magnet is not affected, but the magnetic field detected far from the magnet would look smaller.

Yes, this is also what I thought all along. However, I think that us38 might have stumbled on a secondary effect of the usage of permanent magnets. Obviously, any energy lost by a magnet through usage is not entirely related to the extent to which the magnet is being used. However, it is true that moving conductors in magnetic fields have induced EMFs, which might create net magnetic fields. Of course this discussion is purely theoretical, because permanent magnets do not detectably decrease in field strength due to usage (so from a practical standpoint, the answer to the OP is "no"). But in any case, what do you think about my comments in post #7?

 

[Deamiter]

Ah, a fellow...person with a physics degree. Pleased to meet you!



Yes, this is also what I thought all along. However, I think that us38 might have stumbled on a secondary effect of the usage of permanent magnets. Obviously, any energy lost by a magnet through usage is not entirely related to the extent to which the magnet is being used. However, it is true that moving conductors in magnetic fields have induced EMFs, which might create net magnetic fields. Of course this discussion is purely theoretical, because permanent magnets do not detectably decrease in field strength due to usage (so from a practical standpoint, the answer to the OP is "no"). But in any case, what do you think about my comments in post #7?

Hmm... well first of all I want to make it clear that any induced current is not due to a difference in magnetic field strength at different parts of the ball but due to a CHANGING magnetic field strength as the ball moves through the field. But I think you already knew that.

It's been a few years, but I believe that magnets are magnetic due to the allignment of what can be imagined as a TON of little dipoles. Since they're all lined up, their fields add together and you get a net magnetism. To explain to those not blessed with a nerdy degree, a dipole is just like a magnet with a north and south pole -- but in this case we're treating every molecule as a dipole. Each molecule acts like one of these magnets and if they're all aligned the same way they add up and produce a big magnetic field.

If you hold a conductor (say a piece of metal) in a magnetic field long enough, some if IT'S dipoles will line up with the field lines and you get a magnetic field around the conductor.

Now you do NOT need to lose magnetism in the "permenant" magnet to gain a magnetic field on the piece of metal. The energy is again potential -- when the dipoles in the metal line up with the magnetic field, they produce an opposing field that reduces the net magnetic field (measured far from the magnet). When you pull the metal away from the magnet, you're doing work (adding potential energy). If the metal has gained significant magnetism, it'll take more energy to pull them apart than it took to pull them together. That's because some of the potential energy went into slightly rotating dipoles and creating the net magnetic field on the piece of metal.

 

[us38]

Sound about right to you?

Yep, sounds good.

It is true that the overall strength of a permenant magnet can be lessened by disrupting some of the magnetic dipoles within the magnet, but this is a TINY effect and in no way responsible for the deflection of the ball.

By no means is the effect large. You'd probably have to do the same thing over nine thousand times to get even a measurable effect, but it'd still be there.

 

[arunma]

By no means is the effect large. You'd probably have to do the same thing over nine thousand times to get even a measurable effect, but it'd still be there.

Without doubt. A permanent magnet's loss of net magnetization is accounted for mostly by temperature changes, and so the effect that you describe is certainly not detectable, except perhaps by some very sensitive apparatus. Nonetheless, it sounds like an interesting idea. But I'm not yet sure if it actually happens. Does this effect have a name, so that I can research it?

 

[Deamiter]

Without doubt. A permanent magnet's loss of net magnetization is accounted for mostly by temperature changes, and so the effect that you describe is certainly not detectable, except perhaps by some very sensitive apparatus. Nonetheless, it sounds like an interesting idea. But I'm not yet sure if it actually happens. Does this effect have a name, so that I can research it?

Possibly "magnetic induction"? Though that's a somewhat ambiguous term as it's been used in the past for B which is now often called the magnetic field... But I just found it used on a website or two as the induced field in a conductor -- of course I don't think it really applies to a magnetic field that persists when you remove the conductor from the permenant magnetic field.

I don't think there's a better name for the allignment of dipoles. Here's a page that has some good graphical illustrations of all sorts of magnetic stuff -- induced magnetism is shown about a third of the way down the page.

http://sol.sci.uop.edu/~jfalward/magneticforcesfields/magneticforcesfields.html

 

[arunma]

Hey, it's time to resurrect this thread. Anyway, I did a bit of reading, and as far as I can tell, using a permanent magnet has no effect on the magnet's field strength. This isn't to say that the effect doesn't exist. But from what I can tell, it isn't observed. Magnets certainly do wear out over time, but external conditions such as temperature are the primary contributors to this.

So, as Deamiter seems to concur, my original explanation was complete. No, using a magnet won't wear it out. But hey, if anyone has any information, papers, etc., that attest otherwise, let me know.

 

[Rorshack]

Interesting subject.

 

[billwald]

Magnetism is a field that propagates like sound waves or light waves?

 

[arunma]

Magnetism is a field that propagates like sound waves or light waves?

Neither, actually, though magnetism is related much more to light than to sound. Sound is a compressional wave, meaning that it is propagated when air molecules bounce into one another. The magnetic field, on the other hand, isn't something tangible. Rather, it is something that we use to describe the effect that a magnet has on other objects.

Magnetic fields can travel as waves. When this happens, the magnetic field actually generates an electric field wave as well. This is an electromagnetic wave, which you know of as light. Depending on the frequency, electromagnetic waves can also be observed as radio waves, x-rays, gamma rays, microwaves, etc.