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aga
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Physics : Magnetics
If you hold a magnet close enough to an iron nail, the nail moves and sticks to the magnet.
Where does the Energy come from to overcome the intertia of the nail and make it move towards the magnet ?
Edit:
I mean a Permanent magnet rather than an electromagnet.
[Edited on 9-5-2016 by aga]
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careysub
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Quote: Originally posted by aga  | If you hold a magnet close enough to an iron nail, the nail moves and sticks to the magnet.
Where does the Energy come from to overcome the intertia of the nail and make it move towards the magnet ?
Edit:
I mean a Permanent magnet rather than an electromagnet.
[Edited on 9-5-2016 by aga] |
Energy was stored in the permanent magnet when it was created (magnetized). This is ultimate source of the energy involved in the interaction.
When you hold a nail close to the magnet there is an energy potential between the magnet and the nail, when the nail moves it converts potential
energy into kinetic energy.
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aga
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Makes sense.
Does it take a VAST amount of energy to create a permanent magnet ?
Just that they seem to last a lot longer than batteries.
Edit:
I've had a few stuck to my fridge, clearly accelerating themselves upwards (and a bit sideways) for years and years and years.
[Edited on 9-5-2016 by aga]
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blogfast25
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| Quote: Originally posted by aga | If you hold a magnet close enough to an iron nail, the nail moves and sticks to the magnet.
Where does the Energy come from to overcome the intertia of the nail and make it move towards the magnet ?
|
Magnetism = seriously complicated.
To put some calculus (who?) on Careysub's (correct) bones:
The magnet exerts a force F(r) on the nail. The relationship between F and r is complicated but usually follows an 'inverse
squares' law:
$$F(r)\propto \frac{1}{r^2}$$
This is why magnets tend to have a short acting range.
In the absence of anything more precise about F(r) we can say that if we allow to object to move, work W will be done on it, acc.:
$$dW=F(r)dr$$
$$W=\int_r^0F(r)dr$$
The work energy theorem tells us that the work will be converted to kinetic energy K:
$$W=\Delta K=K_2-K_1=\frac12mv_2^2-\frac12mv_1^2$$
Where v2 and v1 are the end and starting velocities respectively of the object.
Alternatively, we can also describe it in terms of potential energy V being converted to kinetic energy K:
$$\Delta V=\Delta K$$
$$V(0)-V(r)=\frac12mv_2^2-\frac12mv_1^2$$
(This is assuming no friction or drag.)
Of course all this says precious little about what causes the magnetic force to arise in the first place. That's the really hard part!
[Edited on 9-5-2016 by blogfast25]
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aga
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*pop*
i hear agaspace rasing it's ugly head.
Not surpringly there's some maths as well.
First up is that the spatial position 'reduces' to a function:
$$\text where dat fekker is \text = f(x,y,z)$$
Doing that, the maths appear to get easier when spinning the temporal axis through to the magnetic.
No, i've not worked it all out yet, but there's a whole pile of maths in there now.
Probably all wrong, but at least proveably so !
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careysub
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| Quote: Originally posted by aga | Makes sense.
Does it take a VAST amount of energy to create a permanent magnet ?
Just that they seem to last a lot longer than batteries.
Edit:
I've had a few stuck to my fridge, clearly accelerating themselves upwards (and a bit sideways) for years and years and years.
[Edited on 9-5-2016 by aga] |
A permanent magnet is not expending any energy at all so it can truly be permanent.
A magnet motionless, resisting gravity does no work and expends no energy - just as a book sitting on a table resisting the pull of gravity toward the
center of the Earth does no work, and expends no energy (or it you prefer, the table does no work and expends no energy).
Magnetic or ferromagnetic things moving in a permanent magnet's field is having work done on them or, if in free-fall, converting their potential into
kinetic energy until that gets converted to heat when it smacks into the magnet. You do work on the nail when you pull it away again.
When mentioning how long batteries last, are you referring to their storage life or how long they last when energy is being extracted from them?
In the second case, clearly they must be exhausted eventually since their energy storage is finite. In the first case, this is due to "self discharge"
and is related to the stability of the batteries chemistry. There are types of batteries that never self-discharge, because they are completely stable
at room temperature. These are "thermal batteries" that must be heated to activate them.
[Edited on 9-5-2016 by careysub]
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Sulaiman
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most of the work is put into the system each time you remove the nail from the magnet,
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blogfast25
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I agree fully with careysub that a magnet stuck to a fridge does no work, so it won't 'discharge' at all. Permanent means permanent here.
But I do have a niggling little doubt here.
We should consider the magnet plus nail as one system. Then when the nail is some distance away from the magnet the system has potential energy
V(r). Allowing the nail to 'free fall' towards the magnet now converts that V to K, as shown in my post.
Restoring the nail back to its original x = r position reverses everything.
But imagine the nail hitting the magnet, after its free fall, in an inelastic manner. Some friction energy will then be lost. Friction forces are
non-conservative.
Does that mean that doing this repeatedly makes the magnet lose energy, i.e. it becomes gradually 'discharged' over many cycles of free fall/inelastic
collision/restoration to x=r?
[Edited on 10-5-2016 by blogfast25]
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careysub
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| Quote: Originally posted by blogfast25 |
I agree fully with careysub that a magnet stuck to a fridge does no work, so it won't 'discharge' at all. Permanent means permanent here.
But I do have a niggling little doubt here.
We should consider the magnet plus nail as one system. Then when the nail is some distance away from the magnet the system has potential energy
V(r). Allowing the nail to 'free fall' towards the magnet now converts that V to K, as shown in my post.
Restoring the nail back to its original x = r position reverses everything.
But imagine the nail hitting the magnet, after its free fall, in an inelastic manner. Some friction energy will then be lost. Friction forces are
non-conservative.
Does that mean that doing this repeatedly makes the magnet lose energy, i.e. it becomes gradually 'discharged' over many cycles of free fall/inelastic
collision/restoration to x=r? |
When the nail hits the magnet all of K turns into heat (and maybe a little noise - that ultimately decays into heat). Energy is conserved.
When you pull the nail away you are adding energy to the system. And if you let it go again it will fall on to the magnet and produce more heat. If
you did this inside a calorimeter you would see the system getting warmer as you kept this up, cycle after cycle. The magnet loses no energy in this.
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aga
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Still does not sound right somehow.
Considering the simplest system :-
1. magnet is at point A in space (no gravity)
2. nail is placed within the effective range.
3. nail moves towards the magnet.
4. nail sticks to magnet.
Going no further (no pulling nail off magnet) the Energy has transformed into kinetic energy, then heat.
The heat radiates away.
Energy is conserved, so something just lost a bit of energy.
If it was the magnet, then it should now have a weaker magnetic field with that nail stuck to it.
If so, it should have a shorter effective range which is possibly measurable, although maybe not in an amateur setting.
Can anyone think of a do-able experiment to test this ?
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woelen
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A nail has a certain potential energy relative to the magnet.
Imagine a very simple universe with a magnet and a nail and nothing else.
The magnet has a certain potential energy, when it is not touching the magnet. When it moves to the magnet, the potential energy is converted to
kinetic energy, the nail is accelerated towards the magnet.
The energy, mentioned bij aga, is coming from the person who put the nail somewhere far away from the magnet. Each time, when the nail sticks to the
magnet and it needs to be removed, then some energy must be spent by the person separating them. That person then adds potential energy to the
magnet/nail system.
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Oscilllator
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| Quote: Originally posted by woelen | A nail has a certain potential energy relative to the magnet.
Imagine a very simple universe with a magnet and a nail and nothing else.
The magnet has a certain potential energy, when it is not touching the magnet. When it moves to the magnet, the potential energy is converted to
kinetic energy, the nail is accelerated towards the magnet.
The energy, mentioned bij aga, is coming from the person who put the nail somewhere far away from the magnet. Each time, when the nail sticks to the
magnet and it needs to be removed, then some energy must be spent by the person separating them. That person then adds potential energy to the
magnet/nail system. |
Just like with a planet and a ball!
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careysub
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| Quote: Originally posted by aga |
Energy is conserved, so something just lost a bit of energy.
If it was the magnet, then it should now have a weaker magnetic field with that nail stuck to it.
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You are right. It does. You cannot keep extracting more and more energy from the magnet by dropping nails on to it forever.
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blogfast25
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| Quote: Originally posted by woelen | A nail has a certain potential energy relative to the magnet.
Imagine a very simple universe with a magnet and a nail and nothing else.
The magnet has a certain potential energy, when it is not touching the magnet. When it moves to the magnet, the potential energy is converted to
kinetic energy, the nail is accelerated towards the magnet.
The energy, mentioned bij aga, is coming from the person who put the nail somewhere far away from the magnet. Each time, when the nail sticks to the
magnet and it needs to be removed, then some energy must be spent by the person separating them. That person then adds potential energy to the
magnet/nail system. |
No dispute but I'm still not satisfied! 
Even in that universe of magnet + nail + 'something that can pull nail of magnet', each time the nail in 'free fall' hits the magnet potential energy
is converted to heat, so your universe heats up and total energy is conserved.
Then potential energy is restored by work done pulling nail and magnet apart. But since as some of it has been converted to heat, hasn't the magnet
lost something permanently?
Bear also in mind that when the nail clings to the magnet there's still potential energy because there's still force:
$$F=\frac{dV}{dx}$$
Potential energy is that of nail + magnet: in the absence of the nail, the magnet has no potential energy.
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careysub
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| Quote: Originally posted by blogfast25 |
Even in that universe of magnet + nail + 'something that can pull nail of magnet', each time the nail in 'free fall' hits the magnet potential energy
is converted to heat, so your universe heats up and total energy is conserved.
Then potential energy is restored by work done pulling nail and magnet apart. But since as some of it has been converted to heat, hasn't the magnet
lost something permanently?
Bear also in mind that when the nail clings to the magnet there's still potential energy because there's still force:
$$F=\frac{dV}{dx}$$
Potential energy is that of nail + magnet: in the absence of the nail, the magnet has no potential energy. |
The work done pulling the nail away is exactly equal to the work done on the nail by falling to the magnet, which was converted to heat. So nothing
was added to the magnet or taken away at the end of this cycle. The original situation is restored, except for the heat added to the environment.
There is potential energy in the magnet, definitely. Energy is contained in the magnetic field. The magnet also attracts itself, and feels force.
This is similar to self-gravity of the Earth, the energy released when the atoms in the Earth "fell together" from infinity (practically speaking).
The Earth clearly exerts force on itself. When drop weights, and raise them again to our heart's content, but the gravity field of the Earth in
unchanged.
(Not a perfect analogy of course since gravity and magnetism behave differently. Anything falling to Earth adds to its gravity field. Stuff attracted
to a magnet does not, unless it is already a magnet.)
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blogfast25
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@careysub:
I see clearly now.
I was put off track a bit by:
| Quote: Originally posted by careysub |
You are right. It does. You cannot keep extracting more and more energy from the magnet by dropping nails on to it forever.
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But there of course you meant adding nail, upon nail, upon nail, without ever withdrawing one.
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Eddygp
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Just to point out that a magnetic field has an effect ONLY on moving charges!! If you leave a totally static magnet next to a totally static pin in
the middle of space, there is no interaction whatsoever (as opposed to that between charges, that does not depend on the movement of one of them to
exert a force).
there may be bugs in gfind
[ˌɛdidʒiˈpiː] IPA pronunciation for my Username |
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careysub
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| Quote: Originally posted by Eddygp | | Just to point out that a magnetic field has an effect ONLY on moving charges!! If you leave a totally static magnet next to a totally static pin in
the middle of space, there is no interaction whatsoever (as opposed to that between charges, that does not depend on the movement of one of them to
exert a force). |
Magnetic fields exert force on magnetic dipoles that are motionless.
A totally static magnet next to a totally static ferromagnetic pin in the middle of space will experience an attractive force. This is not no
interaction whatsoever.
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aga
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OK. This one appears to have a short sequence showing perpetual motion :-
https://www.youtube.com/watch?v=OH2c9mqSBzw
Just some magnets and a copper coil.
An experiment will be to see how long the 'magnet train' can run inside the loop for.
Theoretically it will grind to a halt at some point as friction induced by gravity robs energy out of the system (IF that's where the energy is at).
The energy required to keep it moving Must end up being quite a lot less than the energy used to build the magnets, or some basic premises of physics
will fall apart.
[Edited on 23-10-2016 by aga]
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j_sum1
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Uh, aga. It has a battery in it.
https://youtu.be/OH2c9mqSBzw?t=36s
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Maroboduus
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| Quote: Originally posted by blogfast25 | | Quote: Originally posted by aga | If you hold a magnet close enough to an iron nail, the nail moves and sticks to the magnet.
Where does the Energy come from to overcome the intertia of the nail and make it move towards the magnet ?
|
Magnetism = seriously complicated.
To put some calculus (who?) on Careysub's (correct) bones:
The magnet exerts a force F(r) on the nail. The relationship between F and r is complicated but usually follows an 'inverse
squares' law:
$$F(r)\propto \frac{1}{r^2}$$
This is why magnets tend to have a short acting range.
[Edited on 9-5-2016 by blogfast25] |
I thought magnets, as dipoles, obeyed the inverse cube law.
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aga
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Ah feck.
I thought that was just there as something to stick the magnets to.
So the magnets act as conductors to induce a current in the copper coil - he does say it's un-enameled wire.
Feckity feck.
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Sulaiman
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Imagine a universe uniformly consisting of nails stably floating in 'space'.
If you input a little energy to magnetise one of these nails,
all others will be atracted to it .... massive free energy ?
How about if just one extra nail is introduced to the otherwise uniform universe ?
so physical separation is a form of energy
CAUTION : Hobby Chemist, not Professional or even Amateur
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Chemetix
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Aga, it's a good thing to ponder. Magnets are just so damn intriguing, with their seemingly endless forces and no way to do any work.
I explain it like this- A magnet is like a little hole in the ground. You put energy into making the hole by removing the dirt, and you can get a
marble to roll down into the hole constantly. But you have to constantly use energy to pull it back up the well of potential energy you created if you
want it to fall back into the hole again. The magnet is a little hole of potential energy you can push up to things and make them fall into the well.
Holding one up to the fridge is like having some of the iron trying to fall into the hole, that lower potential energy is what keeps the 'hole' and
the iron in place. The big question in physics is- can you have a hole in the ground without a pile of dirt next to it? One of these is considered a
'North pole' and the the other a 'South pole'. Both are sources of potential energy, and a magnet must have both apparently.
Someone smarter than me can put the maths to explaining it, EM theory just wasn't my thing.
[Edited on 24-10-2016 by Chemetix]
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woelen
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Indeed, it _can_ be. Only if there is a field acting on the objects, which puts a force on them, the separation is a form of energy. This energy is
called potential energy.
Potential energy can be computed as the integral over a path between the two objects of the inner product of force (which depends on the field
strength and direction and on certain properties of the moving object, such as mass, charge) and tangent along the path. For the common physical
fields we know from daily life, the integral does not depend on the exact path and then we can speak of _the_ potential energy. The field then is
called a "conservative field" and then it can be expressed as the gradient of a scalar function (finding a mathematical expression for this scalar
function may be very difficult, it just tells that there is such a scalar function). This scalar function then is said to be the potential.
[Edited on 24-10-16 by woelen]
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