# Electromagnets and C of EWatch

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#1
Ok, so I asked this before, but I've thought of a better way of phrasing it now as I didn't get any answers. Two electromagnets are attached to identical batteries and placed far apart from each other. Both electromagnets are turned off, and a stationary magnet put near the second one.

Basically, my question would be: (A) what happens when they are turned on, and (B) what is the work being done in each case? As a corollary, will both batteries become flat at the same time, and will both solenoids become equally hot? Because otherwise, I don't understand how come the magnet starts moving in the second one, which seems to generate kinetic energy from (almost) nowhere. I feel like I'm missing something obvious...
0
5 years ago
#2
In order to work out what happens when the magnets are turned on, you need to know about the forces on the object. What do you know about the force due to a magnetic field and its dependence on distance?

With the batteries going flat (assuming their identical in every way), do you think that the proximity of the magnet to an electromagnet affects the power needed to drive it? Why, or why not? This should also answer your heating question.

What do you mean by "the magnet starts moving in the second one"? Kinetic energy due to a magnetic field is converted from the magnetic potential energy you have by placing it further away from the magnet (think about it as like GPE - a weight further away from Earth has a larger potential energy).

(Original post by lerjj)
Ok, so I asked this before, but I've thought of a better way of phrasing it now as I didn't get any answers. Two electromagnets are attached to identical batteries and placed far apart from each other. Both electromagnets are turned off, and a stationary magnet put near the second one.

Basically, my question would be: (A) what happens when they are turned on, and (B) what is the work being done in each case? As a corollary, will both batteries become flat at the same time, and will both solenoids become equally hot? Because otherwise, I don't understand how come the magnet starts moving in the second one, which seems to generate kinetic energy from (almost) nowhere. I feel like I'm missing something obvious...
1
5 years ago
#3
(Original post by lerjj)
Ok, so I asked this before, but I've thought of a better way of phrasing it now as I didn't get any answers. Two electromagnets are attached to identical batteries and placed far apart from each other. Both electromagnets are turned off, and a stationary magnet put near the second one.

Basically, my question would be: (A) what happens when they are turned on, and (B) what is the work being done in each case? As a corollary, will both batteries become flat at the same time, and will both solenoids become equally hot? Because otherwise, I don't understand how come the magnet starts moving in the second one, which seems to generate kinetic energy from (almost) nowhere. I feel like I'm missing something obvious...
1. The first one would produce a magnetic field as normal, and the work done would just be from the battery and any resistance from the wires/resistors you put in place

2. It will interact with the permanent magnetic in order to change some of its energy into kinetic energy, just like GPE changes into KE when you drop something. It's a bit beyond my scope, but you might want to take a look at this.

I'm not too sure about this, but I think they would. The same work is being done on both of the electromagnets, and I can't think of any reason why there would be a difference between them.

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#4
(Original post by Arithmeticae)
1. The first one would produce a magnetic field as normal, and the work done would just be from the battery and any resistance from the wires/resistors you put in place

2. It will interact with the permanent magnetic in order to change some of its energy into kinetic energy, just like GPE changes into KE when you drop something. It's a bit beyond my scope, but you might want to take a look at this.

I'm not too sure about this, but I think they would. The same work is being done on both of the electromagnets, and I can't think of any reason why there would be a difference between them.

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Ok, so to cause the permanent magnet to move in the second case, the work being done is to change magnetic potential to kinetic? That makes sense I suppose...

Thanks, that pretty much answers my question- didn't think about the magnet having some kind of energy. Could this be used up then? I.e. if I was to use the same bar magnet and repeat the 2nd electromagnet test some huge number of times (with new batteries), would I always cause the magnet to move? Would it eventually convert all of its MPE to KE?
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#5
(Original post by tory88)
In order to work out what happens when the magnets are turned on, you need to know about the forces on the object. What do you know about the force due to a magnetic field and its dependence on distance?

With the batteries going flat (assuming their identical in every way), do you think that the proximity of the magnet to an electromagnet affects the power needed to drive it? Why, or why not? This should also answer your heating question.

What do you mean by "the magnet starts moving in the second one"? Kinetic energy due to a magnetic field is converted from the magnetic potential energy you have by placing it further away from the magnet (think about it as like GPE - a weight further away from Earth has a larger potential energy).
I don't think it should- I kinda blanked on the magnet having potential energy . Therefore, I wasn't sure whether the extra energy was coming from the battery somehow working harder, or less heat being dissipated, neither of which seemed likely. I shall read up on magnetic potential energy, not sure why I haven't come across it before.

Two quick questions: can MPE be 'used up', and would demagnetizing a bar magnet (by hitting it/melting) reduce its magnetic potential? Is so, does it go to heat energy? Thanks +1
0
5 years ago
#6
(Original post by lerjj)
I don't think it should- I kinda blanked on the magnet having potential energy . Therefore, I wasn't sure whether the extra energy was coming from the battery somehow working harder, or less heat being dissipated, neither of which seemed likely. I shall read up on magnetic potential energy, not sure why I haven't come across it before.

Two quick questions: can MPE be 'used up', and would demagnetizing a bar magnet (by hitting it/melting) reduce its magnetic potential? Is so, does it go to heat energy? Thanks +1
Magnetic potential energy is quite a difficult concept to understand, compared to other sources of energy, which is probably why you haven't come across it. If a bar magnet demagnetises, then there is no magnetic field and so there can be no magnetic potential (like if there is no gravitational field GPE = 0).

Magnetic potential energy can be converted to other energy by moving the magnets closer together - like when you raise a weight and drop it, the GPE becomes kinetic energy. What precisely the MPE becomes depends on what's happening - if you let the magnets continue towards each other it becomes kinetic energy, but if you hold the magnets apart then it does become hear, yes.
0
5 years ago
#7
(Original post by lerjj)
Ok, so to cause the permanent magnet to move in the second case, the work being done is to change magnetic potential to kinetic? That makes sense I suppose...

Thanks, that pretty much answers my question- didn't think about the magnet having some kind of energy. Could this be used up then? I.e. if I was to use the same bar magnet and repeat the 2nd electromagnet test some huge number of times (with new batteries), would I always cause the magnet to move? Would it eventually convert all of its MPE to KE?
Yeah, I guess.

Yes, I think so. The magnetic potential energy is caused by magnetic dipoles and something to do with electrons aligning which I'm not too sure about (I think one of my old threads talks a bit more in detail about this), so the work is being done on the magnet to get it's domains to align and that's where the MPE comes from. As before, I'm not too sure about this but it seems to make sense.

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