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Accelerating electromagnetic system |
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| Jan2-13, 08:34 PM | #1 |
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Accelerating electromagnetic system
Here is a picture of a situation I thought of:
![]() The circles between the magnets indicate a wire with current flowing in the direction given. This picture is a cross section of a loop of wire, looking down. Not included in this picture is the battery and non-conductive connecting materials linking the magnets and wire together. Since the current in the wire goes through a magnetic field, it should experience a force in the direction given by the right hand rule. If the magnets and wire are held together, then it seems that the entire system should accelerate as long as the current keeps on flowing. This seems like a bizarre conclusion, but I can't think of any way to disprove this conclusion. |
| Jan2-13, 08:45 PM | #2 |
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It will not. The force is from the device back onto itself. Since it isn't exerting a force on anything not part of itself, and it isn't ejecting any mass as propellent, it will not accelerate. It's like connecting a magnet to the end of a stick and holding in front of a wagon, you aren't going to accelerate.
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| Jan2-13, 08:53 PM | #3 |
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For example, a solar sail uses momentum from photons (electromagnetic disturbances) to accelerate. Could this be what is happening here? Edit: Also what is the source of the force that counteracts the force on the wire? Where is it felt? |
| Jan2-13, 09:04 PM | #4 |
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Accelerating electromagnetic system
There is a force upward on the wire, but there is also a force downward on the magnets. If you imagine the magnetic field lines created by the current in the wire, they are forcing the magnets downward.
Edit: it is an example of equal and opposite forces, as Drakkith was implying. |
| Jan2-13, 09:05 PM | #5 |
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| Jan2-13, 09:09 PM | #6 |
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Thanks! Edit: I would also like to point out that this construction is somewhat similar to a simple electric motor. |
| Jan2-13, 09:16 PM | #7 |
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magnetic forces always come in equal and opposite pairs, just like gravity and electrostatic forces. It can be less obvious, because often, we don't specify the exact form of the magnet.
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| Jan2-13, 09:38 PM | #8 |
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| Jan2-13, 09:39 PM | #9 |
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| Jan2-13, 09:48 PM | #10 |
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http://en.wikipedia.org/wiki/Momentu...ectromagnetism |
| Jan2-13, 09:54 PM | #11 |
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Then where is the momentum for the coil coming from? It's got to come from somewhere, it can't just appear. The EM field can transfer momentum but it cannot create it.
Per the third law, the coils feel a force due to the magnetic field of the magnets. In turn the magnets feel a force from the magnetic field of the coil in the opposite direction. Edit: Whoops, momentum of course can be created, but it is conserved. Not sure what I was thinking here. |
| Jan2-13, 10:00 PM | #12 |
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In response to your first two sentences: where does the momentum from an exploding bomb "come from"? |
| Jan2-13, 10:19 PM | #13 |
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However, the force exerted on the coil MUST have an equal and opposite force exerted on the magnet. |
| Jan2-13, 10:24 PM | #14 |
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It is totally possible that some instrument gives off EM energy, which causes the instrument to be accelerated, as you have said. And yes, this doesn't disobey any laws. But specific to the picture you drew in the first post, there is a force up on the wire and down on the magnets. You mentioned that your picture is similar to a simple electric motor. This is true. And in a simple electric motor, you can have a fixed magnet and a spinning wire, or you can have a fixed wire with a spinning magnet. This is because there is a force on the magnet too.
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| Jan2-13, 10:25 PM | #15 |
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| Jan2-13, 10:32 PM | #16 |
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In this case, and in many of the simplest EM problems, magnetic forces come in 'near-instantaneous' pairs.
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| Jan2-13, 10:40 PM | #17 |
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