Can Conducting Current Through a Bar Magnet Cause Movement?

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Discussion Overview

The discussion revolves around whether conducting current through a bar magnet can cause movement of the magnet. Participants explore the implications of current flow, magnetic fields, and forces involved, with a focus on theoretical and conceptual aspects.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants inquire about the method of connecting the battery to the magnet and the expected outcomes of the current flow.
  • There is a suggestion that the current flowing through the magnet will experience a magnetic field that could cause movement due to the Lorentz Force, but this movement may only occur when the current is started or stopped.
  • One participant questions why the magnet would not continue to move once it has started, raising concerns about the forces at play and the establishment of an equilibrium position.
  • Another participant draws parallels to a battery-powered toy car and discusses momentum conservation in the context of energy discharge and force application.
  • Some participants speculate on the implications of the setup in a non-isolated system, particularly in relation to the center of mass and the net forces involved.
  • There is a discussion about the interaction between the wire's magnetic field and the magnet's field, questioning whether attaching the current to the magnet changes the dynamics of the forces involved.
  • One participant expresses uncertainty about their understanding of the physics involved, reflecting on their previous knowledge and the complexities of the topic.

Areas of Agreement / Disagreement

Participants express differing views on whether the setup can lead to net movement of the magnet, with some arguing that it can and others suggesting that it cannot due to conservation laws and the nature of the forces involved. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants highlight the need for clarity in the setup and assumptions, such as the role of the magnetic field, the nature of the forces acting on the magnet, and the implications of the system being isolated or not.

Who May Find This Useful

Individuals interested in electromagnetism, forces in magnetic fields, and theoretical physics may find this discussion relevant.

StoyanNikolov
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Following the Picture below,
If I Conduct current through Permanent magnet Bar and hold the battery still. Will the magnet move in any direction ? Thank you.
ASGi0.jpg
 
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How are you intending to connect the battery to the flat magnet? Where do the wires make conductive contact to the magnet? (top/bottom, side-to-side?)

And I'll assume that you have a current limiting resistor in series with that connection. What are you hoping the magnet does?
 
There is connection . Two opposite sides . Opposite sideways. Just they are not very visible on the picture.
 
Are you expecting the battery to influence the magnet. A battery will produce a field if a current if flowing through it but in your 'circuit' nothing is happening afaics. Draw a circuit.
 
StoyanNikolov said:
There is connection . Two opposite sides . Opposite sideways. Just they are not very visible on the picture.
Oh, I think I see the "wires" now. It's pretty confusing, so let me try to introduce a coordinate system. Let ##W## be the width of the flat magnet and ##T## be the vertical thickness. The ##(x,y,z)## axes point right, back and up.

So your Positive wire connects at ##(0,0,T)## and your Negative wire connects at ##(W,\frac{W}{2},\frac{T}{2})##. Does that sound right?

The current flowing through the flat magnet will experience a vertical magnetic field that will cause a slight horizontal deflection of that current via the Lorentz Force: ##F = q\vec v \times \vec B##, so there will be a slight movement of the free bar magnet in the opposite direction. But the movement only happens when you start or stop the current, much like if you are standing in a canoe and take a step one way and stop, the canoe moves the other way and stops. The center of mass of you + the canoe stays in the same place, by Conservation of Momentum of an isolated system.

EDIT/ADD: https://en.wikipedia.org/wiki/Hall_effect
 
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Faraday Paradox Linear Motion.jpg

Thank you . I've made some changes. Both ends of the Conductor are connected to the center of each opposite side.
Will the magnet experience motion in any direction, considering large enough current passing through? The conductor is is flexible.
Thank you.
 
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StoyanNikolov said:
Will the magnet experience motion in any direction, considering large enough current passing through? The conductor is is flexible.
Yes, the magnetic field of the wires will exert a force on the magnet. The same thing happens if you just have a wire instead of a bar magnet as the field from each part of the wire exerts a force on each other part.
 
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berkeman said:
The current flowing through the flat magnet will experience a vertical magnetic field that will cause a slight horizontal deflection of that current via the Lorentz Force: F=qv→×B→, so there will be a slight movement of the free bar magnet in the opposite direction. But the movement only happens when you start or stop the current
Why wouldn't it keep moving? I'm having a hard time seeing what the restoring force is that establishes an equilibrium position.
 
DaveE said:
Why wouldn't it keep moving?
Conservation of Momentum probably. I think there would be a counter-force from the bunching of the current once that offset forms, but I'd have to think more about it...
 
  • #10
berkeman said:
Conservation of Momentum probably. I think there would be a counter-force from the bunching of the current once that offset forms, but I'd have to think more about it...
On a large scale, I don't see how it's different that a battery powered toy car, but with a linear motor instead of a rotary motor. Momentum isn't conserved when you are discharging energy in a battery to apply force.

It reminds me of a rail gun, although this one is all about the PM B-field, not the induced one.
 
  • #11
The OP's previous threads were about "Reactionless Drives", so I'm guessing that they are asking whether there is a net movement of the CoM in space, not on the surface of the Earth where you have things to push against and this is not an isolated system...
 
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  • #12
berkeman said:
The OP's previous threads were about "Reactionless Drives", so I'm guessing that they are asking whether there is a net movement of the CoM in space, not on the surface of the Earth where you have things to push against and this is not an isolated system...
OK, not so obvious to me now. Let's say the battery, magnet, and wires area all glued to a platform so none of that stuff moves, and it's floating in space. There is a Lorentz force at the magnet, but that B-field wraps around and goes through the battery and the rest. It's stronger at the magnet, so if you integrate around the current loop I think you get a net force.
 
  • #13
Now don't you be posting about Reactionless Drives... :wink:
 
  • #14
berkeman said:
Now don't you be posting about Reactionless Drives... :wink:
Yea, I get it. I'm focused too much on the electrons here. There are also protons present...

This is one of those things that makes me realize I knew a lot more about physics 40 years ago than I do now, and even worse, I'm worse at learning it now. It can't work, because of Newtons' 3rd law. But I do need to think about it some more.
 
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  • #15
If you put a wire in a magnetic field it feels a force, and presumably there's a reaction on the magnet, from the interaction between the wire's magnetic field and the magnet's. If you taped the wire to the magnet, then, presumably nothing would happen (neglecting any momentum carried away by radiation when you turn on the current). Is the circuit described above materially different from a wire taped to a magnet?
 
  • #16
Oh dear.

(1) If I have a current external to a magnetic field, the force on the current is equal and opposite to the force on the magnet.

(2) If I attach the current to the magnet, there is no net force. If I have the current flow through the magnet, that's just a way of attaching them.

(3) "The OP's previous threads were about "Reactionless Drives"" Then why are we wasting everybody's time on this nonsense?
 
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  • #17
With that good summary post by V50, the thread will remain closed. Thanks folks.
 

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