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hokhani

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hokhani

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tech99

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It is interesting that the two forces oppose each other but do not usually balance.

In the old cgs system of units, the forces were used to define the units of electric and magnetic field strength. Maxwell noticed that their ratio was equal to the speed of light, and this led him to suppose that light was electromagnetic.

- #3

hokhani

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Could you please tell which two forces; The forces between the two wires (action and counteraction) or the transverse electric force and magnetic force on the moving charges?It is interesting that the two forces oppose each other but do not usually balance.

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tech99

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Sorry I misunderstood your original question. The Hall effect is surely very small - we know from observation that wires attract.Could you please tell which two forces; The forces between the two wires (action and counteraction) or the transverse electric force and magnetic force on the moving charges?

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https://www.physicsforums.com/threads/hall-voltage-vs-laplace-force.879412/

Read especially post #6.

- #6

sweet springs

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Two wire are coming closer with increasing kinetic energy that is supplied with battery, not from magnetic field generated by the current, I think.Battery works. Magneric field does not work.

Direction of current electrons is curved and deviated by magnetic field. Electrons are constrained within wire. As a simple model electrons collide with wire surface boundary and transmit momentum and energy to wire body. Battery provide energy to help electrons have their original speed relative to wire body. This is interpreted as increase of resistance of wire.

So we may say batteriy lasts longer when wire is alone than there is another current wire nearby.

Direction of current electrons is curved and deviated by magnetic field. Electrons are constrained within wire. As a simple model electrons collide with wire surface boundary and transmit momentum and energy to wire body. Battery provide energy to help electrons have their original speed relative to wire body. This is interpreted as increase of resistance of wire.

So we may say batteriy lasts longer when wire is alone than there is another current wire nearby.

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Two wire are coming closer with increasing kinetic energy that is supplied with battery, not from magnetic field generated by the current, I think.Battery works. Magneric field does not work.

Direction of current electrons is curved and deviated by magnetic field. Electrons are constrained within wire. As a simple model electrons collide with wire surface boundary andtransmit momentum and energyto wire body. Battery provide energy to help electrons have their original speed relative to wire body. This is interpreted as increase of resistance of wire.

So we may say batteriy lasts longer when wire is alone than there is another current wire nearby.

I have my doubts about momentum because internal forces cannot change the overall momentum of a system ( the wire is a system of electrons+positive ions) but yes I believe the force between the electrons and the surface charges at the boundary of wire is what does the work, as internal forces can do work.

I believe the magnetic field force is responsible for the change in momentum of the wire, but internal forces between electrons and ions are responsible for the change in kinetic energy of the wire.

- #8

hokhani

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I think the electrons immediately become gathered by magnetic force on one side of the wire and the electric force of them on the moving electrons cancels the magnetic force and so the moving electrons can go in a straight line without deviation. However, the reaction electric force of these non-deviated electrons, on the electrons gathered on the side of the wire, causes motion of the wire. Therefore, it is the electric field that does work.

On the other hand, we expect the gathered electrons on the sides of the two wires repel each others (suppose the same direction of currents for the two wires) and so a repulsion force between the two wires. It seems a discrepancy here, because on one hand, we expect the two wires become closer but on the other hand they repel each other. The third possibility is that these two forces cancel each other and the wires remain fixed without moving.

On the other hand, we expect the gathered electrons on the sides of the two wires repel each others (suppose the same direction of currents for the two wires) and so a repulsion force between the two wires. It seems a discrepancy here, because on one hand, we expect the two wires become closer but on the other hand they repel each other. The third possibility is that these two forces cancel each other and the wires remain fixed without moving.

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I agree completely with the aboveI think the electrons immediately become gathered by magnetic force on one side of the wire and the electric force of them on the moving electrons cancels the magnetic force and so the moving electrons can go in a straight line without deviation. However, the reaction electric force of these non-deviated electrons, on the electrons gathered on the side of the wire, causes motion of the wire. Therefore, it is the electric field that does work.

That repulsion force (assuming same direction of current for the two wires) is small compared to the Laplace force. However if the wires are placed very close then this repulsion force might not be negligible. But when the wires are placed too close, the Laplace force becomes bigger as well. Maybe it is safe to say that the repulsion force is negligible in comparison to the Laplace Force.On the other hand, we expect the gathered electrons on the sides of the two wires repel each others (suppose the same direction of currents for the two wires) and so a repulsion force between the two wires. It seems a discrepancy here, because on one hand, we expect the two wires become closer but on the other hand they repel each other. The third possibility is that these two forces cancel each other and the wires remain fixed without moving.

- #10

hokhani

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Both Laplace and repulsion forces become stronger as the wires get closer. It seems that these two forces cancel each other. We should see, by experiment, what really happens when the two current-carrying wires are placed close together. I haven't tried this experiment, so far.I agree completely with the above

That repulsion force (assuming same direction of current for the two wires) is small compared to the Laplace force.

- #11

sweet springs

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I have my doubts about momentum because internal forces cannot change the overall momentum of a system ( the wire is a system of electrons+positive ions)

Yea I feel uneasiness about Newton's third law concerning Lorentz force. Say Lorentz force push electron as action, as reaction electron push what ? EM field must be the answer but how it actually is?

- #12

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The Lorentz force can be derived from the action principle, leading to the balance equation for momentum. There you'll see that both matter and the electromagnetic field carry momentum, and the em. forces are given by Maxwell's stress tensor (which is part of the symmetric gauge invariant Belinfante energy-momentum tensor of the field).

- #13

sweet springs

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[tex]\frac{d}{dt}[\ p^\alpha+\frac{1}{c^2}\int N^\alpha dv\ ]=\int T^{\alpha\beta}dS_\beta[/tex]

where alpha,beta =1,2,3 for space region of volume v and surface S, p is momentum of charged particle, N is Poynting vector and T is em stress tensor.

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