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Author Topic: Will a charge near a conductor with a steady current experience a force?  (Read 703 times)

Offline theThinker

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A straight conductor with current will attract another conductor with a parallel current - Lorentz magnetic force empirically tested.

Question: Will  a stationary charge near a  conductor with a steady current experience any force by the conductor?
Answer: No; from theory.

I'll like to know if anyone has come across any empirical observation, either direct of indirect, that confirms there cannot be (absolutely sure) that confirms zero force.

But what has never been verified is only assumed. The thing is, probably no one ever tested it and no one is interested. Furthermore,  it may not be simple task to isolate a charge and make it steady.
« Last Edit: 01/09/2016 08:27:00 by chris »


 

Offline Bored chemist

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Re: Force on a static charge by line current.
« Reply #1 on: 31/08/2016 22:12:47 »
" Will  a stationary charge near a  conductor with a steady current experience any force by the conductor?"
Yes it will.
It will be attracted to the conductor- regardless of any current flowing through it.

Why wouldn't it?
 

Offline PmbPhy

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Re: Force on a static charge by line current.
« Reply #2 on: 01/09/2016 02:07:36 »
Quote from: theThinker
Question: Will  a stationary charge near a  conductor with a steady current experience any force by the conductor?
Answer: No; from theory.
That is correct.

Quote from: theThinker
I'll like to know if anyone has come across any empirical observation, either direct of indirect, that confirms there cannot be (absolutely sure) that confirms zero force.
To be honest that's like asking for empirical evidence that a wheel will roil. It's something that all physicists know by heart and what their research abd experiments are based on as are the things built by engineers. However you could look into the original publications which led to the laws of electromagnetism. I'm just not sure where you'd find it. You could start with Faraday's diaries at http://www.faradaysdiary.com/

The force on a charged particle is known as the Coulomb-Lorentz Force and is defined as

F = q(E + vx[/b]B[/b])

where q is the charge of the particle, v is its velocity and E and B are the electric and magnetic fields respectively. The law is due to Oliver Heaviside so you could look up his work.

Quote from: theThinker
But what has never been verified is only assumed. The thing is, probably no one ever tested it and no one is interested. Furthermore,  it may not be simple task to isolate a charge and make it steady.
All of the laws of physics have been carefully tested so don't worry about that. Especially Maxwell's laws of electrodynamics. A huge amount of work has gone into them. And if any part of them were wrong it'd have shown up by now. Especially what you're asking about. If it was wrong then certain experiments and inventions would not work correctly.
 

Offline PmbPhy

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Re: Force on a static charge by line current.
« Reply #3 on: 01/09/2016 02:20:10 »
Quote from: theThinker
Question: Will  a stationary charge near a  conductor with a steady current experience any force by the conductor?
Answer: No; from theory.
That is correct.

Quote from: theThinker
I'll like to know if anyone has come across any empirical observation, either direct of indirect, that confirms there cannot be (absolutely sure) that confirms zero force.
To be honest that's like asking for empirical evidence that a wheel will roil. It's something that all physicists know by heart and what their research abd experiments are based on as are the things built by engineers. However you could look into the original publications which led to the laws of electromagnetism. I'm just not sure where you'd find it. You could start with Faraday's diaries at http://www.faradaysdiary.com/

The force on a charged particle is known as the Coulomb-Lorentz Force and is defined as

F = q(E + vx[/b]B[/b])

where q is the charge of the particle, v is its velocity and E and B are the electric and magnetic fields respectively. The law is due to Oliver Heaviside so you could look up his work.

Quote from: theThinker
But what has never been verified is only assumed. The thing is, probably no one ever tested it and no one is interested. Furthermore,  it may not be simple task to isolate a charge and make it steady.
All of the laws of physics have been carefully tested so don't worry about that. I mean, how do you think these laws were determined in the first place? They were determined by experiments. It was seen in experiments done in a lab that charged bodies experience no force when in the presence of only a current carrying wire. A current carrying wire generates a magnetic field only. Such wires are electrically neutral because its the negatively charged electrons which move while the positively charged ion lattice which remains stationary. In real life, however, there is always a very small electric field outside current carrying wires. Its a necessity due to the required continuity of the electric field inside the wire.

Especially Maxwell's laws of electrodynamics. A huge amount of work has gone into them. And if any part of them were wrong it'd have shown up by now. Especially what you're asking about. If it was wrong then certain experiments and inventions would not work correctly. There's a good book out called Electrodynamics from Ampere to Einstein by Olivier Darrigol. You should look at this book. I'm sure you'll find it in there. There are of course other history books. Another text to check with is A History of the Theories of Aether & Electricity: Two Volumes Bound As One by Edmund T. Whittaker. You can download these books from here: http://book4you.org/   You have to register first but its free. See also: http://bookzz.org/

" Will  a stationary charge near a  conductor with a steady current experience any force by the conductor?"
Yes it will.
It will be attracted to the conductor- regardless of any current flowing through it.

Why wouldn't it?
Because the force on a charged particle due to a magnetic field is a the cross product of the particles velocity and the magnetic field. In this case the velocity is zero so the force is zero. That's why it wouldn't.
« Last Edit: 01/09/2016 02:37:07 by PmbPhy »
 

Offline theThinker

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Re: Force on a static charge by line current.
« Reply #4 on: 01/09/2016 06:18:03 »
Quote from: theThinker
Question: Will  a stationary charge near a  conductor with a steady current experience any force by the conductor?
Answer: No; from theory.
That is correct.

Quote from: theThinker
I'll like to know if anyone has come across any empirical observation, either direct of indirect, that confirms there cannot be (absolutely sure) that confirms zero force.
To be honest that's like asking for empirical evidence that a wheel will roil. It's something that all physicists know by heart and what their research abd experiments are based on as are the things built by engineers. However you could look into the original publications which led to the laws of electromagnetism. I'm just not sure where you'd find it.


Quote from: theThinker
But what has never been verified is only assumed. The thing is, probably no one ever tested it and no one is interested. Furthermore,  it may not be simple task to isolate a charge and make it steady.
All of the laws of physics have been carefully tested so don't worry about that. Especially Maxwell's laws of electrodynamics. A huge amount of work has gone into them. And if any part of them were wrong it'd have shown up by now. Especially what you're asking about. If it was wrong then certain experiments and inventions would not work correctly.
I know there is a belief that such very fundamental stuffs "should have been tested" or "it must have been tested". If we are interested in a very specific question, it is nothing better to have an incontrovertible answer - eg. "Cavendish use a torsion balance ...",...

I give an example. Aristotle believed that a heavier object falls faster than a lighter one. Everyone just repeated after him for almost 1600 years until Galileo actually demonstrated by letting fall two metal balls, one large and one small. Aristotle  was wrong.

Now Ampere, Biot  and Savart tested magnetic effect of currents. At that time, the electron was not discovered. So they already tested that parallel currents attract - those experiments were always about currents, never using any charge objects .But none seems to have tested by putting a charged  object near a current.

Even the Lorentz formula F = q(E + v X B) was put forward about the time of the discovery of the electrons. He somehow extended the current element (I ds) to a pure charge. But we now know that the element (I ds) is electrically neutral, different from a (di) for a pure line electron current element which would be negatively charged. No one seems to have tested putting a charged object near a current in a wire.
 

Offline hamdani yusuf

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Quote
I give an example. Aristotle believed that a heavier object falls faster than a lighter one. Everyone just repeated after him for almost 1600 years until Galileo actually demonstrated by letting fall two metal balls, one large and one small. Aristotle  was wrong.
Wrong conclusions usually involve false assumptions. In this case, I think it was assumption that air friction is negligible where it is not.
We can see when it rains, large water droplets fall faster than small water droplets, even though they consist of the same material.
 

Offline hamdani yusuf

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I have tried to do the experiment using electrically charged torsion balance. I used ping-pong balls covered with aluminum foil, which then are connected to 4 kV DC source (hence one ball has potential of +2 kV while the other has -2 kV).
But it's hard to reach a firm conclusion since either balls are attracted to even neutral objects.
 

Offline agyejy

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Re: Force on a static charge by line current.
« Reply #7 on: 01/09/2016 10:12:11 »
Because the force on a charged particle due to a magnetic field is a the cross product of the particles velocity and the magnetic field. In this case the velocity is zero so the force is zero. That's why it wouldn't.

While this is true it is an oversimplification. For starters the presence of the charge near the conductor repels the like charges and attracts the opposite charges which changes the charge distribution and results in an attractive force between the conductor and the charge. The flow of current doesn't really change this situation.

However, beyond even that there does seem to be a case for electric fields outside of a current carrying resistive wire due to the current.

Furthermore, in the 2nd edition of Electromagnetic Fields by Wangsness on page 210 it is stated:

Quote from: Wangsness
As we see from the figure, Jf, is directed longitudinally along this uniform conductor, as is E since they are parallel. Thus, at the surface, E will be tangential, and since the tangential components of E are continuous by (9-21), there will be a tangential field outside the conductor which is given by E = Jf/σ. This is in marked contrast to the static case where not only was E = 0 inside the conductor, but it was necessarily normal to the surface, as we saw in (6-2) and Figure 6-1b.

 

Offline theThinker

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I have tried to do the experiment using electrically charged torsion balance. I used ping-pong balls covered with aluminum foil, which then are connected to 4 kV DC source (hence one ball has potential of +2 kV while the other has -2 kV).
But it's hard to reach a firm conclusion since either balls are attracted to even neutral objects.
Hello Hamdani,

I am surprised you did an experiment. Do you mean you did the experiment in the past. If just recently only after reading this thread, then you would be a researcher with access to a  "torsion" balance.

I think there is no need of a precision experiment yet. What we are interested is whether there are other physics besides that of the Lorentz force that predicts a zero force classically.

A simple experiment could be done by someone with some nimble fingers. We use the aluminium foil found in our kitchen. Cut a small light thin I-shaped strip and make it hang very freely somehow, but insulated.  We must then know how to charge it positive and negative (I don't know how to do this). We expect interaction with a current (if any) to change direction when the current/charge  are reversed in sign. So we could easily tell if a force acts on the foil provided the force is of sufficient strength.
 

Offline theThinker

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Re: Force on a static charge by line current.
« Reply #9 on: 01/09/2016 16:20:10 »
Because the force on a charged particle due to a magnetic field is a the cross product of the particles velocity and the magnetic field. In this case the velocity is zero so the force is zero. That's why it wouldn't.

While this is true it is an oversimplification. For starters the presence of the charge near the conductor repels the like charges and attracts the opposite charges which changes the charge distribution and results in an attractive force between the conductor and the charge. The flow of current doesn't really change this situation.

However, beyond even that there does seem to be a case for electric fields outside of a current carrying resistive wire due to the current.

Furthermore, in the 2nd edition of Electromagnetic Fields by Wangsness on page 210 it is stated:

Quote from: Wangsness
As we see from the figure, Jf, is directed longitudinally along this uniform conductor, as is E since they are parallel. Thus, at the surface, E will be tangential, and since the tangential components of E are continuous by (9-21), there will be a tangential field outside the conductor which is given by E = Jf/σ. This is in marked contrast to the static case where not only was E = 0 inside the conductor, but it was necessarily normal to the surface, as we saw in (6-2) and Figure 6-1b.
I think the case about an electric field outside of a conductor with current can come only from relativistic considerations. It would be interesting if there are real test conducted to verify it. Currently, most textbooks give only the physics dependent solely on q v X B.   
 

Offline theThinker

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Apology to  agyejy.

I did not realise you gave a link. I am now reading the article.

Thanks.
 

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