would there be a potential developed ?

[abhilash]

Hello everyone, welcome

 

 

 

 

Consider a straight long tube CD which on conduction electro-deposition of copper takes place at the cathode (-). The cathode is in contact with a long piece of wire AB (as shown in fig 3) which is connected to the negative terminal of the battery and the anode is connected to the positive terminal. PQRS is a piece of wire or a metal plate shaped as shown and is placed near this (AB – CD) arrangement.

 

 

We all agree with relativity, its two postulates and we all agree with the effects of Lorentz contraction on things moving with respect to us.

 

 

 

but why is the effect of Lorentz contraction on moving electrons in a wire often neglected ?

 

 

Often this question is answered this way - well, we know the wire is not charged, so let’s not speak about Lorentz contraction here (and let's neglect the first postulate of relativity here).

Another reason I have encountered is that electrons are considered as point charges, so again let’s not speak about Lorentz contraction here - (relativistic redistribution of electric field lines are possible even there and what about Lorentz contraction of the “lengths” between these points).

 

Aren't’t these like saying “you can disregard relativity in-order to support relativity” or as I quoted in my first post “even though the laws of physics are the same for all inertial reference frames, we can have different laws of physics (for the same phenomenon) in the same inertial reference frame”.

 

 

Now when you consider Lorentz contraction of moving electrons -if there is an increase in charge density of the moving electrons, shouldn't’t the wire acquire a net positive charge?

 

Yes, they will attract positive charges from the outside of the wire.Then if we place a metal strip then it should repel the electrons to the other end of the strip and hence we should measure a potential difference across the strip ? No and the reasons why we can't directly measure its effect that (with reference to the figure)-

1) The electric field which acts on the metallic strip CD is also acting on the probes of the potentiometer (Fig 3) which develops the same potential

across it (i.e., the probes of the wire that are equi-distant from the wire AB are at equi -potential and hence the potentiometer reads zero potential.

This creates a situation where the voltage developed can’t be measured directly. Moreover,

2) The calculated potential developed in case 2) is generally of the order of Pico-volts (for a few amps) and this clubbed with the above fact (1) makes

it even harder to detect this increase in charge density. If this is confusing, please see my orginal post where this is written in terms of force.of magnetic effect and please scroll down a bit until you see the above figure.

 

This was the conditions outside of the wire. Now, will the increased charge density of the conducting electrons draw more (+) charges from a possible reservoir (earth) along the length of the wire? There can’t be any effect to due to increase in charge density of the electrons inside the wire, because this is offseted by the redistribution of the electric lines of force of the moving charges as shown in the figure

 

 

and hence it will not draw opposite charges from the reservoir . (This answer is only partly answered here and I am not quite satisfied with this explanation since there has to be other effects of relativity in this –but this kind of reasoning has the decency that it does not disregard the theory of relativity in order to support it).

 

 

Now,would you please answer this question - would there be a potential developed across PS in the first figure ?

 

 

Thank you.

Edited August 16, 2010 by abhilash

[vuquta]

Hello everyone, welcome

 

 

 

 

Consider a straight long tube CD which on conduction electro-deposition of copper takes place at the cathode (-). The cathode is in contact with a long piece of wire AB (as shown in fig 3) which is connected to the negative terminal of the battery and the anode is connected to the positive terminal. PQRS is a piece of wire or a metal plate shaped as shown and is placed near this (AB – CD) arrangement.

 

 

We all agree with relativity, its two postulates and we all agree with the effects of Lorentz contraction on things moving with respect to us.

 

 

 

but why is the effect of Lorentz contraction on moving electrons in a wire often neglected ?

 

 

Often this question is answered this way - well, we know the wire is not charged, so let’s not speak about Lorentz contraction here (and let's neglect the first postulate of relativity here).

Another reason I have encountered is that electrons are considered as point charges, so again let’s not speak about Lorentz contraction here - (relativistic redistribution of electric field lines are possible even there and what about Lorentz contraction of the “lengths” between these points).

 

Aren't’t these like saying “you can disregard relativity in-order to support relativity” or as I quoted in my first post “even though the laws of physics are the same for all inertial reference frames, we can have different laws of physics (for the same phenomenon) in the same inertial reference frame”.

 

 

Now when you consider Lorentz contraction of moving electrons -if there is an increase in charge density of the moving electrons, shouldn't’t the wire acquire a net positive charge?

 

Yes, they will attract positive charges from the outside of the wire.Then if we place a metal strip then it should repel the electrons to the other end of the strip and hence we should measure a potential difference across the strip ? No and the reasons why we can't directly measure its effect that (with reference to the figure)-

1) The electric field which acts on the metallic strip CD is also acting on the probes of the potentiometer (Fig 3) which develops the same potential

across it (i.e., the probes of the wire that are equi-distant from the wire AB are at equi -potential and hence the potentiometer reads zero potential.

This creates a situation where the voltage developed can’t be measured directly. Moreover,

2) The calculated potential developed in case 2) is generally of the order of Pico-volts (for a few amps) and this clubbed with the above fact (1) makes

it even harder to detect this increase in charge density. If this is confusing, please see my orginal post where this is written in terms of force.of magnetic effect and please scroll down a bit until you see the above figure.

 

This was the conditions outside of the wire. Now, will the increased charge density of the conducting electrons draw more (+) charges from a possible reservoir (earth) along the length of the wire? There can’t be any effect to due to increase in charge density of the electrons inside the wire, because this is offseted by the redistribution of the electric lines of force of the moving charges as shown in the figure

 

 

and hence it will not draw opposite charges from the reservoir . (This answer is only partly answered here and I am not quite satisfied with this explanation since there has to be other effects of relativity in this –but this kind of reasoning has the decency that it does not disregard the theory of relativity in order to support it).

 

 

Now,would you please answer this question - would there be a potential developed across PS in the first figure ?

 

 

Thank you.

 

 

I tried to follow your argument that length contraction somehow affects charge.

 

You then assert one needs to refute SR to prove SR.

 

I am not seeing your logic mathematially.

 

Can you show specifically how these moving electrons refute SR?