E-field in a conductor

lehomme

Joined Mar 2, 2005
2
I am having trouble understanding this concept. I read from my physics book that the E-field = 0 everywhere in perfect conductor even if the terminals of the conductor are connected to any externally applied voltage ( field).

This does not make any sense to me.

1. This is saying that the E-field can be discontinous, because if there is a voltage difference at the terminals, musnt there be an Eletric field between them? (i.e. -V = Int( E . dl))

2. Doesnt the fact that current is flowing imply that there must be an electric field present in the wire.

any help would be much appreciated.

Joined Jan 19, 2004
220
to the best of my knowledge e field is zero inside a conductor for static charges....so flowing charges that constitute current do not obey this rule

e field is zero within a conductor because the charges dont lie in the bulk of the conductor but on the surface..

moreover a potential difference cannot be maintained across a perfect conductor since it will cause infinite current to flow thru the conductor...hence the perfect conductor can only and always be mainted at one single potential..

Brandon

Joined Dec 14, 2004
306
Originally posted by lehomme@Mar 2 2005, 02:31 AM
I am having trouble understanding this concept. I read from my physics book that the E-field = 0 everywhere in perfect conductor even if the terminals of the conductor are connected to any externally applied voltage ( field).

This does not make any sense to me.

1. This is saying that the E-field can be discontinous, because if there is a voltage difference at the terminals, musnt there be an Eletric field between them? (i.e. -V = Int( E . dl))

2. Doesnt the fact that current is flowing imply that there must be an electric field present in the wire.

any help would be much appreciated.
[post=5759]Quoted post[/post]​
1. Don't think of it as discontineous, rather, that the perfect conductor does not exhist as a component. It is as if you just remove the perfect conductor from the circuit and put the circuit back together. For all intents and purposes, a perfect conductor does not disipate heat nor require any work and therefor can be totally neglected. If you connected a perfect conductor between the terminals of a battery, the E field would be between the cell and the cells internal resistance as now the 2 exterior terminals of the battery would be at the same potential. We would have an E field across the internal terminal and the internal resistance but not across the external terminals.

2. Not necessarily. Think of E field in terms of the force that must be applied to get current to flow within a conductor. Air requires a huge E field, i.e., you need tons of voltage to get current to flow in the air. Standard conductors require small E fields, i.e., very little voltage and you can get current to flow. Perfect conductors thereby have no E field since it takes no force to get the current to flow.

lehomme

Joined Mar 2, 2005
2
Thank you all for the detailed responses. Brandon, per your second statement. If in fact a perfect conductor does not require an E-field for current to flow, would that mean you could connect a perfect conductor in a loop and current would flow?

2. Also, in the case that there is a resistor in the cirucit in series with the external supply( connected with a perfect conductor), would that mean then:

a. there is an e-field across the power supply internal to it
b. there is no efield across the perfect conductor wire
c. there is an efield across the resistor

are these statements correct?

Originally posted by Brandon@Mar 2 2005, 01:22 PM
1. Don't think of it as discontineous, rather, that the perfect conductor does not exhist as a component. It is as if you just remove the perfect conductor from the circuit and put the circuit back together. For all intents and purposes, a perfect conductor does not disipate heat nor require any work and therefor can be totally neglected. If you connected a perfect conductor between the terminals of a battery, the E field would be between the cell and the cells internal resistance as now the 2 exterior terminals of the battery would be at the same potential. We would have an E field across the internal terminal and the internal resistance but not across the external terminals.

2. Not necessarily. Think of E field in terms of the force that must be applied to get current to flow within a conductor. Air requires a huge E field, i.e., you need tons of voltage to get current to flow in the air. Standard conductors require small E fields, i.e., very little voltage and you can get current to flow. Perfect conductors thereby have no E field since it takes no force to get the current to flow.
[post=5769]Quoted post[/post]​

Brandon

Joined Dec 14, 2004
306
Originally posted by lehomme@Mar 3 2005, 04:31 AM
Thank you all for the detailed responses. Brandon, per your second statement. If in fact a perfect conductor does not require an E-field for current to flow, would that mean you could connect a perfect conductor in a loop and current would flow?

2. Also, in the case that there is a resistor in the cirucit in series with the external supply( connected with a perfect conductor), would that mean then:

a. there is an e-field across the power supply internal to it
b. there is no efield across the perfect conductor wire
c. there is an efield across the resistor

are these statements correct?
[post=5797]Quoted post[/post]​
Both correct. In theory, since a perfect conductor possess no resistance to charge flow, current would flow freely, but there would need to be some form of difference or disturbance to get the flow initially started, but once started it would go forever. I.e. if you made a perfect conducting ring and passed a magnet through it once, it would have a loop of current flow now forever.

This is a concept many people have tryied to use to do the perpetual motion machines, but since there is no perfect conductor yet and its used as an approximation to make engineering easier, it will likely never happen in our life times.