Dumb question about conductors in electric field

[Kenan]

I've read multiple times that electric field inside of the conductor is always equal to zero because the inner field produced by electrons that moved to one side and left protons on another balance the outer field.
I've also read about Faraday cage.

But I have a few questions about this:

1) So it means that current can only flow on the surface of conductor no matter what?How can electric field be produced on the surface if it is always zero inside then?

Or maybe this is only for electrostatics?But I don't understand how Faraday cage works if clearly there is current on it.

I've also read that dielectrics polarize when they are placed inside of the electric wave but the dipoles are not able to create an inner field that completely balances the outer field,they say that it can only weaken it but not balance it.

2) K,sure,but then it means that in dielectric current can also flow through the inside of the dielectric unlike at the conductor?I don't understand this...

Sorry for dumb questions guys.

 

[mfb]

I've read multiple times that electric field inside of the conductor is always equal to zero because the inner field produced by electrons that moved to one side and left protons on another balance the outer field.

To a good approximation. Real conductors are not perfect, and time-dependent fields lead to deviations as well.

1) So it means that current can only flow on the surface of conductor no matter what?How can electric field be produced on the surface if it is always zero inside then?

Current flows in the inside as well. You just don't get charge concentrations there.

The Faraday cage works fine with currents on the surface only.

I've also read that dielectrics polarize when they are placed inside of the electric wave but the dipoles are not able to create an inner field that completely balances the outer field,they say that it can only weaken it but not balance it.

Right.

 

[Kenan]

To a good approximation. Real conductors are not perfect, and time-dependent fields lead to deviations as well.
Current flows in the inside as well. You just don't get charge concentrations there.

The Faraday cage works fine with currents on the surface only.

Right.

I've read from some page of the site that this works only for electrostatics,when charges are in equilibrium,so if there is current flowing through conductor,there WILL be electric field inside of the conductor.I assume that's correct,right?

But I cannot understand how can they explain how Faraday Cage works with electrostatics if the cage is clearly under the current,it is flowing through the cage.

 

[mfb]

I've read from some page of the site that this works only for electrostatics,when charges are in equilibrium,so if there is current flowing through conductor,there WILL be electric field inside of the conductor.I assume that's correct,right?

If the conductor has a finite resistance and some current is flowing, there is an internal field, yes. This field keeps the current flowing. It is observable as voltage drop at this resistor.

 

[Kenan]

If the conductor has a finite resistance and some current is flowing, there is an internal field, yes. This field keeps the current flowing. It is observable as voltage drop at this resistor.

That's right,but how can the Faraday cage principle be explained with electrostatics if there is an obvious current going through the cage,so it means that it is supposed to get inside of the cage,no?

 

[mfb]

but how can the Faraday cage principle be explained with electrostatics

Who suggested that?
You can describe the shielding of static fields with electrostatics (where you ignore any transients and look at the equilibrium situation only), but not the shielding of time-dependent fields.
The current flow can happen at the outside only, it depends on the cage.

I don't think mixing cables and Faraday cages helps here.

 

[Kenan]

They always explain it the same way on every single page:

"There is an external (outer) electric field that (as an instance) is pointed to the right,electrons on the other hand move to the left and protons stay there where they had been before the electrons moved (at the right side) and so electrons with protons create their own electric field that is opposite to the outer electric field,thus canceling it in a matter of split seconds"

But there WAS current inside it's just it was canceled very,very quickly,that's what makes me puzzled.
If you attach a battery to the cage there is a constant current going through the cage,it just never ends.It's not like it's an open circuit,it is definitely closed but the body still does not get electrocuted.That's what I can't understand.

There IS a current inside but it gets canceled by the internal field in split second,but if there is a generator then the current is constant,no?

 

[mfb]

But there WAS current inside

Not necessarily "inside" in the same way as "in the interior of the material".

Again, I think mixing cables and cages just leads to confusion.

 

[Kenan]

Not necessarily "inside" in the same way as "in the interior of the material".

Again, I think mixing cables and cages just leads to confusion.

I know right but I've seen this video where a man was wearing full metal equipment (I think it was Aluminium or something) and well he was being zapped with the Tesla coil but he felt nothing,that's what I don't understand.
The current clearly goes through the entire equipment including the inner part where his body is kept but he feels no pain of whatsoever.

Same with the Faraday cage - touching it from the inside deals no harm to you.

Even in electrostatics it clearly says that it happens very,very quickly but it is still a current.
I'm sincerely sorry for annoying you,it's just this makes no sense to me...

 

[mfb]

The current clearly goes through the entire equipment including the inner part where his body is kept but he feels no pain of whatsoever.

No it does not. It is limited to the parts that are good conductors - the suit around the man.

Same with the Faraday cage - touching it from the inside deals no harm to you.

Yes, because there is no static net charge on the inside.

Even in electrostatics it clearly says that it happens very,very quickly but it is still a current.

In electrostatic nothing "happens", there are no timescales involved, otherwise it is not electrostatics.

 

[Kenan]

No it does not. It is limited to the parts that are good conductors - the suit around the man.

Yes, because there is no static net charge on the inside.

In electrostatic nothing "happens", there are no timescales involved, otherwise it is not electrostatics.

So the fact that Faraday cage does not conduct electricity from the inside can only be explained via electrostatics and not via electrodynamics?

It's just if we look at it another way via electrodynamics there is current...it's just it seems like this electrostatic explanation is some kind of an excuse,they all explain it the same way but ignore the fact that there is indeed current flowing through the entire cage but somehow there is no electric field inside even though there is current...

Maybe I'm dumb but is not in reality there is current?It is almost impossible to prove the electrostatic explanation in real life,they make it look like it's so perfect...

 

[mfb]

So the fact that Faraday cage does not conduct electricity from the inside can only be explained via electrostatics and not via electrodynamics?

How did you get that impression?
Electrodynamics is more powerful than electrostatics. Electrostatics is a special case of electrodynamics, it can be a useful simplification in many cases.
Both allow to explain why there are no net charges on the inside (this has nothing to do with current flow!).
Both allow to explain why the current flow through the human is tiny (but you don't need a cage for that).

 

[Kenan]

Electrodynamics is more powerful than electrostatics. Electrostatics is a special case of electrodynamics, it can be a useful simplification in many cases.
Both allow to explain why there are no net charges on the inside (this has nothing to do with current flow!)

Could you give me your own explanation of why there is no net charge inside the cage with both electrodynamics and electrostatics then?I just cannot understand why it has nothing to do with the current flow if it clearly exists inside for a brief moment.

The electrostatics explanation is rather simple,especially the part where it clearly states that the inner field balances the outer field but it was the inner field that moved the electrons to one side in first place.

I'm truly sorry for annoying you,it's just this explanation does not answer why a man in metal costume feels no current when it zaps his costume,even if the inner electric field balances the outer field,I mean that it happens after a very short period of time,so there was a current for a very brief moment but only after that moment it was balanced,the current disappeared.

 

[mfb]

If your external field does not change, the charge configuration in the conductor quickly (nanoseconds) reaches an equilibrium. And we don't care about nanoseconds here.

 

[Dale]

I've seen this video where a man was wearing full metal equipment (I think it was Aluminium or something) and well he was being zapped with the Tesla coil but he felt nothing,that's what I don't understand.

Are you familiar with circuits? What happens when you put a big resistor (person) in parallel with a short circuit wire (metal equipment)?

 

[Kenan]

Are you familiar with circuits? What happens when you put a big resistor (person) in parallel with a short circuit wire (metal equipment)?

Well I'm kinda familiar with them...well the connection in this circuit is neither parallel nor series,so current does not split,no?So the current is just supposed to go by the wire to the resistor and then come back to the generator by the other side of the wire,no?But since the circuit is short then it's going to be happening very rapidly.Oh wait...does this somehow have something to do with the fact that there is less current in resistor because it's resistance is much higher than wire's?But is not it still going through the resistor just with much lower power?

It's just what I don't understand is that you guys say that conductor's electrons reach equilibrium very quickly so we just ignore this current and pretend that it does not exist inside but is not current actually flowing like this?It's just the potential difference is being constantly created by the battery,thus not allowing the current to stop flowing,like a never ending cycle.

I DO understand why electrons move to the positive side of the outer field,I do understand why they create an internal electric field that cancels the outer field thus electric field inside of the conductor is zero but what I cannot understand is how can it be like that if there is obvious current flowing through the conductor,is not this basically supposed to mean that current cannot flow inside of the conductor at all if it flows so rapidly that we ignore it?

 

[Dale]

the connection in this circuit is neither parallel nor series

The connection is parallel. The suit is a short circuit for any current that would go through the person.

 

[Dale]

Oh wait...does this somehow have something to do with the fact that there is less current in resistor because it's resistance is much higher than wire's?But is not it still going through the resistor just with much lower power?

Yes. This type of circuit is called a current divider. The current splits between the two paths according to the ratio of their conductances. So the highly conductive suit takes almost all of the current, while the much less conductive human is unharmed.

 

[Kenan]

Yes. This type of circuit is called a current divider. The current splits between the two paths according to the ratio of their conductances. So the highly conductive suit takes almost all of the current, while the much less conductive human is unharmed.

Makes sense,i understand that due to Ohm's law human body takes much less current since the most current is at the low resistant part of the suit,the costume itself.

But here is the hard part that my brain is not able to comprehend:

Someone is inside of the Faraday cage or inside of the suit,the current goes through the suit or the wire (so charges ARE indeed moving and we cannot just say that the electric field inside of the conductor is zero cause there IS current) , that someone touches the cage from the inside or his entire body is touching the suit,why does not that someone feel current going through his body then?Why does that someone feels the current only when he/she/it touches it from the outside and not inside?

There is indeed current going through the metal costume/cage so the charges are NOT in equilibrium,so we cannot assume that electric field inside of the conductor is zero,so we cannot assume that this example is about electrostatics because there is indeed movement of charges.

How can this be explained when we assume that current is moving through the conductor AKA metal suit or Faraday cage?

 

[Dale]

Why does that someone feels the current only when he/she/it touches it from the outside and not inside?

Because then they are in series with the suit instead of in parallel.

 

[Kenan]

Because then they are in series with the suit instead of in parallel.

Yes it makes sense but even when someone is inside of the suit that someone is still touching it from the inside with his hands,it's just like touching the cage while being inside of it and in this case it seems like this circuit is still in series,why?Here:

battery - suit - human

I believe that if it was for the charge equilibrium then the current was not even supposed to be moving due to electroSTATICS,so pretending that current is indeed moving like in real life for me makes most amount of sense,but i don't understand why current does not flow through the inside of conductor...

Feel free to think that I'm a stubborn moron but I can't understand how can moving current examples in real life be explained with electrostatics that clearly state that there is no current at all.

 

[Dale]

I can't understand how can moving current examples in real life be explained with electrostatics that clearly state otherwise.

Personally, I wouldn't use the electrostatic explanation for a non electrostatic scenario. Particularly when the circuit analysis is available instead. The problem is that you are mis analyzing the circuit.

when someone is inside of the suit that someone is still touching it from the inside with his hands,it's just like touching the cage while being inside of it and in this case it seems like this circuit is still in series,why?Here:

battery - suit - human

Current always flows in a closed loop. How does the current get back to the battery?

 

[Kenan]

Personally, I wouldn't use the electrostatic explanation for a non electrostatic scenario. Particularly when the circuit analysis is available instead. The problem is that you are mis analyzing the circuit.

Current always must flow in a closed loop. How does the current get back to the battery?

Right,my bad,I did not mean to say that,now I'm even more puzzled...

It's just my point is that even if you attach the whole suit to a high voltage generator,very high voltage generator the current will not flow inside of the conductor for a certain reason,only outside (UNLESS it melts due to high temperatures)

That's what I don't understand - how can current flow only on the outside of the conductor if it clearly states that if electric field inside of the conductor is zero then there is no current,so it must not be zero so that current can be made.

I just can't find explanation for this,I've searched everywhere.

 

[Dale]

That's what I don't understand - how can current flow only on the outside of the conductor if it clearly states that if electric field inside of the conductor is zero then there is no current,so it must not be zero so that current can be made.

I am trying to explain that but you keep coming back to the electrostatic explanation, which we already agree does not apply. Stop wasting my time and yours by complaining about the electrostatic explanation (which we agree is not valid when there is current), and instead try to learn the circuit explanation (which is valid when there is current).

Please answer the question I posed above. In the circuit you started describing above, how does the current get back to the battery?

 

[Kenan]

I am trying to explain that but you keep coming back to the electrostatic explanation, which we already agree does not apply. Stop wasting my time and yours by complaining about the electrostatic explanation (which we agree is not valid when there is current), and instead try to learn the circuit explanation (which is valid when there is current).

Please answer the question I posed above. In the circuit you started describing above, how does the current get back to the battery?

I'm really sorry for that,i did not mean to make anyone angry here,well now I'll only talk about the circuit:

Well I think that if we attach the entire suit to the battery and put a man inside of the suit,the current will only go through the suit back to the battery and this cycle repeats again and again,does this mean that the man is not a part of the circuit and that's why he's not electrocuted?He is not connected to the battery.

Battery - man - battery is a circuit but it's a completely different case and in our Faraday cage or the suit the electrons only go around the suit because...it leads them back to the battery,they've got nowhere else to go?

But if man touches the suit or the cage that is attached to a battery from the outside he feels being zapped,why does not current just go to the battery ignoring the man?It's not like the man is also connected to the battery.

 

[sophiecentaur]

This rather depends on what the man is attached to. If he is hanging from an insulating rope then he will 'charge up' due to his own capacitance and the PD between him and the battery. The amount of charge he can acquire can be enough to kill him as it flows through him to the various parts of his body (which started at a low initial voltage, relative to Earth). There are occasions when people do not die from high voltage contact when the current happens to pass through a limb and not the brain or heart.
When he is inside the cage, the metal suit is such a low resistance that there can be only a tiny PD between any two parts of the man. Hence, virtually no current will flow through his body.
Dalespam has already explained (much more patiently than I often do) that the suit is effectively in parallel with the man inside and shunts any current. He also explained that the man, outside the suit is in series with it. The suit resistance is so low that it is the same as if he just touched the wire when outside, in fact.
Do not apologise about things - just act more sensibly and read + take on board --in detail -- what people are telling you. (If you actually want to learn something)

 

[Kenan]

When he is inside the cage, the metal suit is such a low resistance that there can be only a tiny PD between any two parts of the man. Hence, virtually no current will flow through his body.
Dalespam has already explained (much more patiently than I often do) that the suit is effectively in parallel with the man inside and shunts any current. He also explained that the man, outside the suit is in series with it. The suit resistance is so low that it is the same as if he just touched the wire when outside, in fact.
Do not apologise about things - just act more sensibly and read + take on board --in detail -- what people are telling you. (If you actually want to learn something)

Aha...I get it,I think - no matter what example do we take (Faraday cage or metal suit) the man inside of it will not feel anything at all because he is is in parallel with the suit (I'm assuming that we could even pretend that he is not a part of the circuit) then the wire takes most amount of current but if he is in series with it (touching the circuit from the outside) then the amount of current going both through the suit and human is the same - that's why touching the electrified suit or the cage outside of it is dangerous.

Am I right now?

 

[sophiecentaur]

Am I right now?

Pretty much.
Once the suit has been connected to the cable and settled down. If the supply is DC then the man could probably get out of the suit and be OK as long as the wire is well away from Earth. If it is close then the E field (Volts per metre) could be problematical.

 

[Dale]

I think that if we attach the entire suit to the battery and put a man inside of the suit,the current will only go through the suit back to the battery and this cycle repeats again and again,does this mean that the man is not a part of the circuit and that's why he's not electrocuted?

Yes. There is a path which involves the man. It goes battery-suit-man-suit-battery. There is also a path which does not involve the man. It goes battery-suit-battery. That is why the connection is a parallel connection. There are multiple alternative paths.

The suit path has much lower resistance than the suit-man-suit path, so most of the current goes that way.

 

[sophiecentaur]

The suit path has much lower resistance than the suit-man-suit path, so most of the current goes that way.

The low suit resistance will ensure that, whatever current is flowing (up to hundreds of Amps) the PD across any two parts of the inside of it, which the wearer may be touching, will only be a few Volts maximum - harmless.
There are a dozen ways of looking at this problem and there is no limit to possible scenarios but the same basics apply and they are the same for basic Resistor - Capacitor networks, once you get the model right.