Why are conductors not used to store electricity?

[NowsTheTime]

I read somewhere that conductors don't really store charge? How can this be correct?

When a conductor is charged by conduction, the electrons spread throughout the surface of the conductor. Doesn't this mean that the capacitor is storing this electricity? What prevents conductors from being used to store electrical energy?

 

[nasu]

Yes, a piece of metal acts like a capacitor and can store some energy.
However the capacitances are too small for practical purposes. Unless the conductors are made really really big in size (see capacitance of the Earth to get some idea)
To increase the capacitance without increasing the size to unpractical values systems of several conductors are used, as in capacitors. Capacitors can store significant amounts of energy.
With the development of super-capacitors, this modality of energy storage has already entered the practical, commercial domain.

 

[NowsTheTime]

Yes, a piece of metal acts like a capacitor and can store some energy.
However the capacitances are too small for practical purposes. Unless the conductors are made really really big in size (see capacitance of the Earth to get some idea)
To increase the capacitance without increasing the size to unpractical values systems of several conductors are used, as in capacitors. Capacitors can store significant amounts of energy.
With the development of super-capacitors, this modality of energy storage has already entered the practical, commercial domain.

I see. What factors determine the capacitance of a single conductor?

 

[nasu]

Size, shape. And the surrounding medium.

https://en.wikipedia.org/wiki/Capacitance

 

[jbriggs444]

When a conductor is charged by conduction...

A conductor is not charged by conduction. It does not build up more charge when a current is passing through it than when there is no current.

 

[russ_watters]

A conductor is not charged by conduction. It does not build up more charge when a current is passing through it than when there is no current.

In other words, the electrons are already there, they just start moving when electricity flows. Like a pipe already full of water before you open the tap.

 

[NowsTheTime]

In other words, the electrons are already there, they just start moving when electricity flows. Like a pipe already full of water before you open the tap.

So why is it that a conductor can go from a charge of 0 coulombs to a charge of -4 coulombs? Aren't electrons entering the conductor and being stored?

 

[jbriggs444]

So why is it that a conductor can go from a charge of 0 coulombs to a charge of -4 coulombs? Aren't electrons entering the conductor and being stored?

They are not. If you shove 4 coulombs of electrons in one end, 4 coulombs come out the other.

 

[Vanadium 50]

Aren't electrons entering the conductor and being stored?

They are exiting as fast as they are entering. Like a pipe.

 

[CWatters]

That's assuming there is somewhere for them to exit to. If the conductor isn't connected to anything then charge can be stored in the small capacitance between it and earth. However the capacitance is too small to make this useful.

 

[jbriggs444]

That's assuming there is somewhere for them to exit to. If the conductor isn't connected to anything then charge can be stored in the small capacitance between it and earth. However the capacitance is too small to make this useful.

Indeed. We're talking about something much less than a picofarad. Storing 4 Coulombs in a picofarad capacitor... That would require billions of volts, the energy equivalent of several tons of TNT and you'd have exceeded the breakdown voltage of the air by a huge factor before even getting close.

 

[nasu]

A conductor is not charged by conduction. It does not build up more charge when a current is passing through it than when there is no current.

I think he means "conduction" as used in some introductory text about electrostatics. Is used as one of the modes to charge a neutral body: by touching it with another, charged body. The other modes may be by induction and by friction.
It is not about electrical conduction with the same exact meaning used in the topics about electric current.
A body can be charged by this "conduction".

http://www.physicsclassroom.com/class/estatics/Lesson-2/Charging-by-Conduction

 

[NowsTheTime]

They are exiting as fast as they are entering. Like a pipe.

If electrons exit the conductor as other electrons enter, then why is it that a conductor can have a charge of "n" coulombs?

 

[NowsTheTime]

I think he means "conduction" as used in some introductory text about electrostatics. Is used as one of the modes to charge a neutral body: by touching it with another, charged body. The other modes may be by induction and by friction.
It is not about electrical conduction with the same exact meaning used in the topics about electric current.
A body can be charged by this "conduction".

http://www.physicsclassroom.com/class/estatics/Lesson-2/Charging-by-Conduction

Yes. That is the definition of conduction that I was referring to.

 

[CWatters]

If electrons exit the conductor as other electrons enter, then why is it that a conductor can have a charge of "n" coulombs?

The flow in and out is only equal once the capacitance of the wire is charged. However that happens very quickly because the capacitance is very small.

You cannot put "n coulombs" of charge into the capacitance of a wire. You can put "n coulombs" through a wire. There is a difference.

A small water pipe cannot store a billion cubic feet of water but a billion cubic feet might pass through it.

 

[anorlunda]

There is at least one context where the capacitance to ground is not small: Long-distance high-voltage AC power transmission lines. For example, a 750 kV, 1000 kilometer line. Underground or underwater power cables are even worse.

If you energize one end of such a line, and leave the far end open circuited, then the voltage rises km by km all along the length of the line because of that capacitance. That configuration must be prevented to protect components from overvoltage damage.

 

[David Lewis]

The flow in and out is only equal once the capacitance of the wire is charged. However that happens very quickly because the capacitance is very small.

Indeed. We're talking about something much less than a picofarad.

The OP did not specify the size of the conductors, how close they were to the ground or to each other.

 

[ZapperZ]

Yes. That is the definition of conduction that I was referring to.

Then you've been using the wrong name! "Conduction" means the FLOW of charges. You are talking about static electricity, which, by definition, isn't a flow charges. This is why your question created such a confusion.

A conductor cannot store energy efficiently because it has mobile charges, which means that it can easily lose or gain charges simply via contact, even with air! And contrary to our ability to cause static charges in conductors, it really is difficult to get it to store a lot of charges. Think of all the effort one needs to put in in a Van de Graaf generator. And then what happens when you turn it off? How quickly are those charges from the metallic dome dissipated?

Zz.

 

[David Lewis]

Thank you. You're bringing up some good points. Would it be correct under the broad definition of conduction to call the plates of a capacitor conductors?

 

[ZapperZ]

Thank you. You're bringing up some good points. Would it be correct under the broad definition of conduction to call the plates of a capacitor conductors?

Conductors are the material. "Conduction" is a process. It is OK to say that the plate of a capacitor is a conductor, because that is the material being used. But under static condition, there is no "conduction".

Zz.

 

[nasu]

The term used in school texts is "charging by conduction". So it is a process. There is electron flow from on conductor to another during the charging process so it is not technically wrong. It may be just confusing or not very common.
I think "charging by contact" (used in other sources) is a less confusing one.

 

[David Lewis]

Very good answer. If my understanding (and terminology) are correct, when you first connect a battery to a capacitor, some current does flow, although it might be for a short amount of time.

 

[ZapperZ]

Very good answer. If my understanding (and terminology) are correct, when you first connect a battery to a capacitor, some current does flow, although it might be for a short amount of time.

That is a transient effect. In fact, this transient current is often used in undergraduate experiments on RC circuits to determine the time constant of that circuit.

But under static condition, which I stated earlier, there is no current flow in the circuit.

Zz.

 

[nasu]

Who said that there is a current under static condition?

 

[NowsTheTime]

Then you've been using the wrong name! "Conduction" means the FLOW of charges. You are talking about static electricity, which, by definition, isn't a flow charges. This is why your question created such a confusion.

A conductor cannot store energy efficiently because it has mobile charges, which means that it can easily lose or gain charges simply via contact, even with air! And contrary to our ability to cause static charges in conductors, it really is difficult to get it to store a lot of charges. Think of all the effort one needs to put in in a Van de Graaf generator. And then what happens when you turn it off? How quickly are those charges from the metallic dome dissipated?

Zz.

That's very interesting. So how should the question have been wording so as to minimize confusion?

 

[NowsTheTime]

The term used in school texts is "charging by conduction". So it is a process. There is electron flow from on conductor to another during the charging process so it is not technically wrong. It may be just confusing or not very common.
I think "charging by contact" (used in other sources) is a less confusing one.

Yes, that is exactly what I was referring to: "charging via conduction". This is the term that I was taught to use when referring to the transfer of electrons to the less negative conductor when two conductors of different charges are brought into physical contact with each other. Is there a different term used to describe this same process?

 

[David Lewis]

And contrary to our ability to cause static charges in conductors, it really is difficult to get it to store a lot of charges. Think of all the effort one needs to put in in a Van de Graaf generator.

Maybe the problem is more storing charge at high voltage than storing a lot of charge per se.