Static electricity, it's all capacitance?

Thread Starter

AngryGecko

Joined Jul 7, 2017
44
Static electricity between objects all comes down to capacitance between them that is charged to a certain voltage, right? Making sure the charge remains constant if the capacitance were to change. Just want to have it confirmed that there is no other effect going on here.
 

#12

Joined Nov 30, 2010
18,210
It seems that way to me. Static electric = charge holding still. Not flowing. No conductance. No conductors. No conductors means no inductors. The only thing left is capacitance.
 

Thread Starter

AngryGecko

Joined Jul 7, 2017
44
Thanks for the reply! How would the capacitor equivalent circuit be when you rub a baloon against your hair and then stick it to a wall? (could not find this on google)
 

Thread Starter

AngryGecko

Joined Jul 7, 2017
44
This is for a conducting sphere but it gives you an idea what the capacitance is.

View attachment 138758
Isn't the capacitance between the charged objects what determines the capacitance? For example when two objects in space build up a charge it's equivalent of having a capacitor with the same capacitance as the two charged objects have together. The voltage should change as the objects get closer or further away from each other (because the capacitance changes) in order to remain the same charge. Is this a proper way of seing this?
 

MrAl

Joined Jun 17, 2014
7,849
Isn't the capacitance between the charged objects what determines the capacitance? For example when two objects in space build up a charge it's equivalent of having a capacitor with the same capacitance as the two charged objects have together. The voltage should change as the objects get closer or further away from each other (because the capacitance changes) in order to remain the same charge. Is this a proper way of seing this?
Hi,

You do know that for two plates that form a capacitor and has a charge there will be a physical force between the two plates right? So that means that in order to move one or both plates you'd have to overcome this force. Doing that would require extra energy (or energy removal) which would change the equilibrium state of the system because you will be changing the total energy of the system. You'd have to figure out how this changes the charge distribution. Capacitance increases as the plates move closer together, so the charge gets more compact. A larger capacitance with the same charge would have to have a different voltage. So i guess as you move them closer together the energy of the force required to do this will convert into a change in voltage (lower).
You would look into this more carefully though.
 

Thread Starter

AngryGecko

Joined Jul 7, 2017
44
Hi,

You do know that for two plates that form a capacitor and has a charge there will be a physical force between the two plates right? So that means that in order to move one or both plates you'd have to overcome this force. Doing that would require extra energy (or energy removal) which would change the equilibrium state of the system because you will be changing the total energy of the system. You'd have to figure out how this changes the charge distribution. Capacitance increases as the plates move closer together, so the charge gets more compact. A larger capacitance with the same charge would have to have a different voltage. So i guess as you move them closer together the energy of the force required to do this will convert into a change in voltage (lower).
You would look into this more carefully though.
Didn't think of the charge distribution, but does it have an effect when converting these charged objects into an equivallent capacitance or can the charge distribution be ignored for this task? Also, as previously posted by crutschow, the capacitance of one object alone seems to be possible to calculate. This confuses me because i have learned capacitance is determined by both the objects that get charged, their distance, etc and not only by one object or the other?
 

MrChips

Joined Oct 2, 2009
22,113
Not sure where this is leading to but here is the math.

Capacitance C, charge Q, and voltage V are inter-related by the formula:

Q = C x V

If no additional charge is put on the capacitor then the charge Q remains the same.
If you move the plates of the capacitor away from each other, the capacitance decreases while the voltage increases.

The energy in the capacitor is ½ C x V^2 = ½ Q x V.

Since Q is constant and V increases, the energy has been increased. This energy was supplied by the work done in pulling the plates apart against the opposing attractive force created by the electrostatic field as measured by the voltage V.

The opposite occurs when you move the plates closer to each other. Energy is released and absorbed by your arms moving the plates together.
 

OBW0549

Joined Mar 2, 2015
3,566
The voltage should change as the objects get closer or further away from each other (because the capacitance changes) in order to remain the same charge. Is this a proper way of seing this?
Yes; it's also the principle of operation of a capacitor microphone.
 

nsaspook

Joined Aug 27, 2009
7,752
Didn't think of the charge distribution, but does it have an effect when converting these charged objects into an equivallent capacitance or can the charge distribution be ignored for this task? Also, as previously posted by crutschow, the capacitance of one object alone seems to be possible to calculate. This confuses me because i have learned capacitance is determined by both the objects that get charged, their distance, etc and not only by one object or the other?
Both are right. The physics capacitor value capacitance (as normally seen in physics types of questions) is a intrinsic quantity (Electric potential) of space between a point charge at one end of a circuit and infinity (V=0) as the other end of the circuit.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/potpoi.html

The electronics capacitor value of capacitance is normally seen as the equivalent of parallel plates in space of a set size and distance.
 

Thread Starter

AngryGecko

Joined Jul 7, 2017
44
So as a summary: the theory of static electricity all comes down to capacitors connected together, that are charged to diferent voltage potentials at different capacitance values leading to the different charges. If two "capacitors" are connected with one "lead" it will be equivalent of having stray capacitance between the two floating "leads". Have I got it right or am I missing something here? Thank you for all the help!
 

MrAl

Joined Jun 17, 2014
7,849
Didn't think of the charge distribution, but does it have an effect when converting these charged objects into an equivallent capacitance or can the charge distribution be ignored for this task? Also, as previously posted by crutschow, the capacitance of one object alone seems to be possible to calculate. This confuses me because i have learned capacitance is determined by both the objects that get charged, their distance, etc and not only by one object or the other?
Hi,

There is never really one object.
If i say i have a rubber ball and ask you to calculate the potential energy, what can you say about it?
That's just one object. In order to calculate anything, you have to know something else, a second bit of data, and that is how far above ground it is. The difference between the height of the ball and the height of ground (normally taken to bs zero) gives you a means to calculate something, and that means two objects really.

A point charge has field lines that emanate out from it, but where exactly do they go? They do not circulate they start at one place and end at another place. That second place is infinitely far away from the point in all directions. That's where we get the second shell from.

A sphere is almost like a point and the field lines emanate out from it in all directions. Where they go brings in the concept of a ground at infinity. The ground in this case is symmetrical around the sphere and is infinitely far from the sphere in all directions. That makes it a giant sphere.

So there are two things, you just dont see one of them but it is implied.

In most everyday electrical work we measure voltages between two points, but in theory we may not be able to do that without assuming some limit and making use of some theoretical ideas like infinite resistance. If we had a meter with infinite input resistance we may be able to measure some of these things by imposing a limit because the field decreases fast with distance (1/r^2) so we may be able to get useful results with distances that are not infinite but are large enough to swamp other errors.. We can sometimes find other ways to handle these questions too though. For example, a Tesla coil with one end of the coil open produces a plasma at the end which emanates out from the end of the wire. But we know a current flow requires a circulation from one place to another and back again. Just where does the open end of that plasma stream find it's 'ground'? It's almost like a ground at infinity but here it is actually closer, yet we dont really see how it gets from the end to the ground we just see a small stream.

I am not really sure what you are seeking here though. Are you trying to understand capacitance itself or space itself or something else. Space itself has a capacitance also.
 
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Thread Starter

AngryGecko

Joined Jul 7, 2017
44
Hi,

There is never really one object.
If i say i have a rubber ball and ask you to calculate the potential energy, what can you say about it?
That's just one object. In order to calculate anything, you have to know something else, a second bit of data, and that is how far above ground it is. The difference between the height of the ball and the height of ground (normally taken to bs zero) gives you a means to calculate something, and that means two objects really.

A point charge has field lines that emanate out from it, but where exactly do they go? They do not circulate they start at one place and end at another place. That second place is infinitely far away from the point in all directions. That's where we get the second shell from.

A sphere is almost like a point and the field lines emanate out from it in all directions. Where they go brings in the concept of a ground at infinity. The ground in this case is symmetrical around the sphere and is infinitely far from the sphere in all directions. That makes it a giant sphere.

So there are two things, you just dont see one of them but it is implied.

In most everyday electrical work we measure voltages between two points, but in theory we may not be able to do that without assuming some limit and making use of some theoretical ideas like infinite resistance. If we had a meter with infinite input resistance we may be able to measure some of these things by imposing a limit because the field decreases fast with distance (1/r^2) so we may be able to get useful results with distances that are not infinite but are large enough to swamp other errors.. We can sometimes find other ways to handle these questions too though. For example, a Tesla coil with one end of the coil open produces a plasma at the end which emanates out from the end of the wire. But we know a current flow requires a circulation from one place to another and back again. Just where does the open end of that plasma stream find it's 'ground'? It's almost like a ground at infinity but here it is actually closer, yet we dont really see how it gets from the end to the ground we just see a small stream.

I am not really sure what you are seeking here though. Are you trying to understand capacitance itself or space itself or something else. Space itself has a capacitance also.
Without going to deep into what capacitance actually is, I'm trying to find if all electric charges can be represented by an/some equivalent electronic "capacitor(s)" charged to a certain voltage, then do the calculations based around that. Thank you for the reply!
 

MrChips

Joined Oct 2, 2009
22,113
Static electricity does not require capacitors, conductors or voltage.
Static electricity is created by stripping electrons off materials or conversely, adding electrons to materials.

You can demonstrate this easily with two strips of Scotch tape (cello tape or plastic adhesive tape).
Take two strips of cello tape, about 10-15cm length each and tape them separately to a laminate table top.
Pull the tapes sharply off the table top.
Now try to bring the two strips together by holding the strips together only at the top end.
The bottom ends will repel each other because they have both acquired the same charge polarity when they were pulled off the table top.

You can find videos and variations of this experiment on the internet.

http://www.physics.usyd.edu.au/supe...ies/Electricity_and_Magnetism/Tape_Charge.pdf
 

nsaspook

Joined Aug 27, 2009
7,752
Static electricity does not require capacitors, conductors or voltage.
Those are the things normally needed for current electricity. I don't think anyone has implied it was needed for static. We've all responded to the question and yes, static electricity does require capacitance because static is an IMBALANCE (CHARGE SEPARATION) between quantities of positive and negative particles which existed beforehand.

http://amasci.com/redgreen.html
A bit about the misleading label "static electricity"... What's being created here is not an invisible substance called "static." Instead, we are creating an imbalance between the quantities of negative and positive electrical "stuff" which were already there in the matter. Once they are separated, it's not necessary that the charges remain "static" upon the surfaces. The charge does not need to remain unmoving. It's not the static-ness of the charge which creates all the interesting effects, it's the imbalancing and separation of the plus and minus. Also, "Static electricity" is not the complement of "current electricity," since "imbalance" is not the opposite of "motion." Charge-flow and charge-imbalance are actually two different phenomena which can even occur together.
 

MrAl

Joined Jun 17, 2014
7,849
Those are the things normally needed for current electricity. I don't think anyone has implied it was needed for static. We've all responded to the question and yes, static electricity does require capacitance because static is an IMBALANCE (CHARGE SEPARATION) between quantities of positive and negative particles which existed beforehand.

http://amasci.com/redgreen.html
Hi,

I think maybe what MrChips was getting at was that for example we can have a single point charge ffoating around in deep space so there is no apparent capacitance because it's just one thing. We could though say that the other plate is at infinity as we had been talking about before i guess.

But then another aspect comes to mind here. There's no way to get rid of capacitance entirely because it's ever present even if we dont have a physical capacitor in hand. Space between any two objects that have non zero surface area must show the propensity to store charge and therefore must possess the quality we call capacitance. So interestingly, even if we have no charge we can have what we call capacitance because it can be looked at as the ability to store charge, even if none is present just yet.
It's like there is nothing that can happen without it encountering capacitance, as if it was impossible to get rid of or just happens to be a property of the universe like space and time. An automobile takes up space, yet we dont go around stating that a car requires space for example. It's implied, and if we have space available, it has the ability to hold a car even if there is no car yet parked there. There are times when we do state this though, but that's when the application demands that information (parking garage).
So i guess we get more into philosophy again. When we talk about specific circuits we have everything defined better so we always know what we are dealing with.
 

BR-549

Joined Sep 22, 2013
4,938
Capacitance is NOT the ability to store charge. Capacitance is a reactance/response to electric field flux. It's a result, not a cause.

Capacitance is a measure of how much local electric field flux aligns, with an external electric field flux.

The energy stored and expended is not due to charge storage......it's due to field alignment. The dielectric controls the alignment.....not the amount of charge.

A charge is NOT a quantum wave function. It's a real physical entity/object with size, shape and area. That's why it can line up.

An atom is simply a series tuned charge circuit. That's all it is.

Capacitance......electric flux alignment. Inductance.....magnetic flux alignment.
 

nsaspook

Joined Aug 27, 2009
7,752
Hi,

I think maybe what MrChips was getting at was that for example we can have a single point charge ffoating around in deep space so there is no apparent capacitance because it's just one thing. We could though say that the other plate is at infinity as we had been talking about before i guess.
Sure, but that's another non-physical circuit invented to allow the calculation of a physics discovery that people tend to confuse as related to current electricity. As soon as we need to actually measure some electrical quality of the space around this mythical isolated single point charge we need to add a test charge, so it's no longer isolated. I also agree with @BR-549 for once, capacitors don't store charge, they store energy in an electric field.
 
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