# Homework Help: Difference between an electric dipole and a capacitor?

1. Oct 20, 2008

### rogerharris

I have a very basic beginners question in physics and was wondering where to post it.

The post is going to be:

TITLE
What is the difference between an electric dipole and a capacitor ?

I am a trying to understand the difference between an electric dipole and a capacitor. The wikipedia entries for the definition are causing me confusion in that they appear to say they are the same thing.

Capacitor

A capacitor is a passive electrical component that can store energy in the electric field between a pair of conductors (called "plates"). The process of storing energy in the capacitor is known as "charging", and involves electric charges of equal magnitude, but opposite polarity, building up on each plate.

Electric Dipole

An electric dipole is a separation of positive and negative charge. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some, usually small, distance.

Both an electric dipole and a capacitor are defined as a seperation of positive and negative charges seperated by a small distance. Are they actually the same thing ? If not how would they be defined as different to each other.

2. Oct 20, 2008

### confuted

Re: Capacitors

A dipole is just two charges separated by some distance; consider perhaps an electron and a proton at some small distance apart.

A capacitor is a much larger, tangible thing, with two conducting plates separated by a dielectric. Each plate holds some charge (lots of charges), and an electric field exists between the two.

3. Oct 20, 2008

### tiny-tim

Welcome to PF!

Hi rogerharris! Welcome to PF!

I'll just add this to what confuted says:

I think the clue is in the word "pole", which indicates a point …

a capacitor is two areas, with a very nearly uniform field between them,

while a dipole is two points, with a not remotely uniform field.

One can think of lots of difference between the structure of the two, but the difference between the effect of the two is the difference between the very simple uniform field and the highly characteristic dipole field.

Last edited: Oct 20, 2008
4. Oct 20, 2008

### atyy

Re: Capacitors

A capacitor is a device for keeping positive and negative charge separated, just like a length of copper wire is a device for passing current. So a capacitor exists even when it is not charged, just like the wire exists even when no current is passed through it. A capacitor is essentially two conducting plates separated by an insulating gap. In introductory physics texts, the plates are usually taken to be infinitely large, flat, and parallel to each other, because it is easiest to calculate the electric field when those plates are oppositely charged. In general, the two plates can be any strange shapes you want. The key is that charge should not be able to jump across the insulating gap.

A ideal electric dipole is two point charges separated by a distance. The electric potential produced by an arbitrary static charge distribution can be roughly thought of as the sum of the electric fields produced by a single charge + an electric dipole + an electric quadrupole + ... When you are very far away, a strangely shaped charge distribution looks just like a single point charge. As you go closer, you notice that it's not exactly a point charge and looks more like two point charges separated by a small distance. The closer you get, the more details you notice. http://physics.unl.edu/~tgay/content/multipoles.html

Last edited: Oct 20, 2008
5. Oct 21, 2008

### rogerharris

Re: Welcome to PF!

Ok thanks for that..What would i do without physics forums

I guess the problem was really the wikipedia definition. It should say that dipoles are characterised by their magnetic field lines, and they converge or diverge very specifically to poles.

I found a lot of results for quadrupole and dipole capacitors in google. Aggh.

6. Oct 21, 2008

### rogerharris

Re: Capacitors

So am i getting this right. From what you are saying a dipole is defined by a seperation of charges but not completely seperated like a capacitor. They are still connected enough that there is a field interaction between them ?

Thanks for link. It's made me think why multipoles shapes look similiar to electron cloud probability distributions.

7. Oct 21, 2008

### rogerharris

Re: Capacitors

Would you know how the field between a capacitor positive and negative seperation, and a dipoles positive and negative seperation differs ?

This is a dipole

http://en.wikipedia.org/wiki/Image:Electric_dipole_field_lines.svg

This is a capacitor

http://en.wikipedia.org/wiki/Image:Capacitor_schematic_with_dielectric.svg

So one major difference between the two is whether charge converge to points ?

I better explain. I am studying neurosciences, where a great deal of it is defined in terms of charges, polarizations etc. Neuron cell walls could appear to have capacitance and neuron building blocks like tubulin are said to be dipoles, although they don't look like they have clear points.

http://www.physics.upenn.edu/~rossje/Tubulin-ribbons2.gif [Broken]

Actually i have just looked at this more, and it would seem these molecular dipoles are defined by something called a dipole moment. I assume that capacitors do not have this, and if i look into dipole moments it could help explain the difference between charge interactions of a capacitor and a dipole.

If this subject is beyond this forums remit, please let me know.

Last edited by a moderator: May 3, 2017
8. Oct 21, 2008

### tiny-tim

dipole moments in neuroscience

Hi rogerharris!

You may find http://en.wikipedia.org/wiki/Bond_dipole_moment useful

Perhaps you ought to start a new thread in Biology?

Last edited by a moderator: Apr 23, 2017
9. Oct 21, 2008

### rogerharris

Re: dipole moments in neuroscience

Last edited by a moderator: Apr 23, 2017
10. Oct 21, 2008

### tiny-tim

Re: dipole moments in neuroscience

I think so, yes …

to be honest, the real world isn't really my forte.
Threads never get locked unless people start being abusive, or insist on wandering off-topic.

The worst that can happen is that someone might mark it "[SOLVED]" … but that can't stop anyone who chooses to ignore it!

11. Oct 21, 2008

### atyy

Re: Capacitors

In an ideal electric dipole, the two point charges are completely separated by a distance.

If you have two very small metal spheres separated by a distance, they will be a capacitor. If you put positive charge on one sphere and negative charge on the other sphere, the electric field will be very similar to that of an ideal electric dipole. The smaller the charged metal spheres are, the more they will look like point charges. A charged capacitor consisting of two strangely shaped pieces of metal separated from each other by a distance will in create an electric field described by a multipole expansion containing dipole, quadrupole, and higher order terms.

12. Oct 21, 2008

### rogerharris

Re: Capacitors

Ok i think i'm beginning to get a few key things to tell the difference between the two here. Tell me if i have got this wrong.

1. a capacitor exists even when it is not charged so a dipole does not exist without charge.

2. A dipole has a measurable dipole moment which is due to some degree of electron sharing between the poles, whereas a capacitor just forms polarized layers in the dielectric between it's poles.

3. Dipole structures will tend to self organize themselves towards polar points, whereas
capacitance will tend to form charge seperation layers like cell walls. (I say self organize because in neuroscience the bits in question tend to self organize)

I'm still not sure what an example of a point charge is in reality but thats for another day.:zzz: