Difference between an electric dipole and a capacitor?

[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 separation of positive and negative charges separated by a small distance. Are they actually the same thing ? If not how would they be defined as different to each other.

 

[confuted]

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.

 

[tiny-tim]

Welcome to PF!

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

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. ​

EDIT: See also http://en.wikipedia.org/wiki/Electret

 

[atyy]

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

 

[rogerharris]

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. ​

EDIT: See also http://en.wikipedia.org/wiki/Electret

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.

 

[rogerharris]

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

So am i getting this right. From what you are saying a dipole is defined by a separation of charges but not completely separated 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 similar to electron cloud probability distributions.

 

[rogerharris]

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.

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

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.

 

[tiny-tim]

dipole moments in neuroscience

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.
…
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.

Hi rogerharris!

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

and also threads like https://www.physicsforums.com/showthread.php?t=258168&highlight=capacitor" in the Biology sub-forum.

Perhaps you ought to start a new thread in Biology?

 

[rogerharris]

Hi rogerharris!

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

I think I've got this, dipoles are different from a capacitor as they have a dipole moment which is due to some degree of electron sharing between the poles ?

and also threads like https://www.physicsforums.com/showthread.php?t=258168&highlight=capacitor" in the Biology sub-forum.

Perhaps you ought to start a new thread in Biology?

A biology forum ? looks perfect i'll try that.

Whats the situation on keeping this thread open a while in case i need to revert back to just the physics ?

 

[tiny-tim]

I think I've got this, dipoles are different from a capacitor as they have a dipole moment which is due to some degree of electron sharing between the poles ?

I think so, yes …

to be honest, the real world isn't really my forte.

Whats the situation on keeping this thread open a while in case i need to revert back to just the physics ?

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!

 

[atyy]

So am i getting this right. From what you are saying a dipole is defined by a separation of charges but not completely separated 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 similar to electron cloud probability distributions.

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.

 

[rogerharris]

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.

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 separation 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 that's for another day.:zzz: