Dielectrics and dipoles

[David513]

Hello all,

I'm a bit confused on why dielectrics do what they do. I know that when the electric field of a capacitor exists, the dipoles of a dielectric orient themselves in the direction of the capacitor's electric field and, thus, the dielectric produces an electric field that opposes the capacitor's electric field, reducing what I consider the "effective" value of the capacitor's electric field and increasing the capacitor's capacitance.

What I do NOT understand is what makes wood a stronger dielectric than metal. Is it because there is a greater amount of dipoles in wood which creates a stronger overall electric field in wood to oppose the capacitor's electric field? Or is it that there are similar numbers of dipoles in wood and metal but for some reason the force applied by the electric field in wood is of greater magnitude than in metal? (If that's the case, why is the force greater? Are the magnitude of the charges in wood greater than in metal?)

If you could shed some light on this matter it would be extremely helpful. Thank you!

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[aldol16]

Where do you get that wood is a stronger dielectric than a metal? Metals have an infinite dielectric constant...

http://webphysics.davidson.edu/physlet_resources/bu_semester2/c08_dielectric_constant.html

 

[David513]

... hmmm maybe I'm more confused than I thought. I thought the dielectric strength was directly/proportionally correlated with the ability to insulate. In other words, better insulators have higher dielectric constants. Metals are great conductors, so I thought they would have low dielectric constants, i.e. would have a weak opposing electric field to the capacitor's electric field, thus barely interfering with the capacitor's electric field while significantly decreasing the capacitor's capacitance.

 

[aldol16]

I think your question is an interesting one and one that a physicist may be able to explain better. I found this site in which responses seem to be pretty divided between zero and infinite for the dielectric constant of a conductor: https://www.physicsforums.com/threads/relative-permittivity-of-a-conductor.328298/

The problem is, we never really talk about dielectric constants with a conductor. So my own reasoning goes like this. Prima facie, one would expect that capacitance would go to zero because all you're doing is adding (what we assume to be) a perfect conductor to the circuit. In other words, imagine this. You have a simple circle with a voltage source and just a circular loop of wire. The wire is, of course, made of some assumed perfect conductor with zero resistance. Now, you cut the wire at one place and insert a capacitor to build up charge. This has just become the standard case of a simple capacitor. Now, the permittivity of free space is just a measure of the ability of a vacuum to permit electric fields within it. Now if you add something in between the plates of the capacitor, i.e. the dielectric, that has its own dipoles, its dipoles will line up opposite to the electric field set up by the capacitor and thus decrease electric field and voltage, thereby increasing capacitance. Q is constant in this case because I will first assume we are talking about an insulator. This is the typical model of a dielectric.

Now imagine what would happen if your "dielectric" was a conductor instead. Its dipoles would also line up with the capacitor electric field, but since it's a conductor, it will also permit charge to flow through the capacitor until equilibrium is reached. In other words, all you've done is short-circuited your system. Remember the piece of perfectly conducting wire you cut out earlier? Well, you've just replaced it with a perfectly conducting piece of metal, which is functionally identical to the piece of wire. So you've in effect made it so that the capacitor no longer works, i.e. C=0 and you just have a normal voltage source and "wire" again. Therefore, it stands to reason to me that the dielectric constant of your conductor is zero.

But now consider this. What I just explained isn't an explanation of a dielectric at all! A dielectric, as noted above, is something that does not permit charge flow through itself - in other words, an insulator. Therefore, this perfect conductor I just inserted could not be a dielectric by a rigorous definition of the term. So then how do I reconcile this? Well, imagine if you were able to make a perfect conductor that was somehow able to block charge flow through itself (an oxymoron, I know). Now, imagine that this metal can perfectly align its field vectors to cancel out the capacitor's electric field vectors. This results in no field and thus no potential difference in that segment of the circuit. C = Q/V and as V goes to zero, C goes to infinity. Thus, the dielectric constant must be infinite!

But again, this last case is oxymoronic in that a conductor cannot prevent charge flow through itself. Therefore, according to my logic, all you're doing is the first case, namely cutting out a piece of conducting wire and putting a larger, irregularly shaped piece of conducting wire in its place (assuming the dielectric covers the surface area of the capacitor and spans the whole way across, as is common).

 

[David513]

@aldol16 thank you for your thorough explanation. That makes complete sense. I guess on the MCAT I won't have to worry about metals being used as dielectrics, huh? Haha!

I guess my followup question would then be comparing two substances that could potentially be used as dielectrics (without simply completing the circuit like a metal would) and why one acts as a stronger dielectric than the other.

Let's take glass and paper. Glass has a higher dielectric constant than paper. So what is it about glass that makes it a stronger dielectric? Going back to my previous questions: Is it because there is a greater amount of dipoles in glass which creates a stronger overall electric field in glass to oppose the capacitor's electric field? Or is it that there are similar numbers of dipoles in glass and paper but for some reason the force applied by the electric field in glass is of greater magnitude than in paper? (If that's the case, why is the force greater? Are the magnitude of the charges in glass greater than in paper?)

 

[aldol16]

http://physics.info/dielectrics/

That website will be of interest to you. It's basically a function of which material will polarize easier - that depends on several factors actually so you should take a look at the link. A rule of thumb is that polar materials will have higher dielectric constants because they polarize more.

 

[David513]

Excellent, thank you kindly! (As always )