Dielectrics and the K Value Question

[justadream]

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TPR Physics

“Wax paper has a dielectric strength that’s about 5 times greater than that of air…If the maximum E (where E= electric field) is increased by a factor of 5, the maximum V that the plates can support is increased by a factor of 5 [since V = E*distance]. Because PE = .5*C*V^2, a capacitor with wax paper as a dielectric not only has a capacitance 3.5 times greater than an air capacitor (since K = 3.5 for wax paper), but its maximum V is increased by 5. Therefore, a capacitor with a wax paper dielectric can store 3.5 * (5^2) = 87.5 times more energy than the same capacitor with just air between the plates”



So for wax paper, K = 3.5.



If K = 3.5, why is the dielectric strength of wax paper FIVE times that of air (shouldn’t it only be by 3.5 times)?


BTW: Semi-related question: What types of materials make the best dielectrics?
I think TBR alludes to things that are “most polarizable” as being the strongest dielectrics.

Would that mean materials that conduct electricity (conductors) are the best dielectrics?

 

[t5Nitro]

TPR Physics

“Wax paper has a dielectric strength that’s about 5 times greater than that of air…If the maximum E (where E= electric field) is increased by a factor of 5, the maximum V that the plates can support is increased by a factor of 5 [since V = E*distance]. Because PE = .5*C*V^2, a capacitor with wax paper as a dielectric not only has a capacitance 3.5 times greater than an air capacitor (since K = 3.5 for wax paper), but its maximum V is increased by 5. Therefore, a capacitor with a wax paper dielectric can store 3.5 * (5^2) = 87.5 times more energy than the same capacitor with just air between the plates”



So for wax paper, K = 3.5.



If K = 3.5, why is the dielectric strength of wax paper FIVE times that of air (shouldn’t it only be by 3.5 times)?


BTW: Semi-related question: What types of materials make the best dielectrics?
I think TBR alludes to things that are “most polarizable” as being the strongest dielectrics.

Would that mean materials that conduct electricity (conductors) are the best dielectrics?

Your question says about 5 times greater than that of air, and then it proceeds to tell you it is 3.5 times. (K = 3.5 times higher). Just don't get tricked by wording.

For your next question, when you place a dielectric into an electric field, the + and - ends orient to the oppositely charged plates, thereby reducing the electric field, increasing the capacitance. They act as insulators, not conductors.

 

[justadream]

@t5Nitro

With regard to the first question, I'm surprised then that TPR decided to continue using "5" in their subsequent calculations

With regard to the second point, so is it the "most polarizable insulators" that are the best dielectrics then? (and in general, the more polarizable something it, the worse it is as an insulator)

 

[t5Nitro]

Check this out. This is how the polar substances align when in an electric field.

The dipole interactions serve to cancel out parts of the electric field and reduce the overall effective E field strength. E field strength and capacitance are inversely proportional. So since E field strength is decreasing, capacitance is increasing (which you can see that proportional relationship between C and k). More polar things will act like this. Water has a dielectric of 80.4, for example.

 

[justadream]

@t5Nitro

I see. "More polarizable" things also tend to be worse insulators right?

 

[t5Nitro]

Hmmm, maybe somebody can give their input on that. If I had to guess, with something like water having such a high dielectric constant, I would say they become better insulators. Keep in mind that's referring to pure/distilled water.