# I Is Plate Capacitor Gap a "Non-linear" Medium?

1. Feb 14, 2018

### Fluxation

Could the space (air gap) between two DC charged parallel plates be considered to be a "non-linear" medium with respect to an EMF radiated by a coil contained within that space?

2. Feb 14, 2018

### sophiecentaur

Only if the PD were very near the breakdown potential. Superposition applies over a huge range of Field Strengths and Air is more likely to be linear (in the limit) than any solid dielectric, I think.

3. Feb 14, 2018

### Fluxation

In practical HV applications, the inner surfaces of the flat capacitor plates would probably be covered in a material such as mylar or kapton sheet. As I understand its molecules would be distorted by the superposed ES field gradient.

Would this of itself not give rise to some form of non-linearity with respect to intersecting EMF's?

Although not germaine to the OP, it might also be pointed out that in a spherical or cylindrical cap the charge distribution itself is non-linear.

All this no doubt becomes more complex if AC instead of DC is applied.

4. Feb 14, 2018

### Staff: Mentor

An ideal capacitor is linear, by definition. Any nonlinearity is a defect in the capacitor.

Most real capacitors are pretty close to linear if operated within their designed parameters.

5. Feb 14, 2018

### Fluxation

There appears to be a difference of opinion on this since the charge/discharge curves of a cap are exponential. There is no straight line relationship between V and I unless plotted together.

The dielectric material also needs to be factored in. Does it have ferroelectric properties?

http://www.eecg.toronto.edu/~ali/ferro/model.html

Here is a thought. What would happen if I sandwitched a flat spiral coil of insulated wire between two oppositely charged disks and fed it with a non-repetitive signal?

Could such a configuration be used to simulate a ferroelectric dielectric?

6. Feb 14, 2018

### Staff: Mentor

The defining equation $i=C\frac{d}{dt}v$ is a linear differential equation. As I said above, a capacitor is linear by definition

7. Feb 14, 2018

### Fluxation

I did not mean to disagree with you, but was rather inquiring about any predictable effect upon the ideal linearity of a plate cap that might be caused by insertion within it of an EMF radiating coil.

8. Feb 14, 2018

### Staff: Mentor

Regarding that, I agree with @sophiecentaur’s assessment in post 2.

9. Feb 15, 2018

### sophiecentaur

I think you mean an EM radiating coil. It's not appropriate terminology to talk of radiating an EMF.