Capacitor and observer, what does he measure?

[BrianConlee]

An observer is standing right outside of a charged capacitor. Can he feel a negative or positive (or both) charge from the plate?

 

[K^2]

In the ideal capacitor, the external field is zero, so the observer wouldn't notice anything. However, real capacitor has an edge. The charges at the edges of the capacitor do produce an electric field that can be detected from outside the capacitor. You'd be seeing effect of both positive and negative charges then.

 

[BrianConlee]

When you say edges, do you mean the top and bottom edges or the outside edges?

Also, with the electric field, if I'm standing next to the negative plate of the capacitor, I'm going to measure the negative stronger since I'm closer?

I'm guessing as you increase the distance between the plates (all other things constant) you feel the field of the plate you're closer too more strongly than the other?

 

[K^2]

When you are measuring an electric field, you can't tell if it's caused by positive charge on one side, or negative on the other. For example, if you have entire space filled with negative charges, and then make a "hole" in the middle, it will create a field identical to a positive charge in the same spot.

By edges I mean the place where the plates end. The "ideal" capacitor is assumed to be "infinite", so there are no edges. Just two plates extending out to infinity. A real capacitor has to end somewhere. That's where the edge is.

 

[BrianConlee]

Thanks for the info K^2.

A few more questions maybe you can answer though...

With a capacitor, is the dielectric where the charge imbalance is developed, or do the actual plates have a charge imbalance?

I kinda understand what you mean by the hole... and the measuring... let me try this example.

I'm holding an electron standing outside a capacitor, next to the negative plate. Are you telling me the electron won't feel a force pushing it away?

And if I can ask this too, is there a way to positively charge an object other than rubbing it with something to induce static electricity? I want a solid object charged, so high heat ionization won't work.

Thanks again so much for your insight!

 

[K^2]

If you place an electron next to a negative plate, you'll have a little bit of force pushing it away. But that doesn't mean that it's just sensing the negative charges. It just tells you that the electric field points towards the negative plate.

The charges in the dielectric shift, but within dielectric, that doesn't really do anything. All it does is partially cancel the charges at the plates. You might want to read up on linear media to understand this a little better.

 

[BrianConlee]

If you place an electron next to a negative plate, you'll have a little bit of force pushing it away. But that doesn't mean that it's just sensing the negative charges. It just tells you that the electric field points towards the negative plate.

Isn't sensing the negative charges and being effected by the electric field lines the same thing in this instance?