Another conductor near a charged conductor

[ananthu]

I need clarifications on the following points regarding a capacitor in general.
1.When you bring another conductor near a charged conductor, the capacitance of the latter is said to increase, because the presence of the second conductor near the first one lowers the potential of the former. Otherwise it should imply that the ability of the latter to hold charges should increase. But it is said that the capacitance of a conductor purely depends on its shape and size and the medium but not on the charge or potential. Like the capacity of a water tank depends only on its size but not on the amount of water! If it is so, in this case, how the presence of a neighboring conductor can increase the capacity of the conductor as its shape or medium in which it is placed no where changes?
2. It is said that the charges in a parallel plate capacitor is stored inside the dielectric portion but not on the plates. How could it be? Is it not the natural choice of the charges to go and stay only on the surface of a conductor?
3. Since the field inside a closed conductor is zero it is always safe to be inside a closed car rather than under a tree during lightning. OK. But if it is an enclosure made by a nonconductor such as a plastic, then what will happen to a person inside it during lightning? What will be the explanation?
I will be grateful to the forum members if they share their elaborate views on the above doubts that linger in me for quite a long time.

 

[Simon43254]

To the first, this occours because capcitance is about the total area, and the distance between the plates. So for a given circuit it's about the total area of the plates not each individual component as the circuit acts as "one".

To the second, the charges don't stay on the surface of the conductor, they stay everywhere, and I mean everywhere in the ciruit. It's just that on the plates they tend to decay as they're are attempting to complete the circuit by transfering to another part of the circuit. In some circumstances, they will be attracted or ripped off by air molecules, (hence why often metal plates may oxidise and you will find a residue appear on them). And be careful when you use the word choice. Suggesting an electron has a consious choice is a dangerous thing to say.

For your last question I don't quite understand what you mean. Are you asking as if there was a field within the enclosure. If not, then I'd say that nothing will happen, because the lightning won't be attracted to the non-conductive plastic.

 

[sophiecentaur]

The dielectric in a capacitor becomes polarised - all the electrons move a bit to the positive plate (you could say). This is the equivalent of having the plates much closer together but without the possibility of a short circuit.
Also-
If you sit 'inside' an insulator, the field across you will be much smaller than if you were outside because the 'Volts per metre' are higher in the insulator than across you and will limit the current that could flow through you. It's in series with you, effectively.
On the other hand, a conductor around you will conduct any current through itself and reduce the volts across you because it is, in effect, in parallel - the majority of the current will bypass you.
And, of course, there is the skin effect, which means the arc current - which comprises some very high frequencies - will never get that far inside your Faraday Cage.

PS try to avoid anthropomorphism when describing physical processes - it can give you the wrong idea about what's happening. Only humans and a few higher primates, possibly, have much of a 'purpose' in their actions. You can sometimes get away with "Behave as if they want to . . . . . "

 

[ananthu]

Thank you all for your kind reply.
Since I am also having some " writer element" in me, I , perhaps, might have used a word with a little poetic color!

 

[sardar]

1. The capacitance of a conductor is indeed dependent on its shape, size, and medium. However, when a second conductor is brought near a charged conductor, it creates a new electric field that interacts with the existing field of the first conductor. This causes a redistribution of charges on the surface of the first conductor, resulting in an increase in capacitance. Think of it like two magnets attracting each other – the closer they are, the stronger the magnetic field between them. Similarly, the closer the second conductor is to the charged conductor, the stronger the electric field between them, resulting in an increase in capacitance.

2. In a parallel plate capacitor, the charges are indeed stored on the surface of the dielectric material between the plates. This is because the dielectric material has a higher permittivity than air, which allows it to hold more charges on its surface. This arrangement allows for a larger capacitance compared to a single conductor. The charges on the plates themselves are just used to create the electric field that stores the charges in the dielectric.

3. The reason it is safe to be inside a closed car during lightning is because the metal body of the car acts as a Faraday cage, shielding the inside from the external electric field. This principle applies to any conductor, including a metal enclosure or even a metal umbrella. However, a plastic enclosure would not provide the same protection as it is not a conductor and cannot create a Faraday cage. The person inside would still be at risk of being struck by lightning.