# Will the capacitance increase or decrease?

1. Apr 7, 2004

### lamp post

if we introduce a conductor b/w the plates of a capacitor then what will happen? will the capcitance increase or decrease?

2. Apr 7, 2004

### turin

If you mean momentarily, then the capacitor will discharge through the conductor at a rate determined by the instantaneous charge state, capacitance of the capacitor and conductance of the conductor.

If you mean permanently, then what is "the capacitance?" The capacitance across the configuration? If so, then it will remain the same, assuming that the conductor makes good contact with the plates. The overall reactance will become more inductive (I think), and the overall impedance will certainly change (pronounced at lower frequencies). One consequence will be that the capacitor can no longer store charge indefinitely. I suppose if you had some definition of capacitance that was based on the reactance, then it would decrease (that is, if it becomes more inductive as I assume it would).

3. Apr 7, 2004

### jdavel

lamp post,

Are you thinking of a parallel plate capacitor, with inititially nothing between the plates, and then you slide another metal plate into the empty space without touching the original plates?

If so, then I think the capacitance will increase. Originally the capacitance is proportional to 1/d where d is the plate separation. With the metal plate stuck in between, it will be 1/(d-t) where t is the thickness of the plate.

Last edited: Apr 7, 2004
4. Apr 7, 2004

### Integral

Staff Emeritus
Is this metal plate touching both plates of the capacitor? If the answer is yes, you now have a conductor not a capacitor.

If it is only touching one plate then you have simply moved that plate. you must recompute the capacitance using the new area and separation.

If it is not touching either plate then you have constructed a pair of series capacitors, each with capacitance that can be found by consideration, as above, with the separation and area. The total capacitance will be found using the formula for addition of series caps.