What is the capacitance?

What is the capacitance if one plate is bigger than the other?

• 1) If you build a capacitor with two rectangular plates of different dimensions, how do you calculate its capacitance 2) If you know the capacitance between 2 plates, and you want to replace one of the plates with a different shape (like a sphere) what dimensions do you consider in order to keep the same capacitance as it was before (when replacing one of the plates with a sphere). I would REALLY appreciate this because it would help my understanding of capacitors. Thanks in advance.

• Answer:

  1. A capacitor works by storing energy in the electric field between the two plates. If the two plates are not the same size, the field between the overlapping areas will be more or less constant; in the region that does not overlap, the field will quickly fall to zero as the distance away from the overlapping areas increases. In other words, the capacitance will be slightly larger than if the two plates were the same. 2. As described above, the electric field is what's important. So you need to figure out what the field will be.