# Formula for electric field for capacitor and cylinder

1. Feb 19, 2014

### derek181

My prof gave two formulas without much explanation as to where he got them from. For a parallel plate capacitor he gave it as E=k2∏q/A and for a cylinder he gave E=kq2/Lr where L is the length of the cylinder and r is the radius and A is the area. Can someone please explain how he derived these.

2. Feb 20, 2014

### tiny-tim

hi derek181!

he gets it from the electric displacement field, D, which is always equal to the surface charge density, ie charge/area …

D = q/A or D = q/2πrL

(and see here)