Electric field formula vs coulombs law, contradiction?

1. Feb 22, 2008

Kurret

1) between two charged plates, the electric field is given by E=U/d=F/Q->F=QE
that means that the force on a particle between these two plates is independant on the position between them. But consider that you have two plates separated by a huge distance, then the force is constant wherever we place a charged particle between the plates. But if we use coulombs law and treat the plates as other charged particles we will get different results. How come?

2) in a plate? condensator we have C=eA/d (e=permitivity constant, d=distance, A=the plates area)
we also have C=Q/U
putting these equal to eachother gives:
eA/d=Q/U -> Q/(eA)=U/d=E
ie the electrical field, and the force on a prarticle in between the plates, is independant on the distance between the plates. I cant really get the logic in this one. for example lets say we have two charged plates with a distance of 1 cm from eachother, and then we move one plate away to the andromeda galaxy. That the force on a particle wherever in between these two plates still should be the same as with the distance of 1 cm is just absurd.

Can someone explain these results? im only still in high school so I havent studied so much advanced physics. Is it maybe because the formula for the electric field is just an apporximation of the real world?

Last edited: Feb 22, 2008
2. Feb 22, 2008

monish

These are reasonable questions. All of these parallel plate capacitor equations only work when the dimension of the plates is much greater than the spacing between the plates. With these constraints, the cases you describe follow as the formulas say they should.