# Help - Electric Field Questions

1. Sep 7, 2006

### DetroitDJ

There's two questions on my current homework assignment that I keep getting the wrong answer too (the answer input site tells me it's wrong, but has nothing on why or any hints or anything). If someone could be so kind to walk me through how to solve these, I'd be really grateful.

The electric field strength 4.00 cm from a very wide charged electrode is 4000 N/C. What is the charge (in nC) on a 1.10 cm diameter circular segment of the electrode?

My method for that one so far has been utilizing the formula for the electric field of a disk, solving for Eta and using Eta in Eta=Q/A to find Q - but that's somehow wrong.

An electron is released from rest 2.0 cm from an infinite charged plane. It accelerates toward the plane and collides with a speed of 1.0*10^7 m/s. What is the surface charge density of the plane? What is the time required for the electron to travel the 2.0 cm?

Basically the same problem, but from a different point of view. Same trouble.

2. Sep 7, 2006

### Tomsk

For the first part, your equation is wrong for E, it should be E=Q/2$\epsilon_{0}$A, or E=$\sigma$/2$\epsilon_{0}$, where $\sigma$ is the surface charge density. What is Eta? Find out the surface charge density, then use that to find the charge on the circular segment. I'm not sure about this actually, as I haven't used the distance from the electrode- I assumed the electric field was constant, I can't see why it wouldn't be.

For the second part, you can find the acceleration, and hence the field strength, by considering the force. Then getting $\sigma$ should be fairly simple, it's the same eqn as the first part.