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.