# Thread: 2 physic excercises

1. ## 2 physic excercises

First of all. I would not ask it if I had not missed consultation class today. Unfortunately I was late about 15 minutes and when I reached the class, the professor had left.

1) Charge of 30 nanoCulomb is at the center of the sphere. The radius of sphere is 10 cm. Electric field strength 20 cm from center is 1500 V/m and it is pointing away from center. What is charge of the net ?
I found that example http://physics.bu.edu/~duffy/semeste...rge_shell.html but it does not seem to be suitable for my case because I don't have a hollow sphere.

2) Between a capacitor plates flies an electron parallel to the plates. Speed of electron is 10 000 km/s. The distance between two parallel plates is 2,0 cm and length of the plates is 5 cm. The voltage of the capacitor is 30 V.
What is the speed of the electron at that moment when electron comes out from two prallel plates ?

2. Ok, I figured out the solution of the second excercise.
I am still in need of help for the first excercise.

3. Originally Posted by totalnewbie
1) Charge of 30 nanoCulomb is at the center of the sphere. The radius of sphere is 10 cm. Electric field strength 20 cm from center is 1500 V/m and it is pointing away from center. What is charge of the net ?
I found that example http://physics.bu.edu/~duffy/semester2/c06_pointcharge_shell.html but it does not seem to be suitable for my case because I don't have a hollow sphere.
It doesn't matter whether the sphere is hollow or not: this follows from Gauss' Law for Electrostatics.

You want the net charge inside a spherical Gaussian surface that encloses the entire sphere. You know that E is 1500 V/m a distance of 0.20 m from the center of the sphere. Since we are given only a radial distance from the center I am assuming that this E value holds for all positions on the Gaussian surface. So:
EA = q(enc)/epsilon0

q(enc) = (epsilon0)EA = (epsilon0)E(4*pi*R^2)
where R = 0.20 m is the radius of the Gaussian surface.

-Dan