A proton is travelling at a speed of 4 x 105 m/s moves into a series of charged parallel plates as shown below.

Question

I have the first part of question the answer is 6.7*10^5 m/s The plates were 0v -1000v and -1500v

Part 2) b. The same proton is fired again at the same speed towards a very large nucleus (parallel plates removed) and comes to rest 0.15 nm away from the atom.  What is the charge of the atom?  

Answer 1

I will set this up for you.

Q = charge of the target NUCLEUS

e = charge of proton (look it up in your book)

m = mass of proton (look it up in your book)

k = electric force constant (look it up in your book)

v = initial speed of proton (given)

x = final distance between proton and nucleus (given)

Conservation of energy: Assume the proton started from a very large distance away from the nucleus so that its initial potential energy is negligible. When it comes to rest near the target nucleus, its initial kinetic energy is converted totally to electrostatic potential energy.

mv²/2 = keQ/x

You know m, v, k, e, and x. Now solve for Q and finish from here. Be careful with the units!

NOTE: If the problem stated an initial separation distance between the proton and nucleus, the solution is a little more complicated, but you would still use conservation of energy. Without a diagram to examine, I assumed the proton approaches the nucleus from infinity.