# Homework Help: Proton trajectory grazing a charged sphere

1. Mar 3, 2017

### vishnu 73

1. The problem statement, all variables and given/known data

2. Relevant equations
potential energy = -kQq/r
potenial = kQ/r

3. The attempt at a solution
i am not quite sure how does l play a part in this experiment since it is far away l is insignificant and the only initial energy is kinetic energy and final is kinetic and electric potential energy i can calculate the final electric potential energy and initial kinetic energy is given so hence at most i can only calculate final kinetic energy but how is that going to help me find l

2. Mar 3, 2017

### ehild

It is a central force field, there is one more conserved quantity, what is it?

3. Mar 3, 2017

### Staff: Mentor

Moderator note: Thread title changed to better describe the problem. Previous title was too general (forum rules on thread titles).

4. Mar 3, 2017

### vishnu 73

is it the work done on the proton?

5. Mar 4, 2017

### ehild

Why should it be constant during the motion of the proton?

6. Mar 4, 2017

### ehild

In a central force field, like that, the angular momentum also conserves.

7. Mar 5, 2017

### vishnu 73

oh wow that is really smart is it because the force is constantly radially out ward hence no net torque on proton about the centre of sphere
so ,
initial energy = final energy
2000eV = 1000ev + 1/2 m vf2
vf = √(2000ev/m)

then by what you said conservation of angular momentum i am assuming you meant about the centre of sphere

hence
m ri x vi = m rf x vf
ri x vi = rf x vf
letting distance o to o' be s
√(l2 + s2) √(4000ev/m) sinθ = R √(2000ev/m)
√(l2 + s2) l/(√(l2 + s2)) = R /√2
l = R/√2
am i right for part a by the way thanks for the amazing insight really inspiring question

8. Mar 5, 2017

### ehild

It is right, good work!

9. Mar 6, 2017

### vishnu 73

omg thanks so much that one hint of yours helped me so much i was initially initially thinking of calculus