# Homework Help: Proton moving towards center of ring

1. Nov 6, 2012

### Zarrey

1. The problem statement, all variables and given/known data
A proton is moving along the main axis of a uniformly charged thin ring. The charge density on the ring is 5.0 nC / cm and the ring radius is 1.0 cm. Initially the proton is 2.0 cm (along the axis ) from the center of the ring with the velocity towards the center of the ring. What initial speed should the proton have such it will cross the plane of the ring?

im a little lost on this one. any help would be wonderful.

2. Nov 6, 2012

### haruspex

Consider PE and KE.

3. Nov 6, 2012

### Zarrey

The proton needs to increase its KE to a point greater than its PE to move correct? so would i solve for the potential energy of the proton and the ring?

4. Nov 6, 2012

### haruspex

Doesn't sound right. It already has some PE. It needs enough KE to do what?

5. Nov 6, 2012

### Zarrey

the proton needs KE in order to move. but what about the charge density? how does that play into KE when it is closer to the center of the ring? that is what i think i am confused about?

6. Nov 6, 2012

### haruspex

It doesn't need much KE merely to move. What else does it need KE for? What is happening to its PE as it gets closer? What conservation law can you apply?

7. Nov 6, 2012

### Zarrey

the law of conservation of energy?

as it moves closer the PE will decrease and the KE will increase.

8. Nov 6, 2012

### haruspex

Yes.
Why will the PE decrease?

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