# Homework Help: Finding Electric Field intensity

1. Jul 17, 2010

### pat666

1. The problem statement, all variables and given/known data
A proton travelling along the x-axis is slowed by a uniform electric field E. At x = 20 cm, the proton has a speed of 3.5 x 106 ms-1 and at x = 80 cm, its speed is zero. Find the magnitude and direction of electric field intensity.

2. Relevant equations
W=Fx
W=Eqx
W=ΔK_E
ΔK_E=1/2 mv^2-1/2 mu^2

3. The attempt at a solution
1st i found the change in energy (work) to slow the proton to a stop. then thats equal to Eqx. I found E to be 1.06*10^5 N/C (positive direction) please check my answer if you have time - its worth marks.

2. Jul 17, 2010

### kuruman

The magnitude of the electric field is correct. In what direction should the force on the proton be and why?

3. Jul 17, 2010

### pat666

thanks kuruman - I thought positive (is that whats meant by direction??) because they need to be "like" to repel??

4. Jul 17, 2010

### kuruman

I am talking about the force on the proton. If it moves left to right (x is increasing), in what direction should the force be to stop it?

5. Jul 17, 2010

### pat666

towards the origin - - but left or right would be dependent on the way the cc were defined???? +x could be towards the left or right couldn't it???

6. Jul 17, 2010

### kuruman

Yes it could, so let me rephrase my question. Relative to the direction of the proton's velocity, in what direction should the force on it point? Same or opposite direction? The answer to this question has nothing to do with which way is positive and which way is negative.

7. Jul 17, 2010

### pat666

opposite to slow it down??

8. Jul 17, 2010

### kuruman

Yes. Now how is the direction of the force on a positively charged particle related to the direction of the electric field? Same or opposite?

9. Jul 17, 2010

### pat666

same- the direction of an electric field is the way a positively charged test particle would go

10. Jul 17, 2010

### kuruman

So put it together. Is the direction of the electric field the same as or opposite to the direction of the proton's velocity?

11. Jul 17, 2010

### pat666

so the full answer is 1.06*10^5 N/C in the opposite direction of the protons velocity.

12. Jul 17, 2010

### kuruman

That is correct.

13. Jul 17, 2010

### pat666

thanks kuruman!!!!!!!!!!!!!!!!!