# Electric Field Problem

1. Apr 4, 2006

### spoonthrower

A uniform electric field has a magnitude of 2.3*10^3 N/C. In a vacuum, a proton begins with a speed of 2.1*10^4 m/s and moves in the direction of this field. Find the speed of the proton after it has moved a distance of 2.0 mm please help. Thanks.

2. Apr 4, 2006

### Galileo

What are your thoughts on it?

3. Apr 4, 2006

### spoonthrower

i know F=qE, so is there a value of q for a proton that i dont know about. once i know the force i guess i need to find the acceleration to calculate the final speed. so i would need the mass of a proton???

4. Apr 4, 2006

### Galileo

Yeah, you need the mass and charge of the proton (which you can look up anywhere). So now you know F (which is constant) and it's just a classical mechanics exercise, analogous to a particle in a uniform gravitational field.

It is more efficient to use an energy approach though. The potential energy lost in moving the 2mm is gained by the kinetic energy of the proton.

5. Apr 4, 2006

### spoonthrower

so I would use .5mvf^2-.5mvo^2=mgh

The mass cancels i know that. I use gravity?????

I am still getting the wrong answer

6. Apr 5, 2006

### Galileo

No! The only thing analogous is that the force acting on the particle is constant. You mustn't use gravitational potential energy (which is proportional to the mass) but electrostatic potential energy (Which is proportional to the charge). I made the analogy to simplify the view on the problem, but I see it's only confusing. Forget I said the whole thing!

If you don't know electrostatic potential energy, just use the work-energy theorem. What is the work done by the field in moving the particle those 2mm?

Last edited: Apr 5, 2006