# Total potential difference of an electron

1. Apr 25, 2015

### Dancing_Queen

So this is the problem that I got on LonCapa:

1. An electron starts from rest 66.1 cm from a fixed point charge with Q=-0.120 μC. What total potential difference accelerates the electron from being very far away from Q?

I understand the equation that I have to use, which is V=k*Q/R. I plugged everything in like this:
V=[(9.0*109 N*m2/C2)*(-0.120*10-6 C)]/(0.661 m)
V=-1.63*103 V

When I was plugging my answer into LonCapa, it said it was wrong, so I changed the sign from a negative to a positive and it accepted that. I was wondering if someone could explain why that is to me. Are all answers supposed to be positive?

[Mentors Note: Thread moved from General Physics]

2. Apr 25, 2015

### haruspex

I find the wording strange. What does it mean to accelerate the electron "from being very far away"? Is that the exact wording? A translation perhaps?
Maybe it doesn't matter. It asks for a potential difference. Unless the wording that puzzles me is supposed to indicate it somehow, there's no specification as to which way the difference should be taken. That being so, it just wants a magnitude, which would generally mean a positive value.

3. Apr 25, 2015

### Dancing_Queen

Nope, this is the exact wording (although I agree it's kind of weird)! Here is what the problem looks like on LonCapa: http://i.imgur.com/FOrbThj.jpg

I guess it's pretty obvious now when I think about it. :D
I just wasn't sure if there was something I was missing or doing wrong. Thank you!

4. Apr 25, 2015

### BvU

There is a slight difference between "from far away" and "to far away"

$V = {k Q\over R}$ is negative at the outset and zero "far away". The unit is Volt = Joules per Coulomb.

The electron (with a negative charge) has a positive energy $E_{\rm electric field} = q V$ in the beginning and that gets converted into a positive kinetic energy "far away".

5. Apr 25, 2015

### Dancing_Queen

BvU, I didn't even catch that, thanks!

6. Apr 25, 2015

### haruspex

Given the revised wording, I agree. The answer is negative.