Electric Potential at a Distance

[xxabr]

Homework Statement

Find the electric potential difference at a point 4.0x10^-2m away from an alpha particle.

Homework Equations

V= kq/r

The Attempt at a Solution

k= 9x10⁹
r= 4.0x10^-2

I'm not quite sure how to get this answer considering I don't have any charge and there's nothing that says to use the charge from the previous question...
Help, please?

 

[tiny-tim]

hi xxabr!

an alpha particle consists of two protons and two neutrons, so it has a positive charge whose magnitude is twice the charge on an electron …

see http://en.wikipedia.org/wiki/Alpha_particle" for more details

 

[xxabr]

Oh! Okay, thanks (:

So...
V= kq/r
= (9x10⁹)(3.2x10^-19)/ (4x10^-2)
= 7.2x^-8V ?

 

[tiny-tim]

V= kq/r
= (9x10⁹)(3.2x10^-19)/ (4x10^-2)
= 7.2x^-8V ?

yup, looks ok! (assuming that the zero of potential energy is at infinity)

(or is it minus that … i get confused … isn't PE minus kq/r ? )

 

[xxabr]

Uh... No, I don't think so.
At least my textbook doesn't say that..

 

[tiny-tim]

hi xxabr!

(just got up :zzz: …)

Uh... No, I don't think so.
At least my textbook doesn't say that..

hmm … let's work this out from scratch …

https://www.physicsforums.com/library.php?do=view_item&itemid=269"

work done is the work bringing a positive test charge from infinity (because that's zero PE) to distance r from a positive charge q …

force is outward (both charges positive), movement is inward, so work done is negative, so PE is positive …

yes, you're right!