What is the potential energy of an electron

[Foxhound101]

Homework Statement

Part A - (Already solved)
The electric potential at point A is 1800V.

Part B -
What is the potential energy of an electron at point A in the figure?

Point A is .01m away from a charge of +2nC

Homework Equations

V=Uq

U = (Kq₁q₂)/r

The Attempt at a Solution

I think the problem is, is that I am confused about the difference between electric potential and potential energy. This problem shouldn't be difficult.

So I tried the second equation

U = (9.00*10^9)(1.6*10^-19)(2*10^-9)/.01
U = 2.88*10^-18

But that isn't the answer...I don't know what equation I should use or what I am doing wrong.

 

[alphysicist]

Hi Foxhound101,

Homework Statement

Part A - (Already solved)
The electric potential at point A is 1800V.

Part B -
What is the potential energy of an electron at point A in the figure?

Point A is .01m away from a charge of +2nC

Homework Equations

V=Uq

U = (Kq₁q₂)/r

The Attempt at a Solution

I think the problem is, is that I am confused about the difference between electric potential and potential energy. This problem shouldn't be difficult.

So I tried the second equation

U = (9.00*10^9)(1.6*10^-19)(2*10^-9)/.01

There's not much detail about the situation here, but one thing I noticed is that you are not putting the sign of the electron in the potential energy equation. For the potential and potential energy the sign is required (unless of course they are just asking for the magnitude for some reason).

 

[Foxhound101]

This time I tried the other equation...

1800 = U / (2*10^-9)
3.6*10^-6 = U

still wrong.

*edit* I made my post while you made yours...>_> Let me look into your post*

 

[Foxhound101]

http://img410.imageshack.us/my.php?image=54425247kj6.gif

Here is the entire problem. I can do the other 2 parts if I know how to do this part.

 

[alphysicist]

http://img410.imageshack.us/my.php?image=54425247kj6.gif

Here is the entire problem. I can do the other 2 parts if I know how to do this part.

I think there are two things wrong here. The first is the sign error I mentioned. But also, this calculation from your first post:

U = (9.00*10^9)(1.6*10^-19)(2*10^-9)/.01
U = 2.88*10^-18

It looks like a calculation error here; it should not be to the -18 power.

Do you get the right answer now?

 

[Foxhound101]

Alright, thanks for the help. I have the correct answer now. I have the rest of the problem worked out as well.

Man...I wasted so much time on this problem when the only major mistake I made was the negative sign.

Normally MasteringPhysics tells me if the sign is incorrect...

 

[alphysicist]

Alright, thanks for the help. I have the correct answer now. I have the rest of the problem worked out as well.

Man...I wasted so much time on this problem when the only major mistake I made was the negative sign.

Normally MasteringPhysics tells me if the sign is incorrect...

Glad to help! And I know it's discouraging to spend so much time on something that turns out to be a seemingly small matter. However, not including the sign is a very common and persistent mistake for potential/potential energy calculations, and it's better to get a warning about it in your mind now before any exams.