# Potential Energy of an Electron near a negative source charge

• B
If an electron starts from rest 85.0 cm from a negative fixed source charge of -0.135 micro coulombs we use the equation ke final - ke intial + pe final - pe intial. In solving you use ke = pe which appears to cancel out the inital kinetic and potential energy but how can the initial potential energy of an electron near a negative source charge be zero?

Chandra Prayaga
You said,
In solving you use ke = pe
. Why is that so?

You said, . Why is that so?
Well maybe you don't have to, but on the key where I got the question that's what was done.

Chandra Prayaga
Well maybe you don't have to, but on the key where I got the question that's what was done.
So, if you don't use it, how will you solve the problem?

ZapperZ
Staff Emeritus
If an electron starts from rest 85.0 cm from a negative fixed source charge of -0.135 micro coulombs we use the equation ke final - ke intial + pe final - pe intial. In solving you use ke = pe which appears to cancel out the inital kinetic and potential energy but how can the initial potential energy of an electron near a negative source charge be zero?
This is VERY confusing, mainly because you did not indicate the exact problem and what exactly you were asked to find!

First of all, if the electron is initially at rest, then what's the brouhaha over "initial KE"? Isn't this zero?

Secondly, where is this "final" condition occurring? Is it at some arbitrary distance from the fixed source charge?

What you should have been aware of (and this is the physics part of the problem), is that the change in PE will equal to the change in KE. It is not the absolute value of each of these quantities. Since initial KE is zero, then this change in PE will be equal to the final KE. This is the whole point of the problem, and it is the concept that is being tested.

This sounds like a HW/Coursework problem, and it is, it has been posted in the wrong part of PF.

Zz.

This is VERY confusing, mainly because you did not indicate the exact problem and what exactly you were asked to find!

First of all, if the electron is initially at rest, then what's the brouhaha over "initial KE"? Isn't this zero?

Secondly, where is this "final" condition occurring? Is it at some arbitrary distance from the fixed source charge?

What you should have been aware of (and this is the physics part of the problem), is that the change in PE will equal to the change in KE. It is not the absolute value of each of these quantities. Since initial KE is zero, then this change in PE will be equal to the final KE. This is the whole point of the problem, and it is the concept that is being tested.

This sounds like a HW/Coursework problem, and it is, it has been posted in the wrong part of PF.

Zz.
Right it wasn't the problem I was wondering about so much as the concept of a negative charge with initial potential energy being zero near another negative charge, but you're saying it does ( have initial potential energy) and it's just the change in potential energy equals the change in kinetic, which makes sense now.
Also yes the final position is arbitrary. I understand the question which was why I posted here not in homework I was unsure.

Last edited:
ZapperZ
Staff Emeritus
Right it wasn't the problem I was wondering about so much as the concept of a negative charge with initial potential energy being zero near another negative charge, but you're saying it does and it's just the change in potential energy equals the change in kinetic, which makes sense now.
Also yes the final position is arbitrary. I understand the question which was why I posted here not in homework I was unsure.
No, I never said that the initial potential energy is zero.

I said that since the electron started at rest, the initial KINETIC ENERGY is zero.

PE is zero only at zero electrostatic potential, and in this case, it is zero at infinity. The electrostatic potential isn't zero when the electron is near the fixed charge.

Zz.

No, I never said that the initial potential energy is zero.

I said that since the electron started at rest, the initial KINETIC ENERGY is zero.

PE is zero only at zero electrostatic potential, and in this case, it is zero at infinity. The electrostatic potential isn't zero when the electron is near the fixed charge.

Zz.
Sorry I (didn't) mean to say that you said initial PE was zero.

I was confused because in the key they said initial PE was zero and they used the final PE for the equation KE final - KE intial + PE final - PE intial = 0

Last edited:
ZapperZ
Staff Emeritus
Sorry I mean to say that you said initial PE was zero.
BUT I NEVER SAID THAT!

I was confused because in the key they said initial PE was zero and they used the final PE for the equation KE final - KE intial + PE final - PE intial = 0
I do not know how else to say this. INITIAL POTENTIAL ENERGY IS NOT ZERO!

Do I have to use bigger fonts now to make my point?

Zz.

BUT I NEVER SAID THAT!

I do not know how else to say this. INITIAL POTENTIAL ENERGY IS NOT ZERO!

Do I have to use bigger fonts now to make my point?

Zz.
I apologize for my typing errors, I know initial potential energy is not zero but I am being told otherwise, I think I get it now thanks for your help.

Chandra Prayaga