. Why is that so?Murphy.T said:In solving you use ke = pe
Chandra Prayaga said:You said, . Why is that so?
So, if you don't use it, how will you solve the problem?Murphy.T said:Well maybe you don't have to, but on the key where I got the question that's what was done.
Murphy.T said: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?
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.ZapperZ said: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.
Murphy.T said: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.
Sorry I (didn't) mean to say that you said initial PE was zero.ZapperZ said: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.
Murphy.T said:Sorry I 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
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.ZapperZ said: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.
The potential energy of an electron near a negative source charge is the amount of energy that the electron possesses due to its position in the electric field created by the negative charge. This potential energy is usually measured in joules (J).
As the distance between the electron and the negative source charge increases, the potential energy of the electron decreases. This is because the electric field strength decreases with distance, resulting in a weaker force on the electron and thus less potential energy.
Yes, the potential energy of an electron near a negative source charge is always negative. This is because the electron and the negative source charge have opposite charges, and the potential energy is defined as the work required to bring a positive test charge from infinity to a certain point in the electric field. Since the electron is a negative charge, work must be done against the electric field to bring it closer to the negative source charge, resulting in a negative potential energy.
The potential energy of an electron near a negative source charge can be converted into its kinetic energy. As the electron moves closer to the negative source charge, its potential energy decreases and its kinetic energy increases. This relationship is described by the conservation of energy principle.
Yes, the potential energy of an electron near a negative source charge can be calculated using the formula U = kqQ/r, where U is the potential energy, k is the Coulomb's constant, q is the charge of the electron, Q is the charge of the negative source charge, and r is the distance between them. This formula assumes that the electron is a point charge and the negative source charge is a fixed point charge.