# Finding kinetic energy

Hi there,

I am thinking about a problem here,

A particle q produces an electric potential of 6.30E2 V at a distance of 1 m away. If a particle of charge Q=0.00300 C is released from rest at a distance of 8.00 m from q, what will be its kinetic energy after moving very far from q?

My question is, can I use the work-energy theorem here? As in
Ki + Ui = Kf + Uf ?????
If I choose a point at 8.00m, then the Ki will be zero, am I right? Then Ui would be Ui = Vi/Q, is this the right equation to use? And then at infinity or very far away, Uf will be zero too. Is this the right approach?

Related Introductory Physics Homework Help News on Phys.org
Doc Al
Mentor
evgeniy said:
My question is, can I use the work-energy theorem here? As in
Ki + Ui = Kf + Uf ?????
That's conservation of energy: sure you can use it.
If I choose a point at 8.00m, then the Ki will be zero, am I right?
Right. It's released from rest at that point.
Then Ui would be Ui = Vi/Q, is this the right equation to use?
No. U = VQ. But how can you find the potential at 8 m? You are only given the potential at 1 m. (Hint: How does potential from a point charge depend on distance?)
And then at infinity or very far away, Uf will be zero too. Is this the right approach?
Yes.

I see what you are saying.
I kinda get it now. So I am supposed to find a potential at 8m. But....
using the equation V = U/qo = 1/4(pi)(epsilon-naught) * q/r...r is the distance from the point charge q to the point at which the potential is evaluated right? But I do not know the value of a charge of q. So I need to find a charge first since I know the potential at 1m. And after it, I will find the potential at 8m, since now I know the charge. And at the end I will use U = Vq. Am I correct?

Doc Al
Mentor
Yes, you are correct. But don't waste any time calculating the charge. Use ratios! (You know that the potential is proportional to 1/r; that's all you need to know.)