# Electrical Potential Energy?

1. Jul 29, 2011

### canoluk2003

Hi guys, have some problem with Electrical PE.

My textbook said sth like this " for the electric field produced by a point charge, we usually choose the energy at infinity to be zero"

What does that mean? What is "energy at infinity"?

Thx!

2. Jul 29, 2011

### mathfeel

Potential (like gravitational potential) is determined up to a constant. So, $\phi_1 = \frac{1}{r}$ is just as good as the potential $\phi_2 = \frac{1}{r} + 1$.

But it is often nice to chose one with appropriate boundary condition, such as [\itex]\lim_{r \to \infty) \phi (r) = 0[/itex]. One of the two above satisfies this condition.

3. Jul 29, 2011

### canoluk2003

So...(don't quite understand), is the "infinity" mentioned in my text book away from the fixed charge or very near to the fixed charge?

Can you explain in a simpler way? :D

Thx!!

4. Jul 29, 2011

### rcgldr

Potential and potential energy are relative. Between two points, there is an absolute difference in potential and potential energy. If an appropriate reference point is chosen, then potential and potential energy can be treated as absolute values instead of relative ones. Since potential from a point charge is relative to 1/r, then choosing ∞ as the reference point make sense because 1/∞ = 0.

For a infinite plane with some finite positive charge per unit area, or between two plates of a capacitor, potential is relative to distance from the positive plate (r), so it make sense to use the surface of the plate (r = 0) as the reference distance.

For an infinite wire, potential is relative to ln(... r), so it makes sense to choose some distance r where ln(... r) is zero as the reference distance.

5. Jul 29, 2011

### Bloodthunder

easier way to understand is "for the electric field produced by a point charge, assume Energy = 0 at a distance $\infty$ away from the point charge."

6. Jul 29, 2011

### canoluk2003

thx dude! i was really confused with the word "infinite" haha

7. Jul 29, 2011

### Studiot

hello canoluk, welcome to Physics Forums

The 'assumption' that there is no interaction beteen charges an infinite distance apart is not at all unreasonable.

It is very important that you do not fall into the trap of saying

"The electric potential is energy required to separate two charges"