# Electric potential

1. Nov 30, 2009

### tag16

1. The problem statement, all variables and given/known data
What is the electric potential a distance x above the center of a ring of charge Q with a
radius R? Derive the electric field from this relationship.

2. Relevant equations

V=k_e$$\int$$dq/r = k_e$$\int$$dq/($$\sqrt{}$$a^2+x^2)

3. The attempt at a solution

V= k_e/($$\sqrt{}$$a^2+x^2)$$\int$$ dq=k_eQ/($$\sqrt{}$$a^2+x^2)

E_x=-dV/dx= -k_eQ(d/dx)(a^2+x^2)^-1/2
= -k_e(-1/2)(a^2+x^2)^-3/2(2x)
E_x= k_ex/(a^2+x^2)^3/2 Q

Is this right? If not what is?

2. Nov 30, 2009

### kuruman

Your expressions are correct although, strictly speaking, the given radius of the ring is R not a.

3. Nov 30, 2009

### tag16

opps...I think I was looking at another problem when I wrote that. Thanks