Electric potential of electric dipole at large distance

[blueyellow]

Homework Statement

write down an expression for the electric potential of an electric dipole at a large distance

Homework Equations

V=(qa cos(theta))/(4 pi epsilon r^2)

The Attempt at a Solution

is 'r' the distance they are talking about or is it 'a'?
either way, if i substitute in a=infinity or r=infinity, the above equation just blows up

 

[tiny-tim]

hi blueyellow!

(have an infinity: ∫ and a theta: θ and a pi: π and an epsilon: ε and try using the X² icon just above the Reply box )

is 'r' the distance they are talking about or is it 'a'?
either way, if i substitute in a=infinity or r=infinity, the above equation just blows up

a is the separation of the two ±q charges (so qa is the dipole moment)

(and 1/∞² = 0 )

 

[cupid.callin]

and you can also find it like this:

as r>>a ... so distance of point at r is approx same from both charge ie ≈ r

so V = kq/r + k(-q)/r = 0