What is the torque on an electric dipole in an electric field?

[says]

Homework Statement

A charge Q is fixed at the centre of a train track, radius R. An electric dipole with charges Q, -Q, separated by distance d. Show that the net force on the dipole is given by F = Q²d/(4πε₀R³)

b) What direction is this force?
c) What is the torque on the dipole?

Homework Equations

F = (1/4*π*ε₀) * (Qq/R²)
F = qE₊-qE_-

The Attempt at a Solution

sinθ = d/2R

F = (Q*Q / 4*π*ε₀R²) (d/2R) - (-Q*Q / 4*π*ε₀R²) (d/2R)

F = (Q² d / 8*π*ε₀R³) + (Q² d / 8*π*ε₀R³)

F = (2*Q² d / 8*π*ε₀R³)

F = Q² d / (4*π*ε₀R³)

b) The direction of the force is in the counter-clockwise direction (CCW). The charge on the left side of the dipole is repulsive, so it points away from the charge in the middle, while the charge on the right side is attractive, so it points towards the charge. The net force is then pointed towards the left.

c) I'm a bit lost with c) and I'm not really sure where to start. Any guidance would be much appreciated. :)

 

[equation3]

What is the triangle for?

 

[kuruman]

The torque on an electric dipole $\vec{p}$ in an electric field $\vec{E}$ is given by $\vec{\tau}=\vec{p} \times \vec{E}$.