A point charge Q_1 is distance r from the center of a dipole consisting of charges (+-) q_2 separated by distance s. The charge is located in the plane that bisects the dipole. At this instant, assuming r s, what is the magnitude of the torque on the dipole? Express this factor in terms of q_2, Q_1, s, pi, and r.

The answer I found for the force magnitude was: Q1q2s/(4pi(r^3)).

I am unsure why my answer for the torque = sF is incorrect.

Question

A point charge Q_1 is distance r from the center of a dipole consisting of charges (+-) q_2 separated by distance s. The charge is located in the plane that bisects the dipole. At this instant, assuming r >>s, what is the magnitude of the torque on the dipole? Express this factor in terms of q_2, Q_1, s, pi, and r.

The answer I found for the force magnitude was: Q1q2s/(4pi(r^3)).

I am unsure why my answer for the torque = sF is incorrect.

anonymous · Answer

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So as i see it.. you gotta calculate what is the field at point P due to charge Q1
Then use the formula 
$$\tau = pE \sin (\theta)$$