Question

1uC and -1uC are placed at a distance of 5 cm forming a dipole. What is the amount of torque required to place the dipole perpendicularly to an electric field of 3*10^5 N/C?

(a) 5*10^-3 N. m

(b) 15*10^-3 N. m

(c) 1*10^-3 N. m

(d) 10*10^-3 N. m

This question was addressed to me during an online exam.

My question is based upon Dipole in a Uniform External Field topic in portion Charges and Fields of Physics – Class 12

Answer 1

The correct choice is (b) 15*10^-3 N. m

Easy explanation: Dipole moment of the dipole = 1*10^-6*5*10^-2 C. m=5*10^-8 C. m. Given the electric field-intensity is E=3*10^5 N/C. Therefore required torque will be p*E*sinθ where θ is the angle between the electric field and the dipole moment = 90 degrees. Therefore torque required in this case will be 5 *10^-8*3*10^5*sin 90 N. m=15*10^-3 N. m.