Question

A dipole is perpendicular to an external electric field (θ = 90°). What are the torque and potential energy?

Options

• Torque = maximum, Potential energy = maximum

• Torque = 0, Potential energy = 0

• Torque = 0, Potential energy = maximum

• Torque = maximum, Potential energy = 0

Answer 1

When θ = 90°, the dipole is perpendicular to the field. The torque is \[\tau = pE \sin(90°) = pE\] (maximum), and the potential energy is \[U = -pE \cos(90°) = 0\]. The maximum torque causes the dipole to rotate toward the lower energy (aligned) state, but at this instant, the potential energy is zero (our reference point).