The Parallel Plate Capacitor

Correct answer: A. When the dipole is parallel to the field

Explanation: For a dipole in a uniform electric field, the potential energy is U = −pE cos θ. If the dipole is parallel to the field, θ = 0° and cos θ = 1, so U = −pE, its minimum possible value for fixed p and E. At 90° the energy is zero, while at 180° it is +pE and maximum. Thus option A is correct.

Correct answer: A. When the dipole is opposite to the field

Explanation: The orientation energy of a dipole is U = −pE cos θ. For the dipole opposite to the field, θ = 180° and cos 180° = −1; hence U = +pE, the maximum value when p and E are fixed. Parallel orientation gives −pE, the minimum, and perpendicular orientation gives zero. The opposite arrangement is therefore an unstable equilibrium.