Explanation:

An electric dipole is a pair of electric charges of equal magnitude but opposite sign separated by a distance. When an electric dipole is placed in a uniform electric field, it experiences a torque that tends to align it with the field.

Potential Energy of an Electric Dipole:

The potential energy of an electric dipole in a uniform electric field is given by the formula:

PE = -pEcosθ

where p is the dipole moment, E is the electric field strength, and θ is the angle between the dipole moment and the electric field.

Unstable Equilibrium and Stable Equilibrium:

An electric dipole placed in a uniform electric field is said to be in unstable equilibrium if it is perpendicular to the field. In this position, the slightest disturbance will cause the dipole to rotate either clockwise or counterclockwise until it aligns with the field.

On the other hand, an electric dipole is said to be in stable equilibrium if it is aligned with the field. In this position, the dipole experiences no torque and will remain in this position unless acted upon by an external force.

Effect of Rotation on Potential Energy:

When an electric dipole is rotated from unstable equilibrium to stable equilibrium in a uniform electric field, its potential energy decreases. This can be explained as follows:

- When the dipole is in unstable equilibrium, its dipole moment is perpendicular to the electric field. In this position, the potential energy of the dipole is maximum.
- As the dipole rotates towards stable equilibrium, the angle between the dipole moment and the electric field decreases. This results in a decrease in the potential energy of the dipole.
- When the dipole is aligned with the electric field, the angle between the dipole moment and the electric field is zero, and the potential energy of the dipole is at its minimum.

Therefore, the potential energy of an electric dipole placed in a uniform electric field decreases when it is rotated from unstable equilibrium to stable equilibrium. The correct answer is (b) decreases.