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An electric dipole of moment P is placed in a uniform electric field E. The potential energy of the dipole is minimum when the angle between P and E is

1. Zero
2. pi/2
3. pi
4. None of these

Correct answer: Zero

Solution

The potential energy of a dipole in a uniform field is U = -P.E = -P*E*cos(theta). This is most negative (minimum) when cos(theta) = +1, i.e. theta = 0, meaning the dipole is aligned with the field. This is the stable equilibrium position.

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