What should be the orientation of an electric dipole in a uniform electric field, that corresponds to stable equilibrium ?
A
θ=90∘
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B
θ=180∘
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C
θ=30∘
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D
θ=0∘
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Solution
The correct option is Dθ=0∘ When the electric dipole aligns itself along the direction of electric field i.e →p||→E, then torque on dipole becomes zero.
Since, we know
τ=pEsinθ
⇒τ=0, when θ=0∘ or θ=180∘
The net force on electric dipole is always zero in an uniform electric field.
Thus, τnet=0&Fnet=0 corresponds to equilibrium position for dipole's orientation when θ=0∘ and θ=180∘.
Now electrostatic potential energy of dipole,
U=−→p.→E=−pEcosθ
⇒U=−pEcos0∘=−pE
⇒U=−pE (when θ=0∘)
Similarly, U=−pEcos180∘=−(−pE)
⇒U=+pE (when θ=180∘)
Therefore, for θ=0∘, the potential energy is minimum.
Umin=−pE
∴θ=0∘ corresponds to stable equilibrium position of dipole.
Why this question?Tip: The equilibrium position corresponding to θ=180∘ is unstable equilibrium because Potential energy of dipole is maximum