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If this dipole were kept in a uniform external electric fieldE0​, diagrammatically represent the position of the dipole in stable and unstable equilibrium and write the expressions for the torque acting on the dipole in both cases.

Answer
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Hint In this question, the dipole is kept in a uniform electric field. And we have to write the expressions for them in two different cases. So for solving this, we will keep the dipole at two positions and then, will find the dipole at θ=0 and atθ=1800.
Formula used:
The potential energy of the dipole will be equal to the
U=PE0cosθ
And for torque, the formula will be
τ=PE0sinθ
Here,
U, will be the potential energy of the dipole
τ , will be the torque
P, will be the electric dipole of the dipole moment
E0, will be the uniform electric field

Complete Step By Step Solution Let us assume that the dipole we are using is kept at two positions namely (a) and (b) which is placed in a constant electric field E0 as it is shown in the below diagram.
seo images

In the first case: When the angle between the dipole and electric field will be zero
Then, the Potential energy of the dipole will be equal to the
U=PE0cosθ
Therefore,
U=PE0cos0
And on simplifying the equation, we get
Since cos0=1
U=PE0
Therefore, we can say that the dipole will be in stable equilibrium.
Now we will find the torque,
And for torque, the formula will be
τ=PE0sinθ
τ=PE0sin00
τ=0
In the second case: When the angle between the dipole and electric field will be 180
Then, the Potential energy of the dipole will be equal to the
U=PE0cosθ
Therefore,
U=PE0cos180
And on simplifying the equation, we get
Since cos180=1
U=PE0
Therefore, we can say that the dipole will be in unstable equilibrium.
Now we will find the torque,
And for torque, the formula will be
τ=PE0sinθ
τ=PE0sin1800
τ=0

Note Stable equilibrium means the lowest potential energy at the equilibrium point. Therefore the stable equilibrium the torque should be zero and the potential energy of the dipole should be minimum. Whereas the unstable equilibrium torque will be zero and the potential energy should be maximum or positive.
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