An electric dipole AB consisting of charge +q and −q and of length 2a is placed in uniform electric field E making an angle θ with the direction of electric field.

Force acting on −q is −qE
Force acting on +q is qE
These two forces are equal and opposite to each other. Hence, a torque on the dipole is developed.

Torque = Force × perpendicular distance between the forces
‌ Or, ‌‌τ=qE×2asin‌θ
‌ Or, ‌‌τ=(q×2a)Esin‌θ
∴‌τ=‌ PE ‌sin‌θ
∴‌‌τ=PE‌s‌i‌n‌θ‌‌‌ (where ‌P‌ is dipole moment) ‌
Dipole will attain stable equilibrium when it will be oriented along the direction of electric field.

OR
A capacitor is connected across the terminals of a d.c. battery.

The energy stored on a capacitor is equal to the work done by the battery.

Work done to move a small amount of charge dQ from the negative plate to the positive plate of the capacitor is equal to V‌dQ, where V is the voltage across the capacitor.
dU=VdQ=‌

Q

C

‌dQ
∴ Energy stored =
U=∫VdQ=‌

1

C

‌∫QdQ =‌

1

2

‌

Q2

C

=‌

1

2

CV2 ......(i)
Energy density is defined as the total energy per unit volume of the capacitor.
For a parallel plate capacitor,
C=‌

Aε0

d

Putting in eqn (i)
U=‌

1

2

‌

Aε0

d

V2 =‌

ε0

2

‌Ad(‌

V

d

)2
U=‌

ε0

2

Ad2(‌ putting ‌‌

V

d

=E)
A ×d= Volume of space between plates
So, energy stored per unit volume