Question

derive an expression for torque experienced by electric dipole in external electric field.

Answer 1

The torque experienced by electric dipole in external electric field is    τ = pEsinθ

Explanation:

• Consider an electric dipole placed in a uniform electric field of strength E.
• The charges of dipole are - q and + q at separation 2l the dipole moment of electric dipole.
• The force on charge - q is, vector F2 = qE, opposite to the direction of field vector E.
• Hence net force on electric dipole in uniform electric field is  

F = F1 - F2 = qE - qE = 0 (zero)

The vector forces  F1 and F2  form a couple which rotates the dipole along the direction of electric field.

This couple is called the torque and is denoted by τ.  

Torque τ = magnitude of one force x perpendicular distance between lines of action of forces  

τ = qE(BN)

τ = qE(2lsinθ)  

τ =(θ2l)Esinθ  

τ = pEsinθ

In vector form vector τ = vector p x vector E

Thus the torque experienced by electric dipole in external electric field is    τ = pEsinθ

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