A molecule of a substance has permanent electric dipole moment equal to 10 –29 C-m. A mole of this substance is polarized (at low temperature) by applying a strong electrostatic field of magnitude (10 6 Vm –1 ). The direction of the field is suddenly changed by an angle of 60º. Estimate the heat released by the substance in aligning its dipoles along the new direction of the field. For simplicity, assume 100% polarization to the sample.
Text Solution
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( 
J = 3.016 J (Approx.))
Sol. Dipole moment of molecule of substance = 10 –29 C-m
applied = 10 6 Vm –1 Change of angle of electric field = 60º
No of molecules in one mole ⇒ n = 6.023 × 10
23 ∴ Amount of heat released in aligning the dipoles along new direction
⇒ + Δ U = Δ Wext
∴ = + [Uf – Ui]
∴ = n [–PE cos60º – (–PE)]
∴ = n 
= 
∴ = 
∴ = 
= 3.0115 J
∴ 
= 3.0115 J (approx)
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