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A molecule of a substance has a perman...

A molecule of a substance has a permanent electric dipole moment of magnitude `10^(-29) `C m. A mole of this substance is polarized at low temperature by appling a strong elecrostatic field of magnitude `10^(6) V m^(-1)`. The direction of the field is suddenly changed by an angle of `60^(@)`. Estimate the heat released by the substance in aligning its dipole along the new direction of the field. For simplicity, assume `100%` polarisation of sample.

A

`-6J`

B

`-3J`

C

`3J`

D

`6J`

Text Solution

Verified by Experts

The correct Answer is:
B
Dipole moment of each molecules ` = 10^(-30)` C m. As 1 mole of the substance contains `6 xx 10^(23)` molecules. Total dipole moment of all the molecules ` p = 6 xx 10^(23) xx 10^(-23) C m = 6 xx 10^(-7) C m`
Initial poteinal energy `U_(i) = - p Ecos theta`
` = - 6xx 10^(-7) xx 10^(7) cos 0^(@)`
` = - 6 J`
Final potential energy, `U_(f) = - 6 xx 10^(-6) xx 10^(6) xx cos 60^(@)`
`= - 3J`
Work done = - [Change in potential enery ]
` = - [(-3)-(-6)] = - 3J`
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