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A molecule of a substance has permanent ...

A molecule of a substance has permanent dipole moment p. A mole of this substance is polarised by applying a strong electrostatic field E. The direction of the field is suddenly changed by an angle of `60^(@)`. If N is the Avogadro's number the amount of work done by the field is

A

`2 NpE`

B

`1/2 NpE`

C

`NpE`

D

`3/2 NpE`

Text Solution

Verified by Experts

The correct Answer is:
B
The potential energy of electric dipole `(vecp)` in an electric field `(vecE) = -vec(p) cdot vec(E ) = -p E cos theta`
So, the work done by the field on N molecules
= change in potential energy
`=-NpE (cos 60^(@) - cos 0^(@)) = 1/2 NpE`.
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Knowledge Check

  • A magnet of magnetic moment M is situated with its axis along the direction of a magnetic field of strngth B the work done by an external agent in rotating it by an angle of 180^@ will be

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