Question

An electric dipole is along a uniform electric field. If it is deflected by ${60}^o$, work done by the agent is $2\times {10}^{-19}J$. Then the work done by an agent if it is deflected by ${30}^o$ further is:

• A. $2.5\times {10}^{-19}J$
• B. $2\times {10}^{-19}J$
• C. $4\times {10}^{-19}J$
• D. $2\times {10}^{-16}J$

Answer 1

Answer: (B)

$2\times {10}^{-19}J$

We know energy of dipole in electric field
$w=\overrightarrow{P}\cdot \overrightarrow{E}$
given $2\times {10}^{-19}=-P.Ecos{60}^{\circ }$
$\Rightarrow P.E=-4\times {10}^{-11}$
Now for further deflection ${30}^{\circ }$
workdone$=(P.Ecos90)-(P.Ecos{60}^{\circ })$
$=0-4\times {10}^{-19}\times \frac{1}{2}$
workdone$=-2\times {10}^{-19}$
w is negative as work is done against electric field.