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Torque on a dipole in a uniform electric field - class-XII

Description: torque on a dipole in a uniform electric field
Number of Questions: 46
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Tags: electrostatics electric charges and fields physics
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An electric dipole of moment $ \vec { p } $ is placed in a uniform electric field$\vec { E }$ . Then
(i) The torque on the dipole is $\vec { p } \times \vec { E } $.
(ii) The potential energy of the system is $\vec { p } \cdot \vec { E } $.
(iii) The resultant force on the dipole is zero.
Choose the correct option.

  1. (i), (ii) and (iii) are correct

  2. (i) and (iii) are correct and (ii) is wrong

  3. Only (i) is wrong

  4. (i) and (ii) are correct and (iii) is wrong


Correct Option: A
Explanation:

In a uniform electric field, $\vec { E } $, dipole experiences a torque $\vec { \tau  } $ given by $\vec { \tau  } =\vec { p } \times \vec { E } $ but experiences no force. The potential energy of the dipole in a uniform electric field $\vec { E } $ is $U=-\vec { p } .\vec { E } $

An electric dipole of length 20 cm having $\pm3 \times { 10 }^{ -3 }$ C charge placed at 60 with respect to a uniform electric field experiences a torque of magnitude 6 N m. the potential energy of the dipole is:

  1. -2$\sqrt { 3 }$ J

  2. 5$\sqrt { 3 }$ J

  3. -3$\sqrt { 2 }$ J

  4. 3$\sqrt { 5 }$ J


Correct Option: A
Explanation:

Here,

Length of dipole $2a= 20\ cm=0.2\ m$
Charge, $q= \pm 3\times 10^{-3}\ C$,             $\theta=60^o$
Torque, $\tau=6\ Nm$
As, $\tau=pEsin\ \theta$
or $E=\dfrac{\tau}{psin \theta}= \dfrac{\tau}{q(2a)sin \theta}(\because p=q(2a)$

$\therefore E=\dfrac{6}{3\times 10^{-3}\times 20 \times 10^{-2} \times sin\ 60^o}= \dfrac{10^5}{5\sqrt 3} NC^{-1}$

Potential energy of dipole, $U=-pEcos \theta=-q(2a) E cos \theta $

$\implies U= -3\times 10^{-3}(20 \times 10^{-2} )\dfrac{10^5}{5\sqrt 3} cos 60^o= -2\sqrt 3\ J$

An electric dipole is kept on the axis of a uniformly charged ring at distance $\frac{R}{\sqrt{2}}$ from the centre of the ring. The direction of the dipole moment is along the axis. The dipole moment is P, charge of the ring is Q & radius of the ring is R. The force on the dipole is ___________________.

  1. (a) $\frac{4 k P Q}{3\sqrt{3} R^{2}}$

  2. (b) $\frac{4 k P Q}{3\sqrt{3} R^{3}}$

  3. (c) $\frac{2 k P Q}{3\sqrt{3} R^{2}}$

  4. (d) zero


Correct Option: A

Two electric dipoles of dipole moment $2 \space cm$ and $4 \space cm$ respectively are kept inside a cube of side $'a' \space m$. Total electric flux linked with the cube is (in SI units)

  1. $\dfrac{6}{\varepsilon _0}$

  2. $\dfrac{2}{\varepsilon _0}$

  3. $\dfrac{4}{\varepsilon _0}$

  4. none of these


Correct Option: D
Explanation:

Since net charge $=0$

$\therefore \phi =0$
$\therefore$ option $D$ is correct answer.

An electric dipole is kept in a non-uniform electric field. It experiences

  1. a force and a torque

  2. a force but not a torque

  3. a torque but no force

  4. neither a force nor a torque


Correct Option: A
Explanation:

In a non uniform electric field, net force$\ne0$, and net torque $\ne 0$.

An electric dipole placed with its axis in the direction of a uniform electric field experiences:

  1. a force but not torque

  2. a torque but no force

  3. a force as well as a torque

  4. neither a force nor a torque


Correct Option: D
Explanation:

Total force of on diopole $= E\ q +E( -q )$ $= 0$
We know,
 Torque $= \vec{P} \times \vec{E}$
but axis of dipole is in the direction of electric field
$\therefore \tau = P.E\ sin\ 0$
$\therefore \tau = 0$
$\therefore $ No torque, No force is acting on dipole

An electric dipole of moment $p$ is lying along a uniform electric field $E$. The work done in rotating the dipole by $90^{o}$ is:

  1. $pE$

  2. $\sqrt{2}\ pE$

  3. $\dfrac{pE}{2}$

  4. $2p\ E$


Correct Option: A
Explanation:

$Work done=\Delta pE$

$pE=-\vec p\vec E$
$pE _f=0$
$pE _i=-pE$
$\Delta pE=pE$

An electric dipole of momentum $3 \times {10}^{-8}\ Cm$ is placed in an electric field of $6 \times {10}^{4}\ N/C$ with is axis making an angle of $30^o$ with the field . Find the torque acting on the dipole.

  1. $ (9 \times 10^{-5} )N-m$

  2. $ (9 \times 10^{-4} )N-m$

  3. $ (9 \times 10^{-3} )N-m$

  4. $ (90 \times 10^{-8} )N-m$


Correct Option: B
Explanation:

We know that Torque $(\vec \tau) = \vec p \times \vec E$


$  \tau = pE\sin \theta$
$ = (3\times 10^{-8})\times ( 6\times 10^4) \sin30^0$
$ = (9 \times 10^{-4} )N-m$

A neutral water molecule $ (H _2 O) $ in its vapour state has an electric dipole moment of $ 2 \times 10^{-24} C-m. $ If the molecule is placed in an electric field of $ 2 \times 10^{4} NC^{ _1} $ , the maximum torque that the field can exert on it is nearly.

  1. $ 4 \times 10^{-20} N-m $

  2. $ 6 \times 10^{-20} N-m $

  3. $ 8 \times 10^{-20} N-m $

  4. $ 2 \times 10^{-18} N-m $


Correct Option: A
Explanation:

$P = 2 \times 10^{-24}Cm$


$E = 2 \times 10^{4}NC$

$\tau = EP = 2 \times 10^{-24} \times 2 \times 10^{4} = 4 \times 10^{-20}Nm$

A dipole is placed parallel to the electric field. If W is the work done in rotating the dipole by $60$, then the work done in rotating it by $180$ is:

  1. $ 2W$

  2. $ 3W$

  3. $ 4W$

  4. $ W/2$


Correct Option: C
Explanation:

Work done in rotating dipole by $\theta \,$ angle from eqbm. ,$W = PE\left( {1 - \cos \theta } \right)$

$\begin{array}{l}W = PE\left( {1 - \cos 60^\circ } \right)\ = PE\left( {1 - \frac{1}{2}} \right) = \frac{{PE}}{2}\or\,\,\,PE = 2W\W' = PE\left( {1 - \cos 180^\circ } \right)\ = PE\left( {1 - \left( { - 1} \right)} \right)\ = 2PE = 2 \times 2W = 4W\end{array}$

Two charges of charge $-4\mu C$ and $+4\mu C$ are placed of the point $A(1, 0, 4)$ and $B(2, -1, 5)$ located in an electric field $E = 0.20\hat {i} V/ C-m$. Then, torque acting on the dipole will be

  1. $2.31\times 10^{-4} N/m$

  2. $7.98\times 10^{-4} C/m$

  3. $7.11\times 10^{-4} C/m$

  4. $7.04\times 10^{-4} C/m$


Correct Option: D

Dipole is placed parallel to the electric field. If W is the work done in rotating the dipole $60^0$, then work  done in rotating it by $180^0$ is

  1. 2 W

  2. 3 W

  3. 4 W

  4. W/2


Correct Option: B

A magnetic dipole is placed at right angles to the direction of lines of force of magnetic induction B. If it is rotated through an angle of $180^0$, then the work done is 

  1. 2 MB

  2. MB

  3. -2 MB

  4. Zero


Correct Option: D

In a certain region of space, electric field is along z-direction throughout. The magnitude of electric field is, however, not constant but increases uniformly along the positive z-direction, at the rate of $10^5 NC^{-1}$ per metre. What is the torque experienced by a system having a total dipole moment equal to $10^{-7}$ C-m in the negative z-direction?

  1. 2 N-m

  2. 3 N-m

  3. 4 N-m

  4. Zero


Correct Option: D
Explanation:

$\tau =\vec P\times \vec E = PE \sin\theta $
Since, $ \theta = 180^o \rightarrow \tau =0 $

For a dipole $q=2\times 10^{-6}C$ , d=0.01 m, find the maximum torque on the dipole if $E=5\times 10^{5}N/C$ :-

  1. $1\times 10^{-3}Nrn^{-1}$

  2. $10\times 10^{-3}Nrn^{-1}$

  3. $1\times 10^{-3}Nrn$

  4. $1\times 10^{-4}Nrn$


Correct Option: C

The work done in rotating a dipole through ${ 180 }^{ \circ  }$ from electric field direction is :

  1. $pE$

  2. $2pE$

  3. $\cfrac { pE }{ 2 } $

  4. $Zero$


Correct Option: B
Explanation:

Work done due to dipole $=W=-pE\cos\theta$ where $p=$ dipole moment, $E=$ electric field
Here, $W=W _{(180)}-W _{(0)}=(-pE\cos180)-(-pE\cos0)=pE+pE=2pE$
Ans: (B)

Total electric force on an electric dipole placed in an electric field of a point charge is :

  1. always zero

  2. never zero

  3. zero when midpoint of dipole coincides with the point charge

  4. zero when dipole axis is along any electric line of force.


Correct Option: B

Work done in turning dipole through an angle 60 is 

  1. $zero$

  2. $pE/4$

  3. $pE$

  4. $pE/2$


Correct Option: C
Explanation:

Work done $=PE\cos{ 60 }^{ 0 }$

This is the working formula

An electri dipole experience linear displacement and  rotational motion in _____ electric field.

  1. uniform

  2. non-uniform

  3. none

  4. a&b both


Correct Option: B

An electric dipole consists of two opposite charges each of magnitude $ 1.0 \mu C $ separated by a distance of 2.0 cm. the diople is placed in an external electric field of $ 1.0 \times  10^5 N/C $ the maximum torque on the dipole is:

  1. $ 0.2 \times 10^{-3} N-m $

  2. $ 1.0 \times 10^{-3} N-m $

  3. $ 2.0 \times 10^{-3} N-m $

  4. $ 4.0 \times 10^{-3} N-m $


Correct Option: C

A dipole is placed in an electric field whose direction is fixed but its magnitude varies with distance. It is possible that the dipole experiences :

  1. no net force and no torque

  2. a net force but no torque

  3. a net force and a torque

  4. no net force but a torque


Correct Option: B,C,D
Explanation:

Dipole means two charges $+q$ and $-q$ are placed at a certain distance.
There  three cases may be occured for electric field whose direction is fixed but magnitude varies with distance.
1) when forces on the two charges are in different direction and different magnitude, they will have a net force and a torque.
2) when forces on the two charges are in same direction but different magnitude, they will have a net force but no torque.
3) when forces on the two charges are in opposite direction but same magnitude, they will have no net force but a torque.

An electric dipole of moment p is kept along an electric field E. The work done in rotating it from an equilibrium position by an angle $\theta$ is :

  1. PE(1 - cos $\theta$)

  2. PE(1 - sin $\theta$)

  3. PE cos $\theta$

  4. PE sin $\theta$


Correct Option: A
Explanation:

Electric field will produce a torque on the dipole.
Torque , $\displaystyle \tau=pE\sin\theta$
The work done in rotating it from an equilibrium position by an angle $\theta$ is $\displaystyle W=\int _0^{\theta}\tau d\theta=pE\int _0^{\theta}\sin\theta d\theta=-pE(\cos\theta-\cos0)=pE(1-\cos\theta)$

An electric dipole is kept in the surrounding of another dipole, it experiences

  1. a force and a torque

  2. a force but not a torque

  3. a force but not necessarily a torque

  4. neither a force nor a torque


Correct Option: C

What will be the magnitude of torque on an electric dipole having dipole moment of  $4 \times 10 ^ { - 9 }  { cm }$  placed in a uniform electric field of intensity of  $5 \times 10 ^ { 4 } { NC } ^ { - 1 }$  making an angle  $180 ^ { \circ }$  with the field.

  1. $10 ^ { - 4 } N - m$

  2. $0$ 

  3. $2\times 10^{ { -4 } }{ N }-{ m }$

  4. $10 ^ { -6 } N - m$


Correct Option: B

An electric dipole, made up of positive and negative charges, each of $1\mu C$ and placed at a distance $2\ cm$ apart. If the dipole is placed in an electric field of $10^{5} N/C$ then the maximum torque which the field can exert on the dipole, if it is turned from a position $\theta = 0^{\circ}$ to $\theta = 180^{\circ}$ is, is

  1. $2\times 10^{-3} N-m$

  2. $3\times 10^{-3} N-m$

  3. $4\times 10^{-3} N-m$

  4. $2.8\times 10^{-3} N-m$


Correct Option: A

If a dipole of dipole moment $\displaystyle \vec { p } $ is placed in a uniform electric field $\displaystyle \vec { E } $, then torque acting on it is given by :

  1. $\displaystyle \vec {\tau } =\vec { p } .\vec { E } $

  2. $\displaystyle \vec { \tau } =\vec { p } \times \vec { E } $

  3. $\displaystyle \vec { \tau } =\vec { p } +\vec{ E } $

  4. $\displaystyle \vec { \tau } =\vec { p } -\vec { E } $


Correct Option: B
Explanation:

Dipole moment of the dipole p and uniform Electric field $E$. we know that dipole moment $p = qa$ (where q is charge and a is the dipole length).And when a dipole of Dipole moment p is placed in a uniform Electric field E , the torque $\tau = Either  force \times \text{perpendicular  distance  between  the  two  forces }= qaE sin \theta$ or $\tau = pEsin \theta$ or $\tau = p \times E$

A dipole is placed parallel to the electric field. If W is the work done in rotating the dipole by 60, then work done in rotating it by 180 is

  1. 2 W

  2. 3 W

  3. 4 W

  4. ${\frac{W}{2}}$


Correct Option: B

An electric dipole is placed in an electric field generated by a point charge.

  1. The net electric force on the dipole must be zero.

  2. The net eiectric force on the dipole may be zero.

  3. The torque on the dipole due to the field must be zero.

  4. The torque on the dipole due to the field may be zero.


Correct Option: D
Explanation:

A and B cannot be true in any case. Because for the net force on a dipole to be zero, the field lines should be parallel to each other and the dipole must be placed perpendicular to the field. However, in case of point charge you cannot have parallel field lines. 
C: Not true again. Because in any orientation (except radial) the force on the positive and negative charge will form a couple and you will have torque.
D: True. If you place the dipole radial from the point object, then there will be no net torque. Since torque $=F\times R\;\sin{\theta}$. If it is radial, $\sin{\theta}=0$, so torque $=0$. 

An electric dipole is placed in non-uniform electric field, then it experiences

  1. only torque

  2. force and torque

  3. only force

  4. neither force nor torque


Correct Option: B
Explanation:

Given an electric dipole placed in a non-uniform electric field. An electric dipole always experiences a torque when placed in uniform as well as non-uniform electric field. But in non-uniform electric field, dipole will also experience net force of attraction. So the electric dipole in non-uniform electric field experiences both torque and force.

State whether True or False :

In a uniform electric field, the dipole experiences no net force; but experiences a torque having a relation with $P$ and $E$ which is given by $\vec{P} \times \vec{E}$ where the parameters $P$ and $E$ have their usual meaning.

  1. True

  2. False


Correct Option: A
Explanation:
Torque ($\tau$)  = Force × distance seperating forces
$\tau = d \space  qE sin \theta$
Since dipole moment is given by $P = qd$
$\therefore \tau =PE sin \theta$ or
$\overrightarrow \tau = \overrightarrow P \times \overrightarrow E$. 
In a uniform electric field, we know that the dipole experiences no net force; but experiences a torque having a relation with P and E is given by $\overrightarrow \tau = \overrightarrow P \times \overrightarrow E$ where the parameters P and E have their usual meaning.

The torque acting on a dipole of momentum $\vec { p } $ in an electric field $\vec { E } $:

  1. $\vec { p } \times \vec { E } $

  2. $\vec { p } .\vec { E } $

  3. zero

  4. $\vec { E } \times \vec { p } $


Correct Option: A
Explanation:

Force acting on dipole due to electric field is given by,

$F=qE$
Torque on a dipole in an electric field is given by,
$T= F\times d = Fdsin\theta=qEdsin\theta$.     .....(i)
But dipole moment is given by,
$P=qd$    ....(ii)
By (i) and (ii) we get,
$\therefore$ $T= pEsin\theta=\overrightarrow{p}\times \overrightarrow{E}$

An electric dipole of moment $\vec { p } $ is placed normal to the lines of force of electric intensity $\vec { E } $, then work done in deflecting it through an angle of ${180}^{o}$ is:

  1. $pE$

  2. $+2pE$

  3. $-2pE$

  4. Zero


Correct Option: A
Explanation:

Work done $W=pE(\cos\phi _1-\cos\phi _2)$

$\phi _1=90^\circ,\phi=180^\circ$
$W=pE(0-(-1))$
$W=pE$

An electric dipole of dipole moment $\vec { P } $ is placed parallel to the uniform electric field of intensity $\vec { E }$. On rotating it through ${180}^{o}$, the amount of work done is ________ .

  1. $2PE$

  2. Zero

  3. $PE$

  4. $-2PE$


Correct Option: A
Explanation:

Work done by external agent in rotating the dipole 
$W=PE\left[ \cos { { \theta  } _{ 1 } } -\cos { { \theta  } _{ 2 } }  \right] $

Consider the initial angle to be
${ \theta  } _{ 1 }=0$ 

and the final angle will be
 ${ \theta  } _{ 2 }={ 180 }^{ o }$

So,$\Rightarrow$ $W=PE[cos\ 0^o-cos\ 180^o]=2PE$

An electric dipole kept in a uniform electric field experiences :

  1. Force and a torque

  2. A force, but no torque

  3. A torque but no force

  4. Neither a force nor a torque


Correct Option: C
Explanation:

In a uniform electric field, both the point charges comprising the dipole will experience force, equal in magnitude and opposite in direction.
Though the net force will always be zero, the torque will be in same direction for both the charges. Hence torque will not be zero.

When an electric dipole $\vec p$ is kept in a uniform electric field $\vec E$ then for what of a value of the angle between $\vec p$ and $\vec E$, torque will be maximum:

  1. ${90^o}$

  2. ${0^o}$

  3. ${180^o}$

  4. ${45^o}$


Correct Option: A
Explanation:

$\overline { P } \times \overline { E } $

$=\overline { P } \overline { E } \sin{ 90 }^{ 0 }$
for maximum $\overline { P } \overline { E } $ then $\theta $ will be ${ 90 }^{ 0 }$.

What will be the magnitude of torque on an electric dipole having dipole moment of $4\times { 10 }^{ -9 }cm$ placed in a uniform electric field of intensity of $5\times { 10 }^{ 4 \,\,}N { C }^{ -1 }$ making an angle ${180}^{o}$ with the field.

  1. ${ 10 }^{ -4 }N-m$

  2. $2\times { 10 }^{ -4 }N-m$

  3. $0$ (zero)

  4. ${ 10 }^{ -6 }N-m$


Correct Option: C
Explanation:

$\overrightarrow { \tau  } =\overrightarrow { p } \times \overrightarrow { E } $

$\overrightarrow{\tau} \longrightarrow$torque
$[\theta \longrightarrow$angle of the dipole moment$(p)$ with the field$(E)]$
$\therefore |\tau |=pE\sin { \theta  } =pE\sin { 180° } =0$

An electric dipole  of dipole moment $\vec {p}$ is placed in uniform electric field $\vec {E}$, with $\vec {p}$ parallel to $\vec {E}$ . It is then rotated by an angle of $\theta$. The work done is

  1. $pE\ \sin \theta$

  2. $pE\ \cos \theta$

  3. $pE\ (1-\cos \theta)$

  4. $pE\ (1-\sin \theta)$


Correct Option: C

A dipole of $2 \mu C$ charges each other consists of the positive charge at the point $P(1, -1)$ and the m=negative charge is placed at the point $Q(-1,1)$ . The work done in displacing a charge of $ + 1 \mu C$ from point $A (-3,-3) $ to $B(4,4) $ is :

  1. $1.6 \times 10^{-19} J $

  2. $ 6.98 \times 10^{-3} J $

  3. Zero

  4. $4.8 eV $


Correct Option: B
Explanation:

Since origin is the central point of the dipole, so electric potential on point ${P _1}\left( { - 3,3} \right)$

${V _1} = \dfrac{{k \times P\cos \theta }}{{{r^2}}}$

${V _1} = \dfrac{{9 \times {{10}^9} \times 2\sqrt 2  \times 2 \times {{10}^{ - 6}}}}{{{{\left( {3\sqrt 2 } \right)}^2}}} \times \dfrac{{\sqrt 2 }}{{\left( {3\sqrt 2 } \right)}}$

Similarly electric potential at point ${P _2}\left( {4,\;4} \right)$

${V _2} = \dfrac{{k \times P\cos \theta }}{{{r^2}}}$

${V _2} = \dfrac{{9 \times {{10}^9} \times 2\sqrt 2  \times 2 \times {{10}^{ - 6}}}}{{{{\left( {4\sqrt 2 } \right)}^2}}} \times \dfrac{{\sqrt 2 }}{{\left( {4\sqrt 2 } \right)}}$

Change in potential

$\Delta V = {V _1} - {V _2}$

$\Delta V = 9 \times {10^9} \times 2\sqrt 2  \times 2 \times {10^{ - 6}}\left( {\dfrac{1}{{18 \times 3}} - \dfrac{1}{{4 \times 32}}} \right)$

$\Delta V = 36\sqrt 2  \times {10^3}\left( {18.52 \times {{10}^{ - 3}} - 7.81 \times {{10}^{ - 3}}} \right)$

$\Delta V = 6.98 \times {10^3}{\rm{V}}$

Since potential at point ${P _1}$ is higher than potential at point ${P _2}$. The charge will move automatically from point ${P _1}$ to point ${P _2}$ under the effect of electric field of the dipole.

Now work done

$W = 6.98 \times {10^3} \times 1 \times {10^{ - 6}}$

$W = 6.98 \times {10^{ - 3}}{\rm{J}}$

Two electric dipoles of moment $\rho $ and $64\rho $ are placed in opposite direction on a line at a distance of $25\ cm$. The electric field will be zero at point between the dipoles whose distance from dipole of moment $\rho $ is 

  1. $5\ cm$

  2. $\dfrac{25}{9}$ cm

  3. 10 cm

  4. $\dfrac{4}{13}$ cm


Correct Option: C

An electric dipole of moment 'p' is placed in an electric field of intensity 'E'. The dipole acquires a position such that the axis of the dipole makes an angle $\theta $ with the direction of the field. Assuming that the potential energy of the dipole to be zero when=${ 90 }^{ 0 }$, the torque and the potential energy of the dipole will respectively be

  1. $p E \sin { \theta } , pE\cos { \theta } $

  2. $p E \sin { \theta } ,-2p E\cos { \theta } $

  3. $p E \sin { \theta } ,2p E\cos { \theta } $

  4. $p E \cos { \theta } ,-p E\cos { \theta } $


Correct Option: A
Explanation:
An electric dipole of moment $=P$
electric field of intensity $=E$
the dipole acquire a position angle $=\theta$
dipole to be zero when $={ 90 }^{ 0 }$
torque and the potential energy of the dipole will respectively 
potential energy $U=PE\sin\theta $ in this situation.
$SIn$ component is benefited for that, perpendicularity and dipole is hence, $PEcos\theta $

An electric dipole is placed in an electric field of a point charge then...........

  1. Force is always zero.

  2. Torque is always zero.

  3. Force is may be zero.

  4. Torque may be zero.


Correct Option: D

Four equal positive charges each of magnitude $q$ are placed at the respective vertices of a square of side length $l$. A point charge $Q$ is placed at the centre of the square. Then

  1. $Q$ must not be in equilibrium

  2. $Q$ must be in stable equilibrium

  3. $Q$ must be in neutral equilibrium

  4. $Q$ must be in unstable equilibrium


Correct Option: B

If we rotate the dipole of moment $p$ placed in an electric field $E$ from an $\theta _1$ to $\theta _2$, the work done by the external force is

  1. $pE(\cos \theta _2 - \cos \theta _1)$

  2. $pE(\cos \theta _1 - \cos \theta _2)$

  3. $pE(\sin \theta _2 - \sin \theta _1)$

  4. $pE(\sin \theta _1 - \sin \theta _2)$


Correct Option: B
Explanation:
Given dipole of dipole moment $p$ in an electric field $E$. It is rotated from $\theta _{1}$ to $\theta _{2}$. We have to find the work done by external force.
When a dipole of dipole moment  $p$ is placed in electric field, work done in rotated the dipole by angle $\theta$ is
$W=-pE \cos{\theta _{1}}$
Now work done in rotating dipole by $\theta _{1}$ is 
$W _{2}=-pE\cos{\theta _{2}}$
Work done in rotating the dipole from $\theta _{1}$ to $\theta _{2}$ is
$W=W _{2}-W _{1}$
$=-pE \cos{\theta _{2}}-(-pE \cos{\theta _{1}})$
$=pE(\cos{\theta _{1}}-\cos{\theta _{2}})$

An electric dipole of dipole moment $p$ is placed in a uniform electric field $E$ in stable equilibrium position. Its moment of inertia about the centroidal axis is $I$. If it is displaced slightly from its mean position find the period of small oscillations.

  1. $2\pi \sqrt{\dfrac{I}{2pE}}$

  2. $2\pi \sqrt{\dfrac{2I}{pE}}$

  3. $2\pi \sqrt{\dfrac{I}{pE}}$

  4. $\pi \sqrt{\dfrac{2I}{pE}}$


Correct Option: C
Explanation:
Dipole moment $=p$
electric field $=E$
centroid axis $=I$
Explanation
When displaced at an angle $\theta $ from its mean position the magnitude of restoring torque is $T=-psin\theta $
For small angular displacement $\sin\theta \approx \theta $
$T=-pE\theta $
$\alpha =\dfrac { T }{ I } =-\left( \dfrac { PE }{ I }  \right) \theta $
    $={ -w }^{ 2 }\theta $
${ w }^{ 2 }=\dfrac { PE }{ I } $
$T=2\pi \sqrt { \dfrac { I }{ PE }  } $
($P.E=$ moment in electric field)

In a certain region of space, electric field is along the z-direction throughout. The magnitude of electric field is, however not constant but increases uniformly along the positive z-direction at the rate ${10^5}\,V/m.$ The force and the torque experienced by a system having a total dipole moment equal to ${10^{ - 7}}C - m$ in the negative z-direction is given by respectively.

  1. 0.01,0

  2. 0.02,0

  3. 0,0.01

  4. None of the above


Correct Option: A
Explanation:

$z$ direction positive rate $={ 10 }^{ 5 }V/m$

torque $=$ M $\times$ $E$
            $={ 10 }^{ 5 }\times { 10 }^{ -7 }$
            $=0.01cm$

 An electric dipole consist of two opposite charges each of magnitude $1\mu C$ separated by a distance of $2\,cm.$ The dipole is placed in an external field of ${10^5}{\text{N/C}}$.The maximum torque on the dipole is:

  1. $2 \times {10^{ - 4}}J$

  2. $2 \times {10^{ - 3}}J$

  3. $4 \times {10^{ - 3}}J$

  4. ${10^{ - 3}}N\,m$


Correct Option: C
Explanation:
An electric dipole consist at two opposite charge each of magnitude $=1\mu C=1\times { 10 }^{ -6 }C$
distance $=2cm$
Exter field $={ 10 }^{ 5 }N/C$
maximum torque on the dipole $=?$
$q=1\times { 10 }^{ -6 }C,\quad 2a=2cm$
                                or,  $=0.02cm$
$\therefore$    $P=q\times 2a$
           $=\left( 1\times { 10 }^{ -6 } \right) \times 0.02$
           $=2\times { 10 }^{ -8 }cm$
Intensity of the external electric field, $E=1.0\times { 10 }^{ 5 }N/C$
(i) ${ Z } _{ max }=pE=\left( 2\times { 10 }^{ -8 } \right) \left( 10\times { 10 }^{ 5 } \right) =2\times { 10 }^{ -3 }N-m$
(ii) Net work done in turning the dipole from ${ 0 }^{ 0 }$ to ${ 180 }^{ 0 }$
i.e  $W=\int _{ { 0 }^{ 0 } }^{ { 180 }^{ 0 } }{ \overline { r }  } d\theta =\int _{ { 0 }^{ 0 } }^{ { 180 }^{ 0 } }{ pE\sin\theta  } d\theta $
           $=pE{ \left[ -cos\theta  \right]  } _{ { 0 }^{ 0 } }^{ { 180 }^{ 0 } }$
           $=-pE\left( { \cos180 }^{ 0 }-\cos{ 0 }^{ 0 } \right) $
           $=2pE$
           $=2\times \left( 2\times { 10 }^{ -8 } \right) \left( 1\times { 10 }^{ 5 } \right) J$
           $=4\times { 10 }^{ -3 }J$
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