Tag: electric charges and fields

Questions Related to electric charges and fields

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

torque on a dipole in a uniform electric field electric dipole electric charges and fields electrostatics physics

  1. 2 W

  2. 3 W

  3. 4 W

  4. W/2

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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 

torque on a dipole in a uniform electric field electric dipole electric charges and fields electrostatics physics

  1. 2 MB

  2. MB

  3. -2 MB

  4. Zero

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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?

torque on a dipole in a uniform electric field electric dipole electric charges and fields electrostatics physics

  1. 2 N-m

  2. 3 N-m

  3. 4 N-m

  4. Zero

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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 $E=5\times 10^{5}N/C$ :-

torque on a dipole in a uniform electric field electric dipole electric charges and fields electrostatics physics

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

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

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

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

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Correct Option: C

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

torque on a dipole in a uniform electric field electric dipole electric charges and fields electrostatics physics

  1. $pE$

  2. $2pE$

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

  4. $Zero$

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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 :

torque on a dipole in a uniform electric field electric dipole electric charges and fields electrostatics physics

  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.

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Correct Option: B

Work done in turning dipole through an angle 60 is 

torque on a dipole in a uniform electric field electric dipole electric charges and fields electrostatics physics

  1. $zero$

  2. $pE/4$

  3. $pE$

  4. $pE/2$

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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.

torque on a dipole in a uniform electric field electric dipole electric charges and fields electrostatics physics

  1. uniform

  2. non-uniform

  3. none

  4. a&b both

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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:

torque on a dipole in a uniform electric field electric dipole electric charges and fields electrostatics physics

  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 $

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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 :

torque on a dipole in a uniform electric field electric dipole electric charges and fields electrostatics physics

  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

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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.