# Physics Questions and Answers – Electric Dipole

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This set of Physics Multiple Choice Questions & Answers (MCQs) focuses on “Electric Dipole”.

1. Which among the following molecule is not a dipole?
a) NH3
b) H2O
c) HCl
d) CH4

Explanation: Though water and ammonia are covalent molecules, they have a net dipole moment due to their distorted structure than the ideal one. Hydrochloric acid has a linear molecular structure but its net dipole moment is towards the Hydrogen atom. But methane has a symmetrical tetrahedral structure and its net dipole moment is zero hence it doesn’t behave as a permanent dipole. Methane solvents are therefore non-polar solvents.

2. What is the dimension of the dipole moment?
a) [L T I]
b) [L T I2]
c) [M L T I]
d) [L T I-1]

Explanation: The dipole moment is defined as the product of a charge and distance. The dimension of charge (current*time) is [I T] and the dimension of distance is [L]. Therefore the dimension of dipole moment is [L T I]. Its unit in the CGS and the SI system are esu*cm and C*m respectively.

3. If the force acting on a point charge kept on the axis of an electric dipole is F, what will be the amount of force if the distance of the point charge is doubled from the dipole?
a) 2F
b) $$\frac {F}{8}$$
c) $$\frac {F}{2}$$
d) $$\frac {F}{4}$$

Explanation: If the distance of a point charge, kept on the axis of a dipole, is sufficiently large than the separation between the charges of a dipole, then it can be shown that electric field E at a distance r from the dipole varies as E α $$\frac {1}{r^3}$$. Now if the distance becomes 2r, electric field intensity will decrease by a factor of 23=8 times. Therefore the force F will now become $$\frac {F}{8}$$.
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4. If electric field intensity at a certain distance from a dipole on its axis is E1 and at the same distance but on the perpendicular bisector of the dipole is E2, then __________
a) E1=E2
b) E1=2 E2
c) E1=4 E2
d) E1=3 E2

Explanation: If the dipole moment of a dipole is p, then electric field intensity at a distance r from the dipole on its axis will be $$\frac {2p}{r^3}$$. But electric field intensity at a distance r from the dipole on its perpendicular bisector is $$\frac {p}{r^3}$$. Both the formulas are derived by assuming r >> length of the dipole. Thus E1=$$\frac {2p}{r^3}$$ and E2=$$\frac {p}{r^3}$$ and hence E1=2 E2.

5. Dipole moment depends on _______
a) Charge only
b) Charge and length of a dipole
c) Charge, length of a dipole and dielectric constant of the medium
d) Charge and dielectric constant of the medium

Explanation: Dipole is defined as two equal but opposite charges, kept at a small distance and having a dipole moment. The dipole moment is simply the product of the electric charge and length of the dipole. The dielectric constant of the medium doesn’t affect the dipole moment i.e. dipole moment of an electric dipole will be the same in water as well as in the air.

6. The electric field at a point on the perpendicular bisector of a dipole is directed ______
a) Along the dipole moment
b) Perpendicular to the dipole moment
c) Opposite to the dipole moment
d) 45 degree to the dipole moment

Explanation:

Due to the two-point charges +q and –q, two separate electric fields will be created at point P. Now we can resolve the field in vertical and horizontal components shown in the figure. If the point P is on the perpendicular bisector of the dipole, E1=E2. So their vertical components cancel out and the resultant electric field has an only horizontal component which is directed opposite to the dipole moment.

7. Two point charges 1C and -1C are placed 5mm away from each other forming a dipole. What will be the field intensity at a point 15 cm away from the dipole on its axis?
a) 1.66*109 N/C
b) 2.66*1016 N/C
c) 3*1018 N/C
d) 1.4*1016 N/C

Explanation: Dipole moment $$\vec{P}$$ = 1C*5mm.
=5*10-3cm.
We know, electric field at a point on the axis of a dipole:
$$\vec{E}=\frac {1}{4\pi\epsilon_o} \frac {2\vec{P}}{r^3}$$[r=distance of the point from dipole]
$$\frac {1}{4\pi\epsilon_o}{P}$$=9*109, P=5*10-3, r=5*10-2 m.
∴ $$\vec{E}$$ = 2.66*1016 N⁄C

8. An electric dipole is placed inside a cube. What will be the nature of electric flux from the cube surface?
a) Coming out of the surface
b) Coming in towards the surface
c) No flux at all
d) Coming out from one half and coming inwards in another half

Explanation: An electric dipole consists of two point charges. The amount of charges is the same but their polarities are different. Therefore the sum of total charges in a dipole is always 0. But flux from a closed surface is related to the total charge inside a surface. As the total charge inside the cube is zero, so there will be no flux coming out or going in towards the surface.

9. Which among the following describes the variation of electric field intensity on the perpendicular bisector of a dipole with distance from the dipole?
a)

b)

c)

d)

Explanation: The electric field on the perpendicular bisector varies with the distance from the dipole as E α $$\frac {1}{r^3}$$. So the graph of the electric field keeps on decreasing with distance from the dipole and it will be zero at infinity. A similar type of graph is observed in the case of points on the axial position of the dipole.

10. The CO2 molecule is a dipole. The statement is ______
a) True
b) False

Explanation: If we observe the structure of the CO2 molecule, we will find that the two C=O bonds are not in a straight line due to the presence of lone pair of electron of C atom. Therefore the dipole moments of the two C=O bonds don’t cancel out each other and hence there is a net dipole moment present in case of CO2 molecule. It means the center of positive and negative charge does not coincide in the case of polar molecules.

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