Physics Multiple Choice Questions on “Electrostatic Potential due to an Electric Dipole”.
1. Which one is the correct expression of electric potential on the axial point of a dipole whose dipole moment is p and length is 2l, at a distance of r?
a) (frac {p}{r^2})
b) (frac {p}{l^2})
c) (frac {p}{(r^2-l^2)})
d) (frac {p}{2lr})
Answer: c
Clarification: Electric potentials due to the two point charges of the dipole are (frac {q}{(r-l)}) and (frac {-q}{(r+l)}). Therefore, total potential = (frac {q}{(r-l)} – frac {q}{(r+l)} = frac {2ql}{(r^2-l^2)}). Now 2ql = dipole moment of the dipole=p. Therefore, the expression becomes (frac {p}{(r^2-l^2)}).
2. What is the electric potential at the perpendicular bisector of an electric dipole?
a) Positive
b) Negative
c) Zero
d) Depends on medium
Answer: c
Clarification: Any point on the perpendicular bisector is equidistant from both the charges of the dipole. Therefore, the electric potential at that point is equal and opposite due to the two different charges. Therefore, the total electric potential at that point is zero.
3. The electric potential at an axial point of a dipole is the maximum.
a) True
b) False
Answer: a
Clarification: Electric field at a distance r from a dipole and at an angle θ is p*(frac {cos theta}{r^2}), where p is the dipole moment of that dipole. If θ becomes 0 degrees, cosθ will be the maximum i.e. 1. Therefore, the electric field is the maximum at the axial points.
4. What is the dimension of the dipole moment?
a) [I L T]
b) [I L T-1]
c) [I L2 T]
d) [I T]
Answer: a
Clarification: Dipole moment = charge*length of the dipole. The electric charge has dimensions [I T] and length has dimensions [L]. Therefore, the dipole moment has the dimension [I T L] and has unit C*m of C*cm.