Physics Multiple Choice Questions on “Electric Field Lines”.
1. Which among the following is false about electric field lines?
a) They are continuous
b) They attract each other
c) They remain parallel in a uniform electric field
d) They diverge from positive charge
Answer: b
Clarification: Electric lines of force are always diverging from positive charge and they converge at a negative charge. Besides, they repel each other because of repulsion between two similar charges. They remain parallel to each other in the uniform field but may divert in case of a non-uniform field. These lines are continuously stretched between two opposite charges and always try to maintain minimum length.
2. At ‘Electrical Neutral Point’ the lines of force _______
a) Are absent
b) Coincide with each other
c) Always converge
d) Always diverge
Answer: a
Clarification: Electrical neutral point is a point in between two positive and negative charges, where the electric field intensity is zero. It means that no lines of force pass through that point because electric field lines are the measure of field intensity i.e. more lines mean more field intensity. The lines never converge or diverge neither they coincide at that point.
3. 1 dyne/esu = _____ N/C
a) 3*104
b) 105
c) 3*105
d) 9*104
Answer: a
Clarification: 1 N/C = (frac {1 N}{1 C} = frac {10^5 dyne}{3*10^9 esu} = frac {1}{3*10^4})dyne/esu. Therefore 1 dyne/esu=3*104 N/C. Both of them are the unit of electric field intensity. But N/C is the unit of electric field intensity in the SI system while dyne/esu is the unit of field intensity in the CGS system.
4. The electric field lines diverge from _______
a) Positive charge
b) Negative charge
c) Dipole
d) Zero potential point
Answer: a
Clarification: Electric field lines always converge at a negative charge and diverge from the positive charge. If a unit positive charge is kept in an electric field, the path of motion of that charge is known as field lines. Now we know that a positive charge is always pushed away by another positive charge. Thus we can find the direction of field lines near a positive charge.
5. Two plates are oppositely charged uniformly and kept parallel to each other at a certain distance. What will be the nature of electric field lines in between them?
a) Circular
b) Parallel to each other throughout the cross section
c) Not uniformly distributed
d) Parallel and uniform in central part but fringes out at the extreme ends
Answer: d
Clarification: The electric field between two uniformly charged plates is uniform. Electric field lines in the uniform electric field are parallel to each other and uniformly distributed. This phenomenon is observed in most of the parts between the plates. But in the two extreme ends, the field line fringes out due to the non-uniform electric field.
6. It is observed that N number of field lines come out perpendicularly from the unit surface area of the sphere. What is the amount of charge inside the sphere? ∈ is the dielectric constant of the medium.
a) -4πR2 N∈
b) 4πR2 N∈
c) (frac {4pi R^2}{Nepsilon})
d) –(frac {4pi R^2N}{epsilon})
Answer: b
Clarification: Field lines come out from the surface. That means the charge must be positive. According to Maxwell’s principle, field lines coming out from a charge q is equal to (frac {q}{epsilon}) where ∈ is the relative permittivity. The total number of field lines coming out of the sphere is 4πR2N. This must be equal to (frac {q}{epsilon}). Therefore q=(frac {4pi R^2}{Nepsilon}).
7. A field line can diverge from and converge at the same conductor. The statement is _____
a) True
b) False
Answer: b
Clarification: For the phenomenon to happen in practice, both positive charge and negative charge must be present on the same conductor. But as a conductor carries electricity and can never store charge at a localized part, this phenomenon can never occur. Field lines are circular for a current carrying conductor and their direction depends on the direction of the flow of current.
8. Two electric field lines ______
a) Always intersect each other
b) Never intersect
c) May intersect sometimes
d) Are always perpendicular to each other
Answer: b
Clarification: We know that the electric field at a point is directed towards the tangent of electric field lines at that point. Now if two field lines intersect at a point, two tangents can be drawn at that point. That means at a particular point there would be two separate directions of the electric field which is impossible. So, we can conclude that two electric field lines never intersect each other.