Consider the following statements about field lines:
(A) Electrostatic field lines can form closed loops when the loop encloses charges.
(B) Magnetostatic field lines can form closed loops around empty space.
(C) The magnetic field lines of a toroid are completely confined within it.
(D) Some fringing of the magnetic field lines between two pole pieces cannot be avoided.
Which of the above statements are incorrect?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A) and (B) only
A closed loop of a static magnetic field line must go round a region through which a current passes. Electrostatic field lines start and end on charges, so they never close on themselves.
(A) Electrostatic field lines can form closed loops when the loop encloses charges. — Incorrect statement. Electrostatic field lines never form closed loops, whether or not the loop encloses charges.
(B) Magnetostatic field lines can form closed loops around empty space. — Incorrect statement. A static magnetic field line cannot close around empty space. It can close only around a region that carries a current.
(C) The magnetic field lines of a toroid are completely confined within it. — Correct statement. The field of a toroid stays inside its windings. Each loop there encloses the current in the windings.
(D) Some fringing of the magnetic field lines between two pole pieces cannot be avoided. — Correct statement. The lines cannot be perfectly straight at the ends of the gap. Otherwise Ampere’s law would be violated.
NCERT (2026-27):
Example 5.3 – “… By contrast, electrostatic field lines can never form closed loops, neither in empty space, nor when the loop encloses charges.”
Example 5.3 – “… Magnetostatic field lines can never form closed loops around empty space. A closed loop of static magnetic field line must enclose a region across which a current is passing.”
Example 5.3 – “(c) Right. Magnetic lines are completely confined within a toroid.”
Example 5.3 – “(g) Wrong. Magnetic field lines between two pole pieces cannot be precisely straight at the ends. Some fringing of lines is inevitable. Otherwise, Ampere’s law is violated.”