The figure shows field lines drawn in seven cases, (a) to (g).
The lines in (a) and (f) cannot be magnetic field lines. They are, in fact, the electric field lines of
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → a long positively charged wire in (a), and a positively charged upper plate with a negatively charged lower plate in (f)
Magnetic field lines form continuous closed loops. They do not start or end at any point. So the net magnetic flux through any closed surface is zero. Electric field lines are different. They start on positive charges and end on negative charges.
Option (1) a long positively charged wire in (a), and a positively charged upper plate with a negatively charged lower plate in (f) — Correct. In (a), the arrows point away from the wire, so the wire is positive. In (f), the lines leave the upper plate and end on the lower plate. So the upper plate is positive and the lower plate is negative.
Option (2) a long negatively charged wire in (a), and a negatively charged upper plate with a positively charged lower plate in (f) — Incorrect. The arrows in (a) point away from the wire. A negative wire would have arrows pointing towards it. The lines in (f) also leave the upper plate, so it is positive.
Option (3) a long negatively charged wire in (a), and a positively charged upper plate with a negatively charged lower plate in (f) — Incorrect. The plates in (f) are read correctly, but the arrows in (a) point away from the wire. So the wire is positive.
Option (4) a long positively charged wire in (a), and two positively charged plates in (f) — Incorrect. The wire in (a) is read correctly, but in (f) the lines end on the lower plate from both sides. A positive plate would send lines out, so the lower plate is negative.
NCERT (2026-27):
Example 5.3 – “(a) Wrong. Magnetic field lines can never emanate from a point, as shown in figure. … The field lines shown, in fact, represent electric field of a long positively charged wire. …”
Example 5.3 – “… The given field lines, in fact, show the electrostatic field lines around a positively charged upper plate and a negatively charged lower plate. …”
FIGURE 5.6