Physics · Magnetism And Matter · NEET
| Start and end | Magnetic: no start or end — closed loop | Electric: starts on + charge, ends on − charge (or infinity) |
| Reason | Magnetic: no isolated monopole exists | Electric: isolated charge can exist |
| Inside the source | Magnetic: lines continue inside the magnet (S to N) | Electric: lines do not loop through the charge |
| Net flux through closed surface | Magnetic: always zero (∮B·dA = 0) | Electric: q_enclosed/ε₀ (can be non-zero) |
| Tangent meaning | Magnetic: gives B direction, NOT force (F = qv×B ⟂ B) | Electric: gives direction of force on + charge (F = qE) |
| Can they cross? | Magnetic: no | Electric: no (same rule for both) |
Correct — a magnetic field line has no beginning and no end. It is a continuous closed loop. Even inside a bar magnet, the lines continue from the south pole back to the north pole, so the loop is complete. This happens because there is no isolated magnetic pole (no monopole) for a line to start from. Electric field lines are different: they can begin on a positive charge and end on a negative charge, because a single charge can exist on its own.
An isolated electric charge exists, so an electric field line has a clear source (the + charge) and a clear sink (the − charge). But you can never get an isolated N pole or S pole — every magnet has both poles together. With no single pole to start or stop on, the only way a line can behave is to close back on itself. This is the same idea as Gauss's law for magnetism: net magnetic flux through any closed surface is zero, meaning every line that enters a surface must also leave it.
No. Two field lines (magnetic OR electric) can never cross. The tangent to a field line at any point gives the direction of the field there. If two lines crossed, the field would have two directions at the same point, which is impossible. This rule is the same for both types of lines, so it is NOT a difference — a common exam trick is to offer it as a 'difference'.
No, and this is a key trap. The tangent to a magnetic field line gives the direction of B (where a tiny compass needle points), but the magnetic force on a moving charge is F = qv×B, which is perpendicular to B. So the line does NOT point along the force. For electric field lines, the tangent DOES give the direction of the electrostatic force on a positive charge (F = qE). This is why NCERT avoids calling magnetic field lines 'lines of force'.
The net magnetic flux through any closed surface is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Magnetic field lines are always closed continuous loops with no start or end, because magnetic monopoles do not exist. Electric field lines start on positive charges and end on negative charges (or go to infinity), because isolated charges do exist.
Yes. Outside the magnet the lines go from the North pole to the South pole, and inside the magnet they continue from South to North, completing the closed loop. Electric field lines do not pass through their source charge in this looping way.
No. Net magnetic flux through any closed surface is always zero (Gauss's law for magnetism). Net electric flux is q_enclosed/ε₀, which is non-zero when the surface encloses a net charge. This is a direct result of magnetic lines being closed and electric lines being open.
Because the tangent to a magnetic field line gives the direction of B, not the direction of the force on a moving charge. The magnetic force F = qv×B is perpendicular to B, so the line does not point along the force. Electric field lines, however, do point along the electrostatic force on a positive charge.
No. Neither magnetic nor electric field lines can cross, because the tangent must give a single, unique field direction at each point. Crossing would mean two directions at one point, which is impossible.
