Lesson 2 of 11 · 6 min
Magnetic field lines
NCERT §5.2, §5.2.1
Nisha lays a glass sheet over her magnet, sprinkles iron filings and taps the glass. A pattern of curves appears. What are these curves telling her?
The lesson in notes
In short
Iron filings sprinkled on glass over a short bar magnet line up in a pattern much like the field of an electric dipole: the magnet has two poles, as the dipole has two charges. A current-carrying solenoid gives a very similar pattern.
The pattern traces the magnetic field lines, a picture of B. They form continuous closed loops, running outside from N to S and inside the magnet from S back to N. Electric field lines of a dipole instead begin on the positive charge and end on the negative one or go off to infinity.
The tangent to a field line at any point gives the direction of the net field B there.
The more lines cross a unit area, the stronger B is: the lines crowd near the poles.
Two field lines never cross, since B would then have two directions at one point.
Field lines can be plotted with a small compass: the needle sets itself along the line at each spot.
They are not lines of force: the force on a moving charge, qv × B, is perpendicular to B, not along it (Example 5.4).