State the properties of magnetic field lines. Draw the field lines of a bar magnet, and give two differences from electric field lines.

1. 1 · State the key idea. Magnetic field lines are closed curves whose tangent gives the direction of B.
2. 2 · List the properties.
   • Closed loops: N to S outside, S to N inside.
   • Crowded lines mean a strong field.
   • No two lines cross.
3. 3 · Draw. [Bar magnet with N and S; symmetric curved lines with arrows, crowded near the poles, continued inside from S to N]
4. 4 · Compare. Electric lines start on positive and end on negative charges; magnetic lines have no ends and pass through the magnet.
5. 5 · Real life. Iron filings on paper over a magnet show these lines directly.
6. 6 · Close in one line. So, the net magnetic flux through any closed surface is zero.

Scoring tip: In the diagram, continue the lines inside the magnet from S to N. Examiners look for the closed loops.