Magnetic Field Lines

• Magnetic field — region where magnetic force acts on a magnet or moving charge.
• Field lines — direction from N to S outside magnet; never intersect.
• Compass needle — aligns along field direction.
• Field due to straight current-carrying conductor — concentric circles (right-hand thumb rule).
• Uniform field between pole pieces.

1. Magnetic field lines — closed curves; outside magnet N → S; inside S → N.
2. Density of lines — stronger field where lines closer.
3. No intersection — would imply two directions at one point.
4. Straight conductor — concentric circles; plane perpendicular to wire.
5. Right-hand thumb rule — thumb = current; curled fingers = field direction.
6. Compass deflection — shows field direction at that point.
7. Solenoid field — similar to bar magnet; uniform inside.
8. Earth's magnetic field — makes compass point north.
9. Oersted experiment — current deflects compass → link electricity and magnetism.
10. Unit of field B — tesla (intro); NCERT focuses on direction and pattern.

Field direction around vertical wire carrying current upward

Step 1 — Point right thumb upward (direction of current I).
Step 2 — Curl fingers around wire — anticlockwise when viewed from above.
Step 3 — At point east of wire, field points north (tangent to circle).
Step 4 — Compass placed below wire deflects accordingly.
Conclusion: concentric circles; direction from right-hand rule.

• Drawing field lines starting and ending (they are closed loops).
• Misconception: field lines are path of iron filings moving (they show direction only).
• Reversing right-hand rule (thumb must be current).
• Thinking no field without iron core (air-core solenoid still has field).
• Allowing two field lines to cross.

• How do magnetic field lines differ from electric field lines (start/end)?
• State right-hand thumb rule for straight conductor.
• What did Oersted's experiment demonstrate?
• Where is magnetic field strongest near a bar magnet?

Open the Practice tab for graded questions on Magnetic Field Lines.