Invisible Lines
Field lines — mapping the magnet's territory
What this lesson covers
Why it matters
Sprinkle iron filings around a magnet and an invisible empire snaps into view — arcs sweeping pole to pole, crowding where the power is. Faraday saw these lines and rebuilt physics around them.
The idea in plain words
Rules of the map. Tap them.
field lines: N → S outside, never crossing
Why do iron filings cluster thickest at the two ends of the magnet?
The filings are drawing the crowding rule for you
- rule 1: direction — the tangent at any point = the field's direction there (where a compass N points)
- rule 2: crowding — closer lines = stronger field — densest at the poles
- rule 3: never cross — two directions at one point is impossible — lines politely avoid each other
- the special case: uniform field — parallel, equally-spaced lines — same strength and direction everywhere
- Field lines converge at the poles
- Crowded lines = strong field = strongest pull
Predict first
Outside a bar magnet, field lines run…
By convention a field line points the way a free N pole would move: out of N, sweeping around, into S — and S to N inside the magnet, closing the loop.
- from the N pole around to the S pole — correct
- from S to N
- in closed circles around the middle
What you do
Walk the plotting compass around the magnet — six stations — then declare the direction rule.
Check yourself
Two magnetic field lines can never intersect because…
A compass at the crossing would have to point two ways at once — geometry forbids it.
INSIDE the bar magnet, the field lines run…
Field lines are closed curves: N→S outside, S→N inside. No line ever just ends.
A compass needle at rest settles…
The needle IS a tiny magnet: the field torques it into alignment — that's what 'plotting' exploits.
- the field would need two directions at one point — correct
- they repel each other
- magnets are too weak
- S to N, completing closed loops — correct
- N to S, same as outside
- nowhere — no field inside
- along the field line through its location — correct
- perpendicular to the field
- randomly