Field Lines — Laboratorium

FIG. 1 A single bar magnet, field shown with iron filings.

View

Filings Field lines Strength map

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Layouts

Magnets

1 magnet on the sheet

Click a magnet to select it.

Strength of the selected magnet —

Controls

Drag a magnet to move it. Clicking an empty spot on the sheet deselects the magnet.
The mouse wheel over a magnet rotates it by 5°, or by 1° with Shift or Alt.
The ring around the selected magnet: grab it (the gold handle is easiest) and turn; 5° steps, 1° with Shift.
Double-click or double-tap a magnet to flip its poles.
The compass follows the cursor: the needle points along the field, and the readout gives the strength |B| in a.u. (arbitrary units) and the angle θ, measured counterclockwise from the horizontal axis. On a touch screen, tap the sheet.

←↑→↓: move the selected magnet by 1 mm (with Shift — by 5 mm)
[]: previous / next magnet
R: rotate by 15° (with Shift — the other way)
F: flip poles
Delete: remove the selected magnet
A: add a magnet
123: filings / field lines / strength map
Esc: deselect the magnet

Notes

The model: each pole is a “magnetic charge” spread over the end face of the magnet (the Gilbert model), and the field of such a charge falls off with the square of the distance. Outside the magnets this model reproduces the field faithfully; inside a magnet it would give the wrong direction of the B field, so we don't draw it there. Values are given in a.u. — arbitrary units.

Did you know…?

You can't cut off a lone N pole. Cut a bar magnet in half and each piece again has an N and an S pole. Nobody has ever found a single “magnetic charge” (a monopole) — in the model on this page it's just a handy calculation trick.
Near Earth's geographic North Pole lies a magnetic S pole. The north-seeking end of a compass needle is an N pole, and opposite poles attract — that's why the needle points north.
Earth's magnetic poles have swapped places many times. Traces of these reversals are recorded in solidified lava on the ocean floor. The last reversal happened about 780,000 years ago.

Challenges

Find the field's null point
Move the compass to the spot where the magnets' fields cancel exactly and |B| drops to zero.
Hint: the “Repulsion” layout, between the N polesnot completed
Make a chain of three magnets
Line up three bar magnets so that the N pole of each touches the S pole of the next (a gap of at most 1 cm).
Add a magnet (A), rotate (R) and dragnot completed
Shift the null point
Change the strength of one magnet so it is at least 1.5 times stronger than the other, and find the null point again. It moves closer to the weaker magnet.
The “Strength of the selected magnet” slidernot completed

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