Lumidisk
Feynman's Disk Paradox Physics
Size guide S–5XL · inches
Lumidisk sits perfectly still, charged studs on her disc and a coil humming at her core. Switch the coil off, and she spins.
The Physics
A disc carries fixed charges and a coil at its centre carries a current. Nothing moves. Switch off the current and the falling magnetic flux induces an electric field that pushes the charges round, so the disc turns. The angular momentum was there all along, stored in the static electric and magnetic fields, and the total of disc plus field is conserved.
L_field = ε₀ ∫ r × (E × B) dV
- L_field: angular momentum of the fields (kg·m²/s)
- ε₀: vacuum permittivity (F/m)
- r: position from the axle (m)
- E: electric field (V/m)
- B: magnetic field (T)
- dV: volume element (m³)
The puzzle and its answer are in The Feynman Lectures on Physics, Vol. II, sections 17-4 and 27-6.
On the shirt
Front: Lumidisk, the pixel-art character, with the title. Back: Faraday's law, the field momentum density, field angular momentum and the conservation line, with the Lectures reference. Printed on a unisex tee.
Shipping calculated at checkout by destinationMade to order, ships in 2–5 business days
Details
Size & fitS–5XL
Unisex heavy cotton (Gildan 5000), classic fit.
| Size | Width | Length | Sleeve |
|---|---|---|---|
| S | 18 | 28 | 15.1 |
| M | 20 | 29 | 16.5 |
| L | 22 | 30 | 18 |
| XL | 24 | 31 | 19.5 |
| 2XL | 26 | 32 | 21 |
| 3XL | 28 | 33 | 22.4 |
| 4XL | 30 | 34 | 23.7 |
| 5XL | 32 | 35 | 25 |
For scale: size M is 20 in (50.80 cm) wide, about 3.143 × 1034 Planck lengths (lP = 1.616255 × 10−35 m). A tape measure is quicker.
Measurements in inches, ±1.5 in tolerance.
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Returns & replacements21 days
Every item is printed to order, so we can’t accept returns or exchanges for change of mind, or if you ordered the wrong size or colour.
Print defect, misprint, damage or the wrong item? We’ll send a free replacement, or a refund if you prefer. Report it within 21 days of delivery with a photo; no need to send it back.
Field notes
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