Homopole
The Faraday Paradox Physics
Size guide S–5XL · inches
Homopole spins a copper war-wheel between magnet poles, and sparks jump from rim to axle.
The Physics
Spin a copper disk in a magnetic field along its axis and an EMF appears between the axle and the rim. Spin the disk with the magnet still: EMF. Spin both together: the same EMF. Spin only the magnet: none. What counts is the disk moving relative to the wires that close the circuit, not the magnet's spin.
EMF = ∫ (v × B) · dr = ½ B ω R² (rim to axle, uniform B)
- EMF: electromotive force between axle and rim (V)
- v: velocity of a point on the disk (m/s)
- B: magnetic flux density along the axis (T)
- dr: radial line element (m)
- ω: disk angular speed (rad/s)
- R: disk radius (m)
Michael Faraday built the copper-disk generator in his first Experimental Researches in Electricity, Phil. Trans. 122, 125 (1832).
On the shirt
Front: Homopole, the pixel-art character, with the title. Back: the disk EMF, the three spin cases and the resolution, with Faraday's 1832 paper. 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.
ShippingBy destination
Shipping calculated at checkout by destination. Printed to order for you.
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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