{"product_id":"lumidisk-feynmans-disk-paradox-physics-t-shirt","title":"Lumidisk – Feynman's Disk Paradox Physics T-Shirt","description":"\u003cp\u003eLumidisk sits perfectly still, charged studs on her disc and a coil humming at her core. Switch the coil off, and she spins.\u003c\/p\u003e\u003ch3\u003eThe Physics\u003c\/h3\u003e\u003cp\u003eA 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.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eL_field = ε₀ ∫ r × (E × B) dV\u003c\/strong\u003e\u003c\/p\u003e\u003cul\u003e\n\u003cli\u003eL_field: angular momentum of the fields (kg·m²\/s)\u003c\/li\u003e\n\u003cli\u003eε₀: vacuum permittivity (F\/m)\u003c\/li\u003e\n\u003cli\u003er: position from the axle (m)\u003c\/li\u003e\n\u003cli\u003eE: electric field (V\/m)\u003c\/li\u003e\n\u003cli\u003eB: magnetic field (T)\u003c\/li\u003e\n\u003cli\u003edV: volume element (m³)\u003c\/li\u003e\n\u003c\/ul\u003e\u003cp\u003eThe puzzle and its answer are in The Feynman Lectures on Physics, Vol. II, sections 17-4 and 27-6.\u003c\/p\u003e\u003ch3\u003eOn the shirt\u003c\/h3\u003e\u003cp\u003eFront: 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.\u003c\/p\u003e","brand":"Printify","offers":[{"title":"S \/ White","offer_id":50936618614887,"sku":"27509394209832700205","price":29.99,"currency_code":"USD","in_stock":true},{"title":"S \/ Black","offer_id":50936618647655,"sku":"24766982188636456859","price":29.99,"currency_code":"USD","in_stock":true},{"title":"M \/ White","offer_id":50936618680423,"sku":"11529455678005806107","price":29.99,"currency_code":"USD","in_stock":true},{"title":"M \/ Black","offer_id":50936618713191,"sku":"87878579773873597792","price":29.99,"currency_code":"USD","in_stock":true},{"title":"L \/ White","offer_id":50936618745959,"sku":"21749175964700962709","price":29.99,"currency_code":"USD","in_stock":true},{"title":"L \/ Black","offer_id":50936618778727,"sku":"27851688973323366483","price":29.99,"currency_code":"USD","in_stock":true},{"title":"XL \/ White","offer_id":50936618811495,"sku":"17614279854513930435","price":29.99,"currency_code":"USD","in_stock":true},{"title":"XL \/ Black","offer_id":50936618844263,"sku":"24836605463205531561","price":29.99,"currency_code":"USD","in_stock":true},{"title":"2XL \/ White","offer_id":50936618877031,"sku":"12064111179633091715","price":29.99,"currency_code":"USD","in_stock":true},{"title":"2XL \/ Black","offer_id":50936618909799,"sku":"27018806927208513052","price":29.99,"currency_code":"USD","in_stock":true},{"title":"3XL \/ White","offer_id":50936618942567,"sku":"25924972147717360680","price":29.99,"currency_code":"USD","in_stock":true},{"title":"3XL \/ Black","offer_id":50936618975335,"sku":"29182633821494744632","price":29.99,"currency_code":"USD","in_stock":true},{"title":"4XL \/ White","offer_id":50936619008103,"sku":"14137059607524678020","price":29.99,"currency_code":"USD","in_stock":true},{"title":"4XL \/ Black","offer_id":50936619040871,"sku":"22815858104599146899","price":29.99,"currency_code":"USD","in_stock":true},{"title":"5XL \/ White","offer_id":50936619073639,"sku":"24471822750761032228","price":29.99,"currency_code":"USD","in_stock":true},{"title":"5XL \/ Black","offer_id":50936619106407,"sku":"21617691547011437966","price":29.99,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0596\/8746\/9159\/files\/feynman-disk-paradox_front_01_white.png?v=1793595678","url":"https:\/\/pixelatedphysics.com\/products\/lumidisk-feynmans-disk-paradox-physics-t-shirt","provider":"Pixelated Physics","version":"1.0","type":"link"}