{"id":467,"date":"2020-04-16T21:25:32","date_gmt":"2020-04-16T21:25:32","guid":{"rendered":"https:\/\/wp.physics.wisc.edu\/ingersollmuseum\/?page_id=467"},"modified":"2026-05-11T06:15:05","modified_gmt":"2026-05-11T06:15:05","slug":"copperdisk","status":"publish","type":"page","link":"https:\/\/wp.physics.wisc.edu\/ingersollmuseum\/exhibits\/em\/copperdisk\/","title":{"rendered":"The Rotating Copper Disk"},"content":{"rendered":"<h1 style=\"margin-top:var(--wp--preset--spacing--50);margin-bottom:var(--wp--preset--spacing--20)\" class=\"is-style-mini-bar wp-block-post-title\">The Rotating Copper Disk<\/h1>\n\n\n<div class=\"wp-block-group alignfull has-base-background-color has-background has-global-padding is-layout-constrained wp-block-group-is-layout-constrained\" style=\"margin-top:0;margin-bottom:0;padding-top:var(--wp--preset--spacing--40);padding-bottom:var(--wp--preset--spacing--60)\">\n<div class=\"wp-block-columns alignnone is-layout-flex wp-container-core-columns-is-layout-b4b75a54 wp-block-columns-is-layout-flex\">\n<div class=\"wp-block-column is-layout-flow wp-block-column-is-layout-flow\">\n<div class=\"wp-block-group is-layout-constrained has-global-padding wp-block-group-is-layout-constrained\">\n<figure class=\"wp-block-image\"><img loading=\"lazy\" decoding=\"async\" width=\"300\" height=\"178\" src=\"https:\/\/wp.physics.wisc.edu\/ingersollmuseum\/wp-content\/uploads\/sites\/10\/2020\/04\/CopperDisk1-300x178.jpg\" alt=\"\" class=\"wp-image-468\" srcset=\"https:\/\/wp.physics.wisc.edu\/ingersollmuseum\/wp-content\/uploads\/sites\/10\/2020\/04\/CopperDisk1-300x178.jpg 300w, https:\/\/wp.physics.wisc.edu\/ingersollmuseum\/wp-content\/uploads\/sites\/10\/2020\/04\/CopperDisk1.jpg 507w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">Turn the crank to set the copper disk rotating. You will see the rotating copper disk drags the compass needle along.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>HOW COME?<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">There must be an external magnetic field&nbsp;<strong>B<\/strong><sub>ext<\/sub>&nbsp;pushing on the compass needle, how does it come about?<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img loading=\"lazy\" decoding=\"async\" width=\"267\" height=\"205\" src=\"https:\/\/wp.physics.wisc.edu\/ingersollmuseum\/wp-content\/uploads\/sites\/10\/2020\/04\/disk-1.jpg\" alt=\"\" class=\"wp-image-469\"\/><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">Electrons which are embedded in the metal of the disk are being dragged along with it with a velocity\u00a0<strong>V.<\/strong><\/p>\n\n\n\n<figure class=\"wp-block-image\"><img loading=\"lazy\" decoding=\"async\" width=\"267\" height=\"205\" src=\"https:\/\/wp.physics.wisc.edu\/ingersollmuseum\/wp-content\/uploads\/sites\/10\/2020\/04\/disk-2.jpg\" alt=\"\" class=\"wp-image-470\"\/><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>2)&nbsp;<\/strong>The end of the compass needle produces a vertical magnetic field.<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img loading=\"lazy\" decoding=\"async\" width=\"267\" height=\"205\" src=\"https:\/\/wp.physics.wisc.edu\/ingersollmuseum\/wp-content\/uploads\/sites\/10\/2020\/04\/disk-3.jpg\" alt=\"\" class=\"wp-image-471\"\/><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>3)<\/strong>\u00a0The vertical magnetic field\u00a0<strong>B<sub>ver<\/sub><\/strong>\u00a0produced by the end of the compass needle induces a force\u00a0<strong>F<\/strong>\u00a0on the moving electrons.<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img loading=\"lazy\" decoding=\"async\" width=\"269\" height=\"205\" src=\"https:\/\/wp.physics.wisc.edu\/ingersollmuseum\/wp-content\/uploads\/sites\/10\/2020\/04\/disk-4.jpg\" alt=\"\" class=\"wp-image-472\"\/><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>4)<\/strong>The force\u00a0<strong>F<\/strong>\u00a0moves the electron and produces a radial Current\u00a0<strong>I<\/strong>.<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img loading=\"lazy\" decoding=\"async\" width=\"267\" height=\"205\" src=\"https:\/\/wp.physics.wisc.edu\/ingersollmuseum\/wp-content\/uploads\/sites\/10\/2020\/04\/disk-5.jpg\" alt=\"\" class=\"wp-image-473\"\/><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>5)<\/strong>&nbsp;The current&nbsp;<strong>I<\/strong>&nbsp;flowing in the disk produces the mystery magnetic field&nbsp;<strong>B<sub>ext<\/sub><\/strong>.<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img loading=\"lazy\" decoding=\"async\" width=\"267\" height=\"205\" src=\"https:\/\/wp.physics.wisc.edu\/ingersollmuseum\/wp-content\/uploads\/sites\/10\/2020\/04\/disk-6.jpg\" alt=\"\" class=\"wp-image-474\"\/><\/figure>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Original 1918 Museum Exhibit<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\"><i><b><a href=\"https:\/\/wiki.physics.wisc.edu\/facultywiki\/Demonstrations\">Physics Lecture Demonstration Database<\/a> <\/b><\/i><\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"<p>Turn the crank to set the copper disk rotating. You will see the rotating copper disk drags the compass needle along. HOW COME? There must be an external magnetic field&nbsp;Bext&nbsp;pushing on the compass needle, how does it come about? Electrons which are embedded in the metal of the disk are being dragged along with it &hellip;<\/p>\n","protected":false},"author":2,"featured_media":0,"parent":63,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"_uw_migration_status":"complete","_uw_gutenberg_post_content_before_migration":"","_uw_seo_meta_title":"","_uw_seo_meta_description":"","_uw_seo_twitter_card_type":"","_uw_seo_meta_image":"","_uw_seo_meta_image_url":"","_uw_seo_meta_image_sizes":[],"_uw_seo_custom_meta_tags":[],"footnotes":""},"class_list":["post-467","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/wp.physics.wisc.edu\/ingersollmuseum\/wp-json\/wp\/v2\/pages\/467","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/wp.physics.wisc.edu\/ingersollmuseum\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/wp.physics.wisc.edu\/ingersollmuseum\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/wp.physics.wisc.edu\/ingersollmuseum\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/wp.physics.wisc.edu\/ingersollmuseum\/wp-json\/wp\/v2\/comments?post=467"}],"version-history":[{"count":6,"href":"https:\/\/wp.physics.wisc.edu\/ingersollmuseum\/wp-json\/wp\/v2\/pages\/467\/revisions"}],"predecessor-version":[{"id":966,"href":"https:\/\/wp.physics.wisc.edu\/ingersollmuseum\/wp-json\/wp\/v2\/pages\/467\/revisions\/966"}],"up":[{"embeddable":true,"href":"https:\/\/wp.physics.wisc.edu\/ingersollmuseum\/wp-json\/wp\/v2\/pages\/63"}],"wp:attachment":[{"href":"https:\/\/wp.physics.wisc.edu\/ingersollmuseum\/wp-json\/wp\/v2\/media?parent=467"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}