{"action":"opened","issue":{"url":"https://api.github.com/repos/ericmazur/PnPbook/issues/6","labels_url":"https://api.github.com/repos/ericmazur/PnPbook/issues/6/labels{/name}","comments_url":"https://api.github.com/repos/ericmazur/PnPbook/issues/6/comments","events_url":"https://api.github.com/repos/ericmazur/PnPbook/issues/6/events","html_url":"https://github.com/ericmazur/PnPbook/issues/6","id":103962205,"number":6,"title":"Validity of Kirchhoff's laws in AC circuits","user":{"login":"ericmazur","id":2316175,"avatar_url":"https://avatars.githubusercontent.com/u/2316175?v=3","gravatar_id":"","url":"https://api.github.com/users/ericmazur","html_url":"https://github.com/ericmazur","followers_url":"https://api.github.com/users/ericmazur/followers","following_url":"https://api.github.com/users/ericmazur/following{/other_user}","gists_url":"https://api.github.com/users/ericmazur/gists{/gist_id}","starred_url":"https://api.github.com/users/ericmazur/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/ericmazur/subscriptions","organizations_url":"https://api.github.com/users/ericmazur/orgs","repos_url":"https://api.github.com/users/ericmazur/repos","events_url":"https://api.github.com/users/ericmazur/events{/privacy}","received_events_url":"https://api.github.com/users/ericmazur/received_events","type":"User","site_admin":false},"labels":[{"url":"https://api.github.com/repos/ericmazur/PnPbook/labels/ch32","name":"ch32","color":"21be1a"},{"url":"https://api.github.com/repos/ericmazur/PnPbook/labels/rethink","name":"rethink","color":"bfe5bf"}],"state":"open","locked":false,"assignee":null,"milestone":{"url":"https://api.github.com/repos/ericmazur/PnPbook/milestones/2","html_url":"https://github.com/ericmazur/PnPbook/milestones/2e","labels_url":"https://api.github.com/repos/ericmazur/PnPbook/milestones/2/labels","id":1280439,"number":2,"title":"2e","description":"second edition","creator":{"login":"ericmazur","id":2316175,"avatar_url":"https://avatars.githubusercontent.com/u/2316175?v=3","gravatar_id":"","url":"https://api.github.com/users/ericmazur","html_url":"https://github.com/ericmazur","followers_url":"https://api.github.com/users/ericmazur/followers","following_url":"https://api.github.com/users/ericmazur/following{/other_user}","gists_url":"https://api.github.com/users/ericmazur/gists{/gist_id}","starred_url":"https://api.github.com/users/ericmazur/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/ericmazur/subscriptions","organizations_url":"https://api.github.com/users/ericmazur/orgs","repos_url":"https://api.github.com/users/ericmazur/repos","events_url":"https://api.github.com/users/ericmazur/events{/privacy}","received_events_url":"https://api.github.com/users/ericmazur/received_events","type":"User","site_admin":false},"open_issues":5,"closed_issues":0,"state":"open","created_at":"2015-08-30T20:19:52Z","updated_at":"2015-08-30T23:14:06Z","due_on":null,"closed_at":null},"comments":0,"created_at":"2015-08-30T23:14:06Z","updated_at":"2015-08-30T23:14:06Z","closed_at":null,"body":"In chapter 32, there are some serious issues with the validity\nof Kirchhoff’s «laws».\n\nThe «laws» were derived in chapter 31 in connection with DC\ncircuits, and I didn't make a fuss about validity back then,\nbecause they are actually more-or-less OK in the DC limit.\nHowever, alas, in chapter 32 the same conclusions are applied\nto AC circuits, blissfully ignoring the fact that they are\nnot reliably valid.\n\nWhen students do this sort of thing -- taking an equation\nout of context and applying it where it's not valid -- we\ncall it \"equation hunting\" and say nasty things about it.\n\nThe only reason Kirchhoff’s «laws» are even approximately\nvalid in real-world AC circuits is that people /like/ the\nsimplicity of Kirchhoff’s «laws» and their corollaries, so\nengineers take pains to minimize the significance of the\nvarious violations.  It must be emphasized that this is\nan engineering choice ... not a law of nature!\n\nWhat's worse, there are plenty of situations where gross\nviolations of Kirchhoff’s «laws» cannot be avoided.\nGround loops are an all-too-familiar example.  Here's\nanother amusing example:\n https://www.youtube.com/watch?v=9tzga6qAaBA&t=0m52s\n\nThe widely-useful method of series/parallel reduction is no \nbetter (and no worse) in this regard.\n\nIt's appalling how many students believe that Kirchhoff’s \n«laws» were graven by God on stone tablets.  This leaves\nthem seriously crippled when trying to deal with ground\nloops et cetera.\n\nThe bad news is that it is remarkably difficult to give\na concise statement of the range of validity of Kirchhoff’s\n«laws».  The best I've been able to come up with is here:\n  https://www.av8n.com/physics/kirchhoff-circuit-laws.htm#sec-kvl-limitations\n  https://www.av8n.com/physics/kirchhoff-circuit-laws.htm#sec-kcl-limitations\n\nThe good news is that the Maxwell equations always work.\nYou can tell students that an engineer with a decent physics\neducation has a significant advantage over one without ... \nespecially when it comes to ground loops, static electricity,\nantennas, waveguides, high-frequency circuits, et cetera."}}