Effects of the Lenz's Law - Page 1
Forum section: electronics / general

author: bitwelder
author_rank: Super Contributor
timestamp: June 10, 2018, 07:49:32 pm
content: Just happened to find this video: a strong magnet falls in a copper tube, but because of Lenz's law, a magnetic field that opposes the changing field of the falling magnet is produced, with the result that the magnet moves/falls more slowly.Fascinating.Quite old video, but it did receive little views and votes.

author: IonizedGears
author_rank: Regular Contributor
timestamp: June 10, 2018, 09:07:05 pm
content: Yeah, circular loops of current called eddy currents develop (seehttp://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magmom.html) in the copper that creates an almost equal but opposite magnetic field. It's not exactly equal because of the resistivity of the copper. The eddy currents develop with a changing magnetic field, or more specifically a changing magnetic flux. It's a neato concept that's specifically used for magnetic braking.

author: dmills
author_rank: Super Contributor
timestamp: June 10, 2018, 10:01:56 pm
content: Also used in a way cool way in some failsafe delay off relays in old railway signalling installations where a heavy copper ring fitted around one of the relay coil pole pieces slows the decay of the field by enough to avoid race conditions in the relay logic.Very steampunk technology, but utterly reliable.Regards, Dan.

author: GerryBags
author_rank: Frequent Contributor
timestamp: June 11, 2018, 02:43:21 am
content: I can't remember where I saw it on YT, but someone got hold of a very large block of copper and a strong magnet on a long string. He swung the heavy magnet towards the copper but as son as it gets within inches it slows down so dramatically that there is no impact, despite the magnet weighing some few pounds. It's very odd to watch the smooth acceleration and then sudden deceleration.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 11, 2018, 03:18:41 am
content: It's really cool when you have a block of silver. The magnet doesn't even slide down the surface, at high angles, it tumbles down!Tim

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: June 11, 2018, 04:20:14 am
content: You don't even need something that fancy, simply rolling a spherical neo magnet down an aluminum block you can see it slows down.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 11, 2018, 05:47:10 am
content: Quote from: dmills on June 10, 2018, 10:01:56 pmThat's very interesting, would like to see one but couldn't find any, because I don't know how they are called, or what to look for.Do you have a link, please?

author: dmills
author_rank: Super Contributor
timestamp: June 11, 2018, 05:51:29 pm
content: I cannot find a picture of a railway version (Which were usually in most impressive glass cases), but there is some discussion of slugged relays herehttp://www.samhallas.co.uk/repository/po_docs/ep_technical_electricity_1_1.pdfAnd here is a modern versionhttps://w3.siemens.co.uk/mobility/uk/en/rail_solutions/Rail_Automation/Documents/QSPA1%20Relay.pdf~0.5 seconds delay on and ~150ms delay on off.BR930 is I think the relay you want, and here is some relay pornhttp://www.wbsframe.mste.co.uk/public/Relays.htmlRegards, Dan.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 11, 2018, 06:53:02 pm
content: So that is how they are called,slugged relay, thank you!Those in boxes are really big ones. To my surprise, it seems slugged relays are still in use and manufactured, yet nowadays are much smaller:http://www.fairs.cgglobal.com/gallery/images/attachments/railway-signalling.pdf

author: dmills
author_rank: Super Contributor
timestamp: June 11, 2018, 08:15:48 pm
content: Those are still not exactly small, being IIRC the best part of 2kg each!Regards, Dan.