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From: John Larkin <jlar...@highlandtechnology.com>
Newsgroups: sci.electronics.design,sci.physics
Subject: Re: Energy in a permanent magnet question
Date: Fri, 13 Apr 2012 12:08:53 -0700
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On Fri, 13 Apr 2012 09:47:06 -0500, Sam Wormley <sworml...@gmail.com>
wrote:

>On 4/13/12 6:29 AM, HardySpicer wrote:
>> Suppose I have a permanent magnet and attract a nail a height h from
>> the ground. If the nail has mass m then I have done mgh of work
>> (potential energy)  which must come from the field. If I then attract
>> a second nail the magnetic attraction will be less since I have used
>> energy when attracting the first one - which is still attached. If I
>> keep doing this there is little energy left to lift anything more.
>
>   No.

Yes. Each nail stuck to the magnet shunts some of the field, reducing
external field, so there's less energy available to lift future nails.

A magnet can't yank up an infinite mass of nails.


-- 

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation