Re: LC circuit/mass-spring analogy

[Leigh Palmer <palmer@sfu.ca>]

>  1)  According to Halliday, Resnick, and Walker's "Fundamentals of
> Physics" (Fourth Edition) the analogy between elements and variables in an
> LC circuit and a mass/spring system goes like this...
>
> LC <--> M-S
> -----------
> q <--> x
> i <--> v
> C <--> 1/k
> L <--> m
>
> My first question is, "What is the mechanical analogy of potential
> difference?"  (This is actually a question in the book.)  I feel that I
> have the answer they are looking for, but I was wondering what other people
> would come up with.

My answer would be "There is none in this case." Since potential difference
is defined as potential energy per unit charge, and the analogy drawn links
charge with displacement, the evident answer would be "potential energy per
unit of displacement". In this case the potential energy is not proportional
to the displacement, so that makes no sense.

It's OK that it makes no sense! You might as well ask "What is the quantity
analogous to the displacement current?" Same answer, less guilt, I suppose.
This analogy, while it is a very good one, is limited. Otherwise I would
have built a kilowatt transmitter back in my ham days with a leaf spring
and an anvil!

Leigh