Re: [Phys-L] Is this lovely video a demonstration of a Faraday Cage or something else?

[John Denker <jsd@av8n.com>]

In the context of:

> http://physicsfootnotes.com/hv-cable-inspector/

On 12/05/2016 02:09 PM, Derek McKenzie asked:

> Is this lovely video a demonstration of a Faraday Cage or something
> else?

I'd say it is a Faraday cage *and* something else.
Several somethings, actually.

> What are the main physics principles at work here?

Here are a few:

1a) The most spectacular part of the video is the humongous
 spark.  There is an interesting principle at work here,
 namely a gross violation of Kirchhoff’s Circuit «Laws».

 Consider the helicopter as a /node/ in the circuit.  The
 spark makes it obvious that a huge current is flowing
 into the node.  There is only the one path, so there is
 no hope of making Kirchhoff's «Laws» work.

 In fact, this video is one of my favorite teaching tools
 for convincing people that those «Laws» did not originate
 on God-graven stone tablets.  There are lots of situations
 where the Maxwell equations give the right answer but
 Kirchhoff's laws do not.

 Also, the Maxwell equations imply conservation of charge,
 which is a simpler notion, more directly usable in the
 introductory course.  Even so, there are lots of situations
 where charge is conserved but Kirchhoff's «Laws» are violated.

 See
   https://www.av8n.com/physics/kirchhoff-circuit-laws.htm#sec-kvl-limitations
 and more directly applicable to the video:
   https://www.av8n.com/physics/kirchhoff-circuit-laws.htm#sec-kcl-limitations

1b) This is a teachable moment in terms of my favorite
 hobby-horse, namely the process of effective learning and
 teaching.  Ask the students, when you saw that video, did
 it seem strange to have /one/ electrical path with a
 current flowing in it?

 The rule is, when you see something new, mull it over in
 your mind, checking to see in what ways it is consistent
 -- or inconsistent! -- with stuff you already know.  Do
 this all day every day.  It is the /connections/ between
 ideas that make the difference between useful knowledge
 and useless knowledge, i.e. between understanding and a
 pile of useless factoids.

 This has been known in the educational psychology literature
 for a good long while (James, 1898).


2a) There is also a tremendous voltage divider and current
 divider.  The woven metal suit would not protect the guy
 from the current flowing into the helicopter.  That's why
 he has the big metal stick.  That gives the current a path
 that does not go through the suit, and lowers the voltage
 and the current that the suit needs to deal with.  Without
 that, the suit might disappear in a bright orange flash,
 and even if it didn't, the guy would have a very bad day.

 Voltage dividers are commonly covered in the introductory
 course.  They are treeeemendously important in practice.
 I wired up three different voltage dividers literally today,
 not counting the ones built into every electrical device
 on earth (sometimes desired and sometimes not, but present
 all the same).

2b) Same shtick:  When you saw the video, did you say Oh
 look, a current divider?


3) And yes, the metal suit can be thought of as a
 Faraday cage, within limits.

 Most of what is said about Faraday cages is valid only
 in the DC limit, strictly speaking ... but much of it
 is valid to a decent approximation for AC.  For high
 voltage and high frequencies the distinction between
 voltage divider and Faraday cage becomes sketchy.

 Did you hear them mention that the suit is made of steel
 wire?  Did you wonder why steel and not copper?  Copper
 is a much better conductor, isn't it?  The answer has to
 do with skin depth.
   http://www.physicspages.com/2014/09/11/skin-depth-of-electromagnetic-waves-in-conductors/

 The suit is not a perfect Faraday cage.  The helicopter
 could kill the guy by getting near him when he's not
 ready with the stick.