Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-L] big circuit

There's a 1941 Three Stooges movie that features a long,
tall, straight staircase, 147 steps long. That staircase
actually exists:

Even so, the movie remains fundamentally a work of fiction.

I mention this because the veritasium video we have been
discussing mentions the Poynting vector, which is something
that actually exists.

Even so, that doesn't make the eleventeen fallacies in the
video go away.

The Pointing vector is neither necessary nor sufficient for
answering the questions posed by the video. You can perfectly
well figure out how the "big circuit" works using a lumped LC
model, which is vastly more appropriate to the introductory
physics course. So the Poynting vector is less important to
this circuit than the staircase was to the Three Stooges.

Intoning the words "Poynting vector" does not mean that wires
are now irrelevant to electrical circuits. Raise your hand
if you believe that energy will flow *directly* from the
battery to the bulb, in a time 1 meter / c, even though the
wires run in a completely different direction.

Understanding propagation in a "big circuit" has tremendous
real-world consequences. For multiple reasons we urgently
need to upgrade the electrical grid. As part of that, there
is a lot of talk about a "smart grid". However, physics
imposes harsh restrictions on how smart the grid can be. For
certain types of fault, such as a short circuit, by the time
you find out about it, it's too late to do much of anything
about it. You can be smart about the second- and third-order
things, but the zeroth- and first-order problems have to be
solved by physics, by massive physical objects storing huge
amounts of energy.

On 11/24/21 1:09 PM, Ian Sefton via Phys-l wrote:

Although for most of us our first acquaintance with the Poynting
vector was for EM waves, Poynting’s original paper* included a
discussion of “A circuit containing a voltaic cell”.

Yes ... but making use of the Pointing vector starts with
calculating the E and B fields, which the veritasium stooge
does not even begin to do.