Re: Power Lines

[Richard Blade <rblade@UCCS.EDU>]

Tina, Rick, et al-

This is my first time to respond to the list, so I hope my contribution is
of some value.

In my intro physics class, I introduce the conceptual basis of electrical
current in the "Blade Scale" that was developed by my brother Bill while in
high school.  It relates magnitude of current to the electrical shock
effect on the human body through a set of desciptive nouns:

bite=1-10ma
zap=10-100ma
scutch=0.1-1amp
sizzle=1-10amp

It seems to be a very useful tool in introducing the students to electric
circuits.  Incidentally, voltage (i.e. electric potential difference) is
introduced as a kind of pressure that pushes the current through
conductors.  Resistance in the context of electrical shock has to be
carefully treated because biological materials are often nonlinear and
almost all the resistance in the human body is in the skin, so probes
placed between different points on the body have roughly the same
resistance between them, independent of distance.  The latter provides a
good question for discussion by the class.  Usually students begin by
thinking about salty sweat, but someone eventually gets it after
considering the conductivity of the blood.  That leads to the introduction
of the "principle path method" of analyzing circuits, which, in contrast to
Kirchhoff's laws, allows the studenst to conceptualize what is actually
going on in a complicated circuit.

Richard Blade
Professor of Physics and Energy Science
Univ of Colo at Colo Springs

At 12:21 PM 2/25/03 -0500, you wrote:
> Tina,
>
> Think POTENTIAL DIFFERENCE here.  The single line may be at 100,000 Volts
> relative to ground, but the current that runs through you will depend on the
> PD between your hands (hanging) or feet (standing).  Is there a PD?  In most
> cases no.  The danger with people and power lines is that we are big enough
> to easily come in contact with TWO lines--two lines at different potentials
> and therefore a significant PD.  The birds are small and will seldom, if
> ever, contact two wires at once.
>
> Rick
>
> *********************************************************
> Richard W. Tarara
> Professor of Physics
> Saint Mary's College
> Notre Dame, Indiana
> rtarara@saintmarys.edu
> ********************************************************
> Free Physics Educational Software (Win & Mac)
> www.saintmarys.edu/~rtarara/software.html
> ********************************************************
> ----- Original Message -----
> From: "Tina Fanetti" <FanettT@WITCC.COM>
> To: <PHYS-L@lists.nau.edu>
> Sent: Tuesday, February 25, 2003 12:01 PM
> Subject: Power Lines
>
>
> > Today in class, we were talking about why birds standing on power lines
> don't get electrocuted.
> > One of my students asked if he grabbed on to a power line would he get
> electrocuted.. yes, because the current would flow through the chest and
> could stop his heart...
> > He then asked if he stood on the power line, like a bird, would he get
> electrictuted.
> > I was like what kind of an idiot would want to stand on a high pwer
> line....(okay maybe not so tactful)
> > But would you get electricuted?
> > I would think that if the current went foot-to-foot you would be okay, but
> who really wants to take that chance?
> > Thanks
> > Tina
> > PS I did search google....not finding anything useful yet...
> >
> > Tina Fanetti
> > Physics Instructor
> > Western Iowa Technical Community College
> > 4647 Stone Ave
> > Sioux City IA 51102
> > 712-274-8733 ext 1429