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Re: Today's jaw dropper




I got one for you from Wilson's COLLEGE PHYSICS 3rd Ed, Q15.64. The
problem states that an electron is moved tthrough a potention of 10^8 V
in a lightning discharge. Ignoring collisions, what KE does it gain?
The supposed e- gains 10^8 eV of KE.

Now, my reasoning is that an e- masses about 5x10^5 eV. Guess how fast an
electron with a KE of 10^8 eV is going.

My *guess* is, pretty close to the speed of light and it is borne out by
my calculation of .999988 c.

Sure, John -- looks good. However, relativity is Chapter 25 in Wilson;
and this problem is in Ch 15. Now guess what velocity my students get
for an e- with 10^8 eV of KE. Is it even appropriate to describe this
electron as having 10^8 eV of Kinetic Energy? I think I would phrase an
energy description of such an e- quite differently.

Finally, we can consider what real e- do in lightning bolts. Leigh
talks about deceptive window-dressing and this is my fave problem this
semester for it.

Dan M