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Re: R = V/I ?



I'll respond just to this bit and then go to bed!

At 15.10 09/05/00 -0700, Leigh Palmer wrote:
Let's not complicate things. My system is a Mixmaster and a bowl of
cake batter. It has two terminals. Mixmasters can be run perfectly
well off DC mains because they have universal (AC/DC) motors, as
anyone watching an old TV next to a running Mixmaster will readily
perceive. Let's say I run my Mixmaster on DC. My system dissipates
energy, it has an IV characteristic, and the power dissipated P = IV.
I can define R for this system by your causal relation: R = P/I^2.
Like the light bulb however, this relation will yield a variable
resistance. Unlike the light bulb, electrical resistivity plays no
important part in dissipating energy in this system.

Of course one would not apply R = V/I to the motor. From "my viewpoint" the
reason is clear. There is an emf in the motor, symptomatic of an energy
conversion which is not (directly) dissipative. One can always choose to
mix eggs and flour, dissipating the energy converted. Or instead one could
use it to accelerate a vehicle. And then perhaps apply the brakes... but
this doesn't produce a resistance. For resistance to be present the charge
carriers must dissipate the work done on them by the E field, directly,
within the device, heating it. Jam a spoon between the Mixmaster paddles to
stop them from turning. This will demonstrate the difference between the
dissipation due to its resistance, and the energy being converted by
working on the pancake mix!

Mark



_____________________________________
Mark Sylvester
United World College of the Adriatic,
34013 Duino TS, Italy.
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