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Re: [Phys-l] F causing a or Delta-V causing I



Robert Cohen wrote:
How's this?

Delta V represents the energy (per charge) that is converted (typically
to heat and light for a resistor) as the moving charge (current)
interacts with the element. The more current, the greater the energy
(per charge) that is converted.

That's an _example_ of a situation that might be covered by Ohm's law.

Thus, it is clear that current through the element causes a voltage
across it (Delta V=IR) and not the other way around. :)

Sorry, that's not clear at all.

It takes more than an example to convince me of the general case.
I know too many counterexamples.

For example, consider this circuit:
http://www.av8n.com/physics/img48/v-ir.png

On the left side of the circuit is a constant-current source that always
puts out current I.

In this circuit, the output voltage is Vds (which we call V for shorthand)
which is the drain-to-source voltage across the FET. (A legend is provided
to explain standard FET terminology.) We let R denote the resistance of the
FET channel. It should come as no surprise that
V = I R

The interesting thing about a FET is that I can control the channel
resistance by fiddling with the gate voltage, Vgs. For a depletion-mode
FET, big gate voltage means small channel resistance, and vice versa.

Now Vgs is under my control, so I can use it to change the R value, and
thereby "cause" the output voltage V to change.

Therefore, if you believe in proof-by-example, it must be "clear" that the
equation V = I R should be interpreted as a statement that R causes V.

===================

BTW this is not a far-fetched circuit. Billions of circuits of this basic
type have been sold.

===================

Please, folks: Tost of the laws of physics -- especially the simple laws
that are encountered in HS physics and first-year college physics -- are
statements of equality, not causality.


If you want to understand causation, that's commendable ... but if you're
going to do it, do it right. And beware that understanding causation is
a whoooole lot more work than understanding equations.