Thursday, June 13, 2013

Varying voltage and zero sum games

Dear reader, off to another experiment.

First, I suppose I should mention my kid sister, who has a Master's degree in Materials Science from some Hole In The Wall University, and after my conversation with her last night on solar cells, suspect she may have even greater electrical Kung Fu than my little brother.  Not sure who will represent her Kung Fu, ya'll can watch and decide.

I confess to minor deviations from the text of the book.  However, isn't that the point?

We are on experiment 3, which you should take the opportunity to review at the HO:ME blog.  I agree wholeheartedly with the author of that blog that it's difficult to read the bands on the resistors.  And whoever thought that particular color counted as purple?  Crazy.  It was bad enough I purchased a magnifying card to help.

No jokes about old eyes, please.

The experiment has you set up a circuit with a set resistor and an LED in series, initially, then with a potentiometer to vary the resistance.

At one point during the experiment the text asks to check the voltage across both the potentiometer and the LED, and look for variation.

Indeed, it does vary.

Here, at the potentiometer:

Note the intensity of the LED increases as the measured voltage across the potentiometer decreases.

Now the voltage drop across the LED itself:

Harder to see here, but the intensity of the LED increases as the measured voltage decreases.

I admit that at first the changes above confused me, especially when I had only measured the voltage at the potentiometer, and not yet at the LED.  I was caught in a mental trap that equated voltage to "available power".  It was not clear to me why I would have increased intensity at the LED--and therefore an obvious increase in "available power"--when I measured a decrease in voltage at the potentiometer.

It was not until I read in the text that the LED and the potentiometer were "sharing" the voltage that things clicked.  This is a zero sum game.

I first heard that concept when working for P. Read Montague (who is much more impressive than the Wiki article implies) at Baylor's Human Neuroimaging Lab.  It means something a little different than sharing; it implies that there is a set and unchangeable amount of stuff, and that if one party takes more it leaves less for others.  See Wikipedia article.

It became clear later that I had not conceptualized the LED as a resistor either.  In my mind, it was some sort of "passive receptor of electrical power that magically transmutes it into light" as opposed to something with inherent resistance.

In addition, I was not looking at the voltage as a "voltage drop", just as a source of power. In this context it's more appropriate to look at the measured voltage on the multimeter as "voltage stolen by the circuit element being measured"--unless you are measuring a battery or solar cell.

Simple stuff, I know.  I'm sure my younger siblings will get a laugh out if this. But good learning, nonetheless!


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