[Catalist] Physics hack to cope with burnt out multimeters on Planck's constant experiment

[Leon Harris]

Soooo ...

Your are a physics teacher, and you want to do the Planck's constant LED 
experiment, where you derive Planck's constant from a plot of the 
wavelength of the LED and the breakdown voltage of the LED.

You know that in a dark room, you can see the LED glowing even at 2 
microamps.

But, but, buuuuuut ...

Some %#@$%!!!! lower school year 9 teachers pack of 32 monsters has been 
unleashed on your beautiful senior school multimeters, and has cooked 
the low range current function. Mmmme, I love the smell of burning 
styrene vapours in the morning! (Said no physics teacher ever).


Here is a fix for you - if you make the circuit below, and replace the 
current meter with a 1000 000 ohm resistor, the voltage across it is 
equal to the current flowing through it in microamps. In the circuit 
below, which might cost under $10 each to make, you have all the kit 
needed to do your lab, without resorting to violence against your y9 
colleagues.

Theory: from ohms law. I = V/R  amps=volts/ohms.   I x 10-6 amps = 
volts/ 10^6 ohms. The following little circuit will give you 0-4.5 
microamps at 0-4.5 volts, enough to measure even the 405nm blue LEDS 
(which start to conduct at 2.42 volts, using a knee of 2.5 uA )

You are welcome !


*No y9 teachers were harmed during these experiments




Typical current/voltage curve for a red 650nm LED.

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