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

[rsskinner at optusnet.com.au]

But do you take the 1.4 value or the 1.6 value? Also note that this experiment has nothing to do with the Work Function as some have mentioned.

Roy

 

From: Catalist <catalist-bounces at lists.stawa.net> On Behalf Of Leon Harris
Sent: Tuesday, 7 September 2021 8:29 PM
To: catalist at lists.stawa.net
Subject: [Catalist] Physics hack to cope with burnt out multimeters on Planck's constant experiment

 

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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