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

[Leon Harris]

As I recall, you pick an endpoint. 2.5 uA of current is good. Somewhere 
at the point of inflection of the curve.
Work function is different, but related. If you want to demonstrate work 
function, make an aluminium can electroscope, with mylised tinsels and 
time the time it takes to spontaneously discharge. Then repeat the 
experiment while burning a strip of magnesium ribbon in front of it. 
Still not so cool though, and from what I see in the physics program, 
this is a useful experiment with good learning in a very arid part of 
the syllabus.

I share the hack because I used it to support a physics teacher I know, 
and because I know the experiment is in fairly widespread use around the 
state.

On 7/09/2021 9:52 pm, rsskinner at optusnet.com.au wrote:
>
> 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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