Measuring Planck's constant with LEDs

Resource: https://www.scienceinschool.org/2014/issue28/planck

What is a diode and how can it be used to measure Planck's constant:

Electric circuit for measuring

the voltage-current response

of each LED. From left to

right: battery, potentiometer

or rheostat, voltmeter,

ammeter, LED.

Students might only use a voltmeter initially and determine the cut-off voltage by observation. Their value for Planck’s will be pretty broad. This then offers a good opportunity to refine and develop as a PI using measures of current and voltage.

Linear regression gives the following value for the gradient (m):

m = 1.24811 x 10-6 Vm (volt metres) From this, the Planck constant can finally be calculated. From above, m = hc/e, so:

h = em/c (4)

= 1.6022 x 10-19 x 1.24811 x 10-6 /

2.9979 x 108

= 6.6704 x 10-34 Js (joule seconds)

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