Ph102 Lab: Plancks Constant Worksheet printable pdf download

Ph102 Lab: Plancks Constant Worksheet

PH102 Lab: Planckʼs constant

Introduction

At this point you should have connected the lab box to a USB port and started the tuto-

rial software (“Bamalab” on the desktop).

You will need: Labjack box, two “clippy” banana cables, four regular banana cables,

red LED, yellow LED, color-wavelength chart.

In this experiment we will determine Planckʼs constant by analyzing the current-voltage

characteristics and light output of light-emitting diodes (LEDs). We have already seen in

previous labs that diodes are non-Ohmic elements; that is, current and voltage are not

proportional, as shown below:

When measuring I versus V, as above, the LED will show almost no current until a spe-

ciﬁc “threshold” voltage is reached, after which the current increases rapidly and the

LED begins to glow. Roughly, what should this threshold voltage be?

In a light emitting diode, electrical energy is converted into light energy. How this hap-

pens we will learn next week when we discuss quantum physics, but we can already

apply conservation of energy. We know that when the LED is glowing, when the voltage

is greater than the threshold, only one single color is emitted. This means that all pho-

tons emitted have roughly the same energy, since we know E=hf, where h is Planckʼs

constant and f is the frequency of the photon. If all of the photons have essentially the

same energy, then the increase in brightness of the LED can only come from the LED

emitting more and more photons as voltage is increased.

If photons of only a speciﬁc frequency and energy are emitted, then we should expect

nothing to happen until the electrical potential energy supplied to the LED is equal to the

energy of the photons emitted. In other words, we imagine that the potential energy of

each charge injected into the LED is converted completely to light energy. Right at the

threshold voltage, the electrical energy is just equal to the photon energy:

1/3

Ph102 Lab: Plancks Constant Worksheet