Planck constant V in E=qV question

• Paul37
In summary, Planck's constant, V, is found by measuring the wavelength of a LED and then using the equation E=qV to find it.
Paul37
Planck constant "V" in E=qV question

1. Homework Statement

I am trying to prove Planck's constant using LEDs. So far I have measured several LED's wavelengths using a spectrometer and from this their frequency. This will be f in the equation h = E/f. For E I put the LEDs in a forward biased circuit and am trying to use the equation E = qV to find E so I can therefore find Planck's constant. My question is which value do I use for V? The minimum voltage needed to produce a current in the circuit or the voltage the circuit eventually tends to.

2. Homework Equations

E=hf
E=qV

Also, do you know of any related experiments that I could do concerning this sort of area. I was going to find the wavelength of a UV LED and find its energy by a different method (not going to look at a UV LED through a spectrometer) but I wasn't sure exactly how to do that.

I think you should use the voltage when the diode starts to emit light.
Once the led is conducting there should not be much variation in voltage for different brightness (current)
This is a standard experiment in England for the IOP based physics course

technician said:
I think you should use the voltage when the diode starts to emit light.
Once the led is conducting there should not be much variation in voltage for different brightness (current)
This is a standard experiment in England for the IOP based physics course

well i tried it today and a blue LED started at 2.50V but the voltage went well above 5V and the LED just became brighter.

I'm in Scotland by the way. Part of an investigation

Paul37 said:
I am trying to prove Planck's constant using LEDs. So far I have measured several LED's wavelengths using a spectrometer and from this their frequency. This will be f in the equation h = E/f. For E I put the LEDs in a forward biased circuit and am trying to use the equation E = qV to find E so I can therefore find Planck's constant. My question is which value do I use for V? The minimum voltage needed to produce a current in the circuit or the voltage the circuit eventually tends to.
Not sure, but maybe a look at the diode current equation would offer some insight.
Also, do you know of any related experiments that I could do concerning this sort of area. I was going to find the wavelength of a UV LED and find its energy by a different method (not going to look at a UV LED through a spectrometer) but I wasn't sure exactly how to do that.
If I recall correctly, over the years Scientific American's Amateur Scientist column has reported a method or two for measuring h using ingenious setups. A search there might turn up something useful.

The Planck constant, denoted as "h", is a fundamental constant in quantum mechanics that relates the energy of a photon (E) to its frequency (f). The equation you have mentioned, h = E/f, is the correct formula to use in your experiment.

In order to find the value of Planck's constant, you need to measure the energy of the photons emitted by the LED. This can be done by using the equation E = qV, where q is the charge of the electron and V is the voltage applied to the LED. The key here is to use the voltage at which the LED is emitting light, which is known as the "operating voltage". This is the voltage at which the LED is producing the desired frequency of light and is typically higher than the minimum voltage needed to produce a current in the circuit.

Therefore, to accurately determine Planck's constant, you should use the operating voltage of the LED in your calculations. This can be found by measuring the voltage across the LED using a voltmeter while the LED is emitting light.

As for other related experiments, you could try investigating the relationship between the energy of a photon and its wavelength by measuring the energy of different colored LEDs and comparing it to their respective wavelengths. This could also be extended to include UV LEDs, as you have mentioned. Additionally, you could also explore the photoelectric effect and how it relates to the quantization of light energy.

I hope this helps and good luck with your experiments!

What is Planck constant?

The Planck constant, denoted by the symbol h, is a fundamental physical constant that relates the energy of a photon to its frequency. It is named after the physicist Max Planck and has a value of approximately 6.626 x 10^-34 joule seconds.

What is the significance of Planck constant?

The Planck constant plays a key role in quantum mechanics and is used to calculate the energy levels of atoms and molecules. It is also used to determine the wavelength of light emitted by a blackbody, and is essential in understanding the behavior of particles at the atomic and subatomic level.

What is the relationship between Planck constant and voltage in the E=qV equation?

In the equation E=qV, where E is energy, q is charge, and V is voltage, the Planck constant is used to convert the frequency of a photon (v) to its energy (E). This is because the energy of a photon is directly proportional to its frequency, and the proportionality constant is the Planck constant.

How is Planck constant measured?

The value of Planck constant was initially determined by Max Planck through experiments involving blackbody radiation. Today, it is measured using a device called a Kibble balance, which uses the relationship between the Planck constant and the mass of an object to determine its value.

What are the units of Planck constant?

The units of Planck constant are joule seconds (J·s) in the International System of Units (SI). However, in quantum mechanics, it is often expressed in units of electron volts (eV) or inverse centimeters (cm^-1) to make calculations more convenient.

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