1. Introduction

[    ] Test a hypothesis: Hypothesis-driven research

[ X ] Measure a value: Experimental research (I)

[    ] Measure a function or relationship: Experimental research (II)

[    ] Construct a model: Theoretical sciences and applied mathematics

[    ] Observational and exploratory research

B. Type & Category

Type of research: 2  (Write down one number from 1 to 6)

Category  –  19 (Write down one number from 7 to 20)

Sub-category –  g (Write down the sub-heading alphabet)

Aim / question being addressed 

What is Planck’s constant?
“Planck's constant relates the energy of a photon with the frequency of light. It allows the precise calculation of the energy of light emitted or absorbed and thereby permits the determination of the actual energy of the photon. Along with constant for the speed of light, Planck's constant (h = 6.626 10-34 joule-second in the meter-kilogram-second system of measurements) is a fundamental constant of nature.”  (Lerner, K. L., 2004)

During the twentieth century, German physicist, Maxwell Planck, “proposed that atoms absorb or emit electromagnetic radiation only in certain units or bundles of energy termed quanta.” (Lerner, K. L. ,2004)  Planck developed a quantum theory that account for a wide range of physical phenomena with acceptance of his experimental results regarding radiation emitted by an object and its temperature increases.

Electromagnetic radiation was thought to travel in waves with a never ending number of frequencies and wavelengths. “Planck determined that energy of light was proportional to its frequency. As the frequency of light increases, so does the energy of the light.”  (Lerner, K. L. ,2004)

How to accurately measure Planck’s constant?
Use a moving coil watt balance and electric power measured in terms of Josephson and quantum Hall effect is compared with mechanical power measured in terms of meters, kilograms and seconds. We find the Planck constant h=6.62606891(58)×10−34Js. The quoted standard uncertainty (1 standard deviation estimate) corresponds to (8.7×10−8)h. Comparing this measurement to an earlier measurement places an upper limit of 2×10−8/yr on the drift rate of the SI unit of mass, the kilogram.”( Edwin R. Williams, Richard L. Steiner, David B. Newell, and Paul T. Olsen. ,1998 ,September 21)

Why do we use LEDS to measure Planck’s constant?

Planck’s constant is the relation between the energy of a photon and the frequency of light. It is a constant value that should not change, therefore allows the calculation of the actual energy of a photon.

When an electric current runs through a LED (Light-Emitting Diode), energy is released in the form of photons. A LED can produce different colours. The colour of the light is determined by the energy in the photons. The colours can be altered by the chemical composition of the semiconductor materials.

The colour of light is determined by the wavelength. We use LEDs in this experiment because each colour of LED has a different threshold voltage at which electrons start being produced. Measuring this voltage, together with known values for the emission wavelengths, provides a path to finding a value for the Planck constant.

What is Quantum?

“Quantum is the Latin word for amount and, in modern understanding, means the smallest possible discrete unit of any physical property, such as energy or matter.” (Rouse,M. ,2006, July) Planck wanted to discover why radiation “from a glowing body changes in color from red, to orange, and, finally, to blue as its temperature rises. He found that by making the assumption that radiation existed in discrete units in the same way that matter does, rather than just as a constant electromagnetic wave, as had been formerly assumed, and was therefore quantifiable, he could find the answer to his question.”(Rouse,M. ,2006, July)
Planck wrote a mathematical equation that involves a figure used to represent individual units of energy and called these units quanta. “Planck assumed there was a theory yet to emerge from the discovery of quanta, but in fact, their very existence defined a completely new and fundamental law of nature. Einstein's theory of relativity and quantum theory, together, explain the nature and behavior of all matter and energy on earth and form the basis for modern physics.” ( Rouse,M. ,2006, July)

Why is Planck’s constant important?
“Planck’s Constant defines the quanta (minimum amount) for the energy of light, and therefore also the energies of electrons in atoms. It also factors into something called the Uncertainty Principle, discovered by Werner Heisenberg in 1927.” (Clear, S,2010 September 7)
A particle is located somewhere and has a position given by x, and is moving with a certain velocity which is given by p (momentum and the velocity multiplied by mass). “The Uncertainty Principle shows that one cannot know the x and p values to a precision greater than h-bar/2. The value h-bar is just Planck’s constant divided by 2π, or h/2π. (The “uncertainty” is shown by a Δ.)” (Clear, S,2010 September 7)

The value of planck’s constant is 6.62607004 × 10-34 m2 kg / s and has a percentage error of less than 10%.

Independent variable

The independent variable are the LED colours: red, green, blue and yellow.

Dependent variable

The dependent variable is the measurement of the voltage, which is used to calculate Planck’s constant.

Controlled variables

(a)  Same brand of 3V batteries

(b)  Same potentiometer/rheostat

(c)  Same voltmeter

(d)  Same ammeter

(e)  Same brand of LEDs   

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