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9.4 From ideas to implementation: 2. The photoelectric effect and black body radiation

Syllabus reference (October 2002 version)
2. The reconceptualisation of the model of light led to an understanding of the photoelectric effect and black body radiation
Students learn to:
E=hf
and
c=f(theta)
Students:

Extract from Physics Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.
[Edit: 30 June 09]

Prior learning: Preliminary module 8.2 (subsections 1, 3 and 4)

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perform an investigation to demonstrate the production and reception of radio waves

Sample procedure

  1. Turn on a radio and tune it to your favourite AM or FM (500 - 1600 k Hz) station. Note any “noise” or interference that is heard in the signal.
  2. Turn on a signal generator that produces an AM modulated radio frequency (RF) signal and adjust the output signal frequency from 500 - 2000 k Hz. Note any effect on reception of your favourite station.
  3. Take a length of copper wire and bend it into a shape that you think will make a good transmitter. Try various shapes in turn, including straight, a loop and a coil.
  4. Turn off the signal generator and connect the transmitter to the output terminals.
  5. Repeat as for step 2. Adjust the output frequency slowly until “noise” or interference is heard on your favourite station. Compare this frequency with the known frequency of the radio stationn when the noise or interference is at a maximum.
  6. Repeat the previous step using different transmitter aerials and after tuning the radio to a different station. (Do this in a systematic way.)

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describe Hertz's observation of the effect of a radio wave on a receiver and the photoelectric effect he produced but failed to investigate

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outline qualitatively Hertz's experiments in measuring the speed of radio waves and how they relate to light waves

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identify Planck's hypothesis that radiation emitted and absorbed by the walls of a black body cavity is quantised

Background information Graph: Y=Energy radiated X=theta
It was thought that the energy absorbed and emitted by a black body should be continuous, that is, could occur in any amount, and should increase as the wavelength became shorter. This was not supported by the experimental data as shown in the sketch. The amount of energy radiated reaches a maximum at a wavelength that depends on the temperature of the black body.

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identify data sources, gather, process and analyse information and use available evidence to assess Einstein's contribution to quantum theory and its relation to black body radiation

This investigation can be conducted by gathering a range of resources including scientific journals, CD-ROM resources and the Internet.

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identify Einstein's contribution to quantum theory and its relation to black body radiation

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explain the particle model of light in terms of photons with particular energy and frequency

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identify data sources, gather, and present information to summarise the use of the photoelectric effect in photocells

Sample information

Photocells are common in electric eyes, radiation detectors and light meters. Many utilise the photoelectric effect to detect the presence of light or radiation at particular wavelengths. For example, a photoelectric photometer is used by astronomers to analyse the frequencies of light received from a star. Others respond to a change in light intensity by detecting a particular photocurrent, such as in an alarm circuit where an intruder cuts a beam of light falling on a photocell.

Photovoltaic devices, use a silicon semiconductor to convert sunlight, or any visible light, into electrical energy. When sunlight falls on a junction between n-type and p-type semiconductor material, electrons are ejected from atoms. These electrons are collected to form a direct electric current (DC).

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identify the relationships between photon energy, frequency, speed of light and wavelength: E=hf and c=f(theta)

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solve problems and analyse information using: E=hf and c=f(theta)

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process information to discuss Einstein and Planck's differing views about whether science research is removed from social and political forces

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References

Heilbron, J., 2000, The dilemmas of an upright man. Max Planck and the fortunes of German science. With a new afterword. Cambridge (MA), London. This biography of Planck lays out the two sides of the issue.

Rosenthal-Schneider, I., 1980, Reality and scientific truth. Wayne State University Press, Detroit, ISBN 0-81-431650-6. This is an engaging collection of essays about and correspondence with Einstein, Planck and von Laue, by Ilse Rosenthal-Schneider, who was taught by these three in Germany before she emigrated to Australia. She spent the second half of her life at the University of Sydney, where she taught the history and philosophy of science.

Walker, M., 1995, Nazi science: Myth, truth and the German atomic bomb. Plenum, New York, ISBN 0-306-44941-2.

American Institute of Physics (AIP), June 2001, Albert Einstein: image and impact, in Public concerns American Institute of Physics web site, USA. The following is a relevant quotation from their AIP web site:

"The outbreak of the First World War brought Einstein's pacifist sympathies into public view. Ninety-three leading German intellectuals, including physicists such as Planck, signed a manifesto defending Germany's war conduct; Einstein and three others signed an anti-war counter manifesto. He helped form a non-partisan coalition that fought for a just peace and for a supranational organisation to prevent future wars. As a Swiss citizen, Einstein could feel free to spend his time on theoretical physics, but he kept looking for ways to reconcile the opposing sides. "My pacifism is an instinctive feeling," he said, "a feeling that possesses me because the murder of men is disgusting. My attitude is not derived from any intellectual theory but is based on my deepest antipathy to every kind of cruelty and hatred".

Sample information

Although there was no direct debate between Einstein and Planck on this issue, it seems that Einstein and Planck took different views about scientists remaining in Germany during the Nazi era and continuing to do scientific research.

Planck stayed on and directed the Kaiser Wilhelm Institute. Einstein and others left Germany. Although there was no direct correspondence between Einstein and Planck, consideration of the actions of each provides a case study of the complexity of evaluating the moral responsibility of science to social orders.

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