Planck's Constant E-practical
A realistic 3D virtual physics experiment
Determine Planck's constant by choosing from a selection of colour LEDs, setting the potentiometer, and making your own measurements of voltage, and current — just as you would in the laboratory.
Realistic experimental technique
E-practicals are 3d, highly realistic emulations based on a blend of theory and the actual practicals, giving an authentic experience of doing the actual practical in a laboratory. Students operate the apparatus, make measurements and obtain their own results. It is designed to reproduce the experience of performing the practical rather than simply demonstrating the underlying physics. Like a real experiment, it can be done badly. Careless setup or measurements can produce poor results, so students have to apply experimental technique and take care with their measurements.
Useful when laboratory access is limited
Ideal for preparation before a practical, revision afterwards, missed practicals, home study, or demonstrating an experiment when setting up the real apparatus isn't practical.
Works directly in a browser
No installation is required. Windows, Mac and suitable mobile devices can run the WebGL e-practical.
Watch it in action
This shows how the e-practical can be used. In particular it shows how the student or teacher interacts with the apparatus and how the interaction can be controlled using the mouse or the alternative on-screen controls.
What you get
The application is supplied as a PowerPoint which provides background, a synopsis of what the student will learn, a description of the Physics involved, a description of the apparatus, the independent and dependent variables, the method, a link to the interactive online e-practical, analysis of the results and further discussion.
School/Department licence £9.99
Valid for 1 year
Introduction
Planck’s constant is a universal constant that has its roots in Planck realising that energy levels could not be continuous but had to be discrete in units of this constant. He had been trying to find a radiation law for black body radiation. Lord Raleigh and James Jeans had proposed a scheme that had worked with the kinetic theory of gases but when waves were used instead of gas particles it was found that it predicted an infinite amount of energy at high frequencies. This was named the ultraviolet catastrophe, and it is exactly what Planck was trying to eliminate by proposing a new radiation law. The old Raleigh and Jeans way of solving it was to assume the equipartition (equal sharing) of energy between all possible frequencies. But with waves you can just keep on fitting more and more into the black body at higher and higher frequencies.
Planck had a great deal of confidence in the experimental results concerning black body radiation, so he became determined to find a law that agreed with the available observations. He eventually found one but had little idea of why it worked. In his quest to justify it he reworked some of Boltzmann’s ideas on entropy from his kinetic theory of gases. It was Boltzmann’s idea of dividing the energy into small but finite levels that was key. Planck called the amounts of energy that could be radiated ‘quanta’ and these were given by:
E = hf
Where E is the energy quantum and h is Planck’s constant and f is the frequency of the radiation. This equation is one of the cornerstones of what was (very rapidly) to become the field of quantum mechanics.
It’s possible to measure Planck’s constant using some relatively simple equipment. An LED will start to emit light of a particular wavelength at a precise voltage, called the threshold voltage, which can easily be measured. The voltage allows us to calculate the energy going into the LED and the wavelength of the light allows us to calculate the light’s energy. Given that the energy of light is Planck’s constant times the frequency (see above equation) we can calculate the value for Planck’s constant. There’s just one slight complication: energy from heat is also going into the LED. We can eliminate this effect by taking the threshold voltage for different coloured LEDs, and assuming that the thermal energy supplied to each LED is the same.
What students do
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The voltage is already set to 6V with a maximum current set to 1A.
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Switch on the power to the circuit by clicking on the power button on the power supply.
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The multimeter measuring the voltage is on a setting for measuring up to 10V, the second multimeter is on a setting to measure milliamps.
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To see the LED light up as soon as possible, turn the room lights down by using the scroll wheel whilst over the light control. You can also lower the blinds by clicking on the blind switch next to the light dimmer switch.
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Determine the point at which the LED starts to emit photons by monitoring the current as it is not possible to see the light from this immediately that the LED starts emitting, the number of photons is just too small. What we observe is that the current being measuring by our ammeter (the multimeter on the right) will go up sharply at the point that the LED starts to emit photons.
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Turn the potentiometer until reading on the right hand multimeter goes up sharply. Move the potentiometer forwards and backwards a little to identify the point at which the current starts as accurately as possible. If you increase the voltage a little further you should see the light from the LED. Note the voltage indicated on the left hand multimeter. This is the threshold voltage.
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Repeat the experiment for both the green and blue LEDs noting the threshold voltages and the wavelengths.
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Create a table in your laboratory book or in Excel with a column for the colour of the LED, a column for the wavelength and a column for the threshold voltage.
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Plot a graph of V against the inverse of the wavelength and measure the gradient. Equate this with hc/e and hence calculate a value for h.
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Create a table in your laboratory book or in Excel with a column for the colour of the LED, a column for the wavelength and a column for the threshold voltage.
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Plot a graph of V against the inverse of the wavelength and measure the gradient. Equate this with hc/e and hence calculate a value for h.
Want to experience an e-practical yourself?
Try our free Magnetic Field for a Coil e-practical.
Other Physics E-Practicals
Explore the other e-practicals available at GCSE, A'level and Undergraduate
This section is adapted from material developed by Dr Robert Lucas and is related to the book High School and Undergraduate Physics Practicals, published by CRC Press.