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Module 4 - Chapter 13 - Quantum physics I - Coggle Diagram
Module 4 - Chapter 13 - Quantum physics I
Photons
EM radiaiton is tiny packets of energy rather than a continuous wave
Photon model is used to explain how EM radiaiton interacts with matter
Wave model explains EM radiation's propogation through space
Photon energy
Energy of a photon is directly proportional to its frequency
E = energy of the photon
f = frequency of EM radiaiton
h = Planck constant
Combine the equation with
Short wavelength X-rays have much more energy than long-wavelength radio waves
Questions of scale
Electronvolt is used to measure energies at quantum scale
1 ev is the energy transferred to or from an electron when it move through a pd of 1V
1ev =
LEDs and the Planck constant
LEDs convert electrical energy into light energy
Visible photons are emitted when the pd across them is above a critical value (threshold value)
When pd reached the threshold pd, LED start to emit photons of a specific wavelength
W = QV, energy is aout the same as the energy of the emitted photon
Use a voltmeter to measure the minimum pd required to turn on the LED
If we know the wavelength of the photons emitted by the LED, we can determine the planck constant
At threshold pd, energy transferred by an electron in the LED is equal to the energy of the single photon it emits
Graphs
Gather data using different wavelengths
Plot a graph of V against 1/wavelength
Planck cosntant = hc/e
Improve accuracy
Small black tube placed around the LED to make it more obvious when it lights up
Use a variety of different wavelengths
Photoelectric effect
Gold lead electroscope
Briefly touch the top plate with the negativ electrode from the high voltage power supply, this charged the electroscope
Excess electrons are deposited onto the plate and stem of the electroscope
Any charge developed on the plate at the top of the electroscope spreads to the stem and the gold leaf
Stem and leaf have the same charge and repel eachother
If a clean piece of zinc is placed on a negatively charged electroscope and UV radiation shines onto the zinc surface, gold leaf falls towards the stem
Electroscope has gradually lost it snegative charge as the incident radiaiton causes the free electrons to be emitted from zinc
Key observations
Photoelectrons were only emitted if the incident radiaiton as above the threshold frequency for each metal
If photons don't have enough energy, no electrons will be able to leave no matter how many photons are incident on the metal
If incident radition was above the threshold frequency, increasing the intensity of the radiaiton didn't increase the max kinetic energy of the photoelectrons, more electrons were emitted
The only way to increase max kinetic energy was to increase the frequency of the incident radiaiton
Threshold frequency - Minimum frequency of incident radiation required to release electron from the surface of a metal in the photoelectric effect
One to one interaction where an electron can absorb EM radiaiton (photon) to gain energy sufficient energy to leave a metals surface
Explanation
EM radiation was a stream of photons rather than continuous waves
Each electron in the surface of the metal must required a certain amount of energy to escape from the metal
If frequency is too low, intenstiy of the light (number of photons per second) doesn't matter as electrons cannot accumulate energy from multiple photons
Rate of emissions of photoelectrons is directly proportional to the intensity of the incident radiaiton
Photon model - Model of EM radiation that treats light as being made up of photons, to explain the photoelectric effect which the wave model can't
