Physics- Unit 1- Quantum Phenomena

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  • Created by: FireDwarf
  • Created on: 02-12-13 17:31
What is the photoelectric effect?
Is emissions when an electron is given off by a metallic surface due to exposure to EM radiation. The electron absorbs a photon, 1 per elecrron, giving it energy required to get over the work function and as a result leave the metal surface.
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What is the order of EM radiations in increasing frequency but decreasing wave lengh?
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What is the threshold frequency?
Min value of frequency to get an photoelectric effect (f0)
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Time of emission?
As soon as exposed
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Effect of radiation intensity?
Greater the intensity, greater the number of electrons emitted. Most be over the threshold however to occur.
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Why do threshold frequencys vary?
Dependant upon material
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What is the formula for energy of a photon?
E = hf
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Therefore when the photon strikes the electron,what occurs?
It recieves the energy from the photon.
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What must the energy of the photon (and as a result the electron) be higher then?
work function
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Therefore, what is the energy of the photonelectron?
Ekmax = hf- work function
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Formula for work function?
hf0 = work function
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Equation for threshold frequency?
Work function/h
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Vaccum photocell?
Application of the photoelectric effect
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What occurs in a vaccum photocell?
Light shown on the photocathode, greater then threshold freq, electronsare emitted and transfer from cathode to anode. Microampere measures photoelectric current.
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What is photoelectric current proportional to?
number of photoelectrons per second =I/e (e = charge of 1 electron) I = current
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What is the number of photoelectrons per second proportional to?
The intensity of the light because each electron absorbs 1 photon.
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How can we construct a graph to show Ekmax = hf - work function?
Freq x-axis, ekmax y axis, gradient is h, y intercept is - work function and x- intercept threshold frequency.
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What is the formula for stoppage voltage?
V = E/Q E= Energy and Q is charge of 1 electron.
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How to get from electron volts to Jf?
x 1.6 x 10 -19
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How do electrons occupy levels in atoms?
They first occupy the closest level to the positive nucleus, grand state. They then begin to fill the states afterwards (excited states)
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How is the energy for each state measured?
By the respect to the ionisation energy needed (eV)
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What are the levels in respect to the ionisati level?
Lower, with the grand state being the lowest.
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What is ionisation?
The energy required to remove one electron from an atom to turn it into an ion.
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How can it be caused?
electric current through s gas, heating, radiation (alpha etc).
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Promotion of an electron to a higher energy level within an atom. Incoming electrons kinetic energy is used to move the electron.
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Excitiation using photons?
Incoming photons may not have enough energy to cause the photoelectric emission but may cause excitation.
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When will this only occur?
Will only occur if the energy of the photon is exaclly equal to the diffrence between the inital and the final energy level.
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Unstable- only last around 10^-16 seconds before going back down to its original energy level. When it goes down a level, it releases a photon with the energy of the diffrence between level and fallen level.
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Example- work out the F of a photon which is emitted from dexciation with a 5.4 ev diffrence.
E = hf (f= E/h). E convert to J and then use forumula.
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What is a flurosecent tube?
Glass tube with a fluorosecent coating on its inner surface.Tube contains mercury vapour at low pressure.
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What occurs when an electric current flows through it?
The mercury atoms collide with the electrons and the mercury atoms. This causes the mercury atoms electrons to become excited and move up to a higher energy level (excited state). They then de-excitate and release a UV photons
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These UV photons are then abosrbed by the flurosecent coating's atoms, causing them to excite, then dexcite, releasing Visible light photons.
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What is the wavelengh for a visible light photon?
400-650 nm
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How can we tell an element from its line spectrum?
Each visible light photon emitted is of a diffrent colour and therefore a diffrent wavelengh. The photons released are due to the specific diffrences between the energy levels. The energy levels of each element are unique and therefore its line spec.
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How did this help discover helium?
The pattern of lines in the line spctrum are unque to the element. They observed the pattern in the line spectrum as unique wavelenghs never seen before. Therefore, new element.
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What is the order of EM radiations in increasing frequency but decreasing wave lengh?



Card 3


What is the threshold frequency?


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Card 4


Time of emission?


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Card 5


Effect of radiation intensity?


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