Chapter 13 Flashcards
What is the photoelectric effect
What are the electrons emitted sometimes called
The idea that EM radiation shone on a bit of metal causes electrons to be emitted from them
Photoelectromd
How can you use the gold foil leaf experiment to show the photoelectric effect
1) briefly touch the top metal part with the negative cathode. Excess electrons should fall here as a result. These will then go to the stem and the leaf too, making them negative
2) as the stem and the leaf are negative , they repel each other , causing leaf to stand
3) however if a piece of metal is placed on top and UV light is shone on it - PHOTOELECTRONS WILL BE EMITTED , and a decrease of electrons will cause the negative charge to lower here, and thus the repulsion decreases
Perhaps if you continued to do it electrons more release making positive and repulsion happen again
Three KEY OBSERVATIONS FROM EXPERIMENT LEARN !
1) photoelectrons only emitted if radiation used was above a specific frequency, the threshold frequency. No matter how intense the frequency was before , if it did not hit threshold not even one electron released
2) when it did hit threshold, effect was instantaneous
3) When it hit threshold And intensity was increased , ONLY MORE ELECTRONS WERE EMITTED , not electrons with HIGHER kinetic energy .
The only way to do this is use a higher frequency
What and who lead to the idea that electromagnetic radiation was actually PHOTONS = particulate nature rather than continuous wave
Planck
Discovered the energy of the EM waves could only come in distinct quanta
- as a result he said em waves had particulate nature being TINY PACKETS of energy rather than a continuous stream of waves
- einstein called this photons
What was the relationship between energy of a photon and the frequency of the wave
(Assuming waves are photons for clarity)
What number is does
E = Hf , h is Plancks constant
6.63 x 10-34 Js
How else can we rewrite equation and why is this interestingness
E = h c / lambda
Interesting because it combines particulate nature of wave being energy of photons and the wave like feature by using wavelentgh in the equation
What is a better unit of energy to use when measuring energy of the waves /photons?
Electron volts ,as even the most energetic photon is in the -19
How can we do a simple pag with LEDS to find Plancks constant , but what assumption do we need
What graph?
We don’t need to know how it happens
Assumption is that all the energy in an electron is transferred to all the energy in a photon WHEN AT THE MINIMUM THRESHOLD PD
1) use different known wavelengths in a circuit connected to a power source and voltmeter etc.
2) an LED works by allowing current after a minimum of the threshold
So find the minimum amount of pd needed for light to turn on and use equation Ve = h+/lambda
Plot graph v agains 1/ lambda and multiply gradient etc
Use means
Why do we need to use precisely the threshold pd for this experiment (as you are not using work function equation) and not go higher
Not using function equation here, the assumption clearly states that only at the THRESHOLD PD is the energy of an electron transferred is equal to the energy of the photon emitted, that’s why do at threshold
Here plot a graph for better accuracy.
From the gold foil experiment and observations, what of three findings Shows waves to be particle like and why
Certain frequency needs to be met
- if a highly intense but low frequency wave shone the effect not observed
- yet if a high frequency but low intense wave the effect was observed
== this should not be if waves have wave like properties, as rate of energy transfer (power) is proportional to the intensity, so if more intensity more energy transferred, but clearly this was not the case !!
So what did Einstein do to explain the findings from the experiment sdm what did this model involve
What he suggest about metal electrons and intensity .
Einstein then builded kn Plancks work on suggesting waves have particle like features due to coming in discrete PACKETS of energy as photons ,
This was photon model
- Einstein said that waves were more a stream of photons (packets with energy ) than a CONTINUOUS WAVE
- He suggested that each electron on metal must have a specific energy needed to escape the metal , and that it could gain this energy exactly from the photon in a ONE TO ONE INTERACTION
- and since energy is proportional to frequency in e = hf if there was a low frequency, it wouldn’t be able to transfer enough energy to free the electron, NO MATTER HOW INTENSE
If one photon didn’t have enough energy to free an electron, then a million won’t make a difference either . Yet if they crossed the frequency, then a million would cause a million to escape in one to one interactions
How does new photon metal explain the second observation thst when threshold frequency met effect observed is instant ?
Einstein said that photons can transfer energy in ine to one interactions, so as soon as frequency met, electrons emitted , ELECTRONS COULDNT ABSORB ENERGY FROM MULTIPLE PHOTONS, so as soon as required frequency = which means reauired energy , electrons would just emit
How did the photon model and Einstein also explain the third observation , that passed threshold frequency increasing intensity increased amount of electrons emitted, not max KE that they had
Define the term he used
Einstein’s said different electrons in the position they are on shells in ions will require different amount of energies to escape . HE MADE A CONSTANT = THE WORK FUNCTION , WHICH IS THE MINIMUM AMOUNT IF ENERGY REQUIRED TO FREE ONE ELECTRON.
- for a given frequency higher than work function you have a given energy. From conservation of energy Einstein worked out that any extra kinetic energy the electron had was the result of the work function first , and then what was left over would be converted to KE.
Thus for a given frequency past the work function, any electron would have a MAXIMUM KE IT COULD HAVE
Finally if increase intensity this would just increase the rate of electrons released, not the max KE they had, the only way to do this was to increase the frequency of the initial radiation
Summary of three observations of golf leaf experiment and how new photon model proves
1) a threshold frequency needed to be met for the observed effect to happen
- wave model suggests rate of energy transfer proportional intensity (power= i/a) so inc energy = inc intensity)
- yet low freq high intensity would not work, high freq low intensity would.
- thus wave more a stream of photons , not a continuous wave = where energy transferred is in Knete one interactions
- einstein said electron needed specific energy to be freed, snd as this proportional to frequency, a specific frequency is needed
2) that past threshold frequency, effect is instant
- electrons couldn’t absorb multiple photons and store energy , transfer was in one to one interactions
- thus as soon as frequency was met = energy required met = and effect happened instantly
3) passed threshold frequency , increase intensity lead to more electrons being emitted rather than being emitted with more Ke
- einstein defined the minimum amount of energy needed to free an electron as work function
- transfer was in one to one interactions ,
- for a frequency higher than work function, first energy would be used to free the electron, and by conservation of energy, any left would be used as KE
- thus for a certain frequency snd work function, there is a MAX Ke of electrons
- increasing intensity only increased number, not value of KE, which could only be done by increasing frequency,
Key point mention Esch time about energy Transfer
One to one interactions
