Chapter 13 Flashcards
What is the photoelectric effect
This is when a form of EM radiation is shone on a metal and electrons escape and emitted
These emitted electrons are known as photo electrons
How to use the GOLD LEAF ELECTROSCOPE experiment to show the photoelectric effect in action
Electrons that escaped?
Perhaps I’d kept doing , then maybe more electroms would be emitted and it becomes positively charge and repulsion happens again?
- if you touch the zinc plate at the top with a negative electrode from a high voltage supply, it will negatively charge it
- the excess electrons spread to the stem and the leaf, charging them bith too
- as they are both negative , they repel, and the leaf can be shown to be repelled
- now if we shine UV radiation on the plate , we observe that the repulsion slowly decreases.
== this is because the photoelectric effect has taken place, which made electrons escape, and thus meaning the stem and leaf lost its charge and repulsion was less
2) these are known as photoelectrons
What three observations were found after doing this experiment about the PHOTOELECTRIC EFFECT?
1) that only when a certain threshold frequency was met would photoelectrons be emitted. No matter how intense the radiation was, if it wasn’t above the threshold, no electrons would be observed
2) As soon as yiu crossed the threshold, the effect seen was INSTANTANEOUS
3) Once crossing the threshold frewuency an increase in intensity didn’t mean an increase in maximum kinetic energy of the electrons released, but rather greater numbers (properinsk to intensity increase )
The only way to increase the maximum kinetic energy was to increase the frequency
Who was the person that actually suggested em radiation actually existed as photons?
What did he discover
What did he want to change as a result and what to
Planck
He discovered that the em energy could only exist as discrete quanta of values in packets. Which were called photons
As a result he said the model of em should be as packets rather than a continuous stream of energy that are waves
What is the equation thst links energy of a photon
What is Planck constant
What new equation can you make
R,emend this is sssumign waves exist as photons lmao
Energy = H x f
Energy is = to Plancks constant x frequency of the wave
Essentially energy directly proprtional to frequency of wave
Planck constant = 6.63x10^-34Js
Thus f = c/wl
E = Hc/Wl
Why intereitjg
Equation has properties of both being modelled as photons with E but also as waves with wavelentgh
It shows energy is inversely proprtional to wvakentgh so short wavlentgh will have more energy than radio waves etc, that’s why they are damaging
A better unit if energy fir describing these ones which are so small at lower levels
As so small, can just divide answer by 1,6x10-19 to get into ev, and multiply by that to get to joules
How to do SIMPLEMPAG to determine Plancks constant
But what assumptions do we need
Basically using LEDS, which only turn on after a certain threshold pd is met. They emit VISIBLE PHOTONS
ASSUMING THAT THE WORK DONE BY ELECTRONS IS THE SAME AS EMITTED PHOTON, by finding the minimum voktage needed to turn LED on, and knowing the wavelength, we can pretty much find out Plancks constant
However using just one datA NEVER GOOD, in all prestcisls should aim to use few data points so graoh can be plotted and value found using gradient, as this provides a good average to eliminate more chance of uncertainties
A simple set up can be used , creating a potentiometer so that values for v can be tested from 0 to terminak pd
For that resistor in series, then safety resistor with component which is LED in parallel to that, and voltmeter in parallel to this (safety resistor limits current actions as safety) . Now arrow half way through resistor to show potentiometer. Now yiu ready
All we do is for different wavelengths find the minimum voltage needed for the LED TO LIGHT UP, this can be done using a black sheet to make it more obvious
Then equation is VQ= hf
Ev =hc/WL
Thus V =hc/eWL
And grsdient = hc/e when plotting v against 1/WL
Will be uncertainties here too but grsdient dividier by c/e gives you Planck constant value!
Like all PAGS use the MEANS OF THE GOLTSGES
PAGS
Need to preferably use data to make a graoh
Assumptions stated
Grsdient l
Means each time repeats
Best resolution and over larger distance to reduce uncertainty
Proper assumption for LED experiment again
Only at the THRESHOLD PD is it assumed that the energy of an electron is fully transferred to the Negev of the photon emitted
How to find out how many photojs emitted per second if you know power and energy of one
Power just means this much energy per second
If you have 20J of energy in a photon yiu essentially saying how many photons to get to thst power
So do power / energy and you get photons per second!
Which observation showed waves to act like particles rather than waves
The first, the fact that a specific frequency hsd to be met, even do thst if a lower frequency but higher intensity wave wa sused nothing would happens, goes against the wave description as here the rate of energy transfer is proprtionsl to intensity for cisntsrncross sectional area
As a result , a new model had to be made. Einstein proposed that waves should be modelled as particles
Einstein
Suggested that each electron inthe surface of thr metal must require a specific amount of energy to escape from the metal,
- and that each photon can could transfer its full energy to an electron in one to one interactions
THEREFORE
assuming that waves have particle behaviour, the energy of a photon was proprtional its frewuency
How did new photon modele dplain first observation
The fact that photons energy can be modelled to be proprtionsl to its frewuency, it means the photons will have to be of a specific frequency in order to allow an electron to escape.
Thus no matter how intense the photons were (which is basically just how many there are per second), if they weren’t of a specific frequency, the electrons would never have the required energy to escape, thus wouldn’t be able to escape
And since these can only happen in one to one interactions, the electrons are unable to COLLECT ENERGY EITHER , as only one to one
Second
This explains the second as as soon as the correct frequency wa meet the electrons could escape as they had the right energy to. As these happen in one to one interactions it happens instaneously, it wasn’t a matter of collecting energy for a while and then escaped = no, just get and go
Third
Einstein also could explai;the third. Here the increased intensity past the threshold frewuency just means more ohtoons per second, and if they all have the required frequency, then it doesn’t means emitted electrons have more KE but rather more one to one interactions happens and more electrons are emitted!
However third observations mentions about max KE?
Einstein said depending on eelctrons position I metal, different metals would need different energies to escape.
However he defined a constant for each metal which was the ABSOLUTE MINIMUM AMOUNT OF ENERGY NEEDED TO FREE AN ELECTRON and called it the work function
Thus for a given energy thst was = to the work function, the electrons would just escape with no KE, fair enough
But when the frequency increased so that energy provided was more than the work function, by conservation of energy Einstein worked out freed electrons would have the MAX KE equal to the difference in starting energy and work function . This would lead to a maximum velocity too.
However this is the MAXIMUM, some electrons durther in the atom may have a higher amount of energy needed to escape them, thus the left over energy is less and velocity too. Thus this value is the MAXIMUM KE that any electron in the metal could ever have , snd the electrons that have these are the ones closest to the surface which are the ones that require the min energy to escape = work function
Thus only way ti increases MAX KE OF THE electrons was not by increasing intensity, but rather frewuency past the threshold.
Increasing intensity would increase s number of photons per second, and as we know that energy transfers could only happen in one to one it ers tions , this just means more photoelectrons emitted per second, rather than. Ore with higher KE
Summarise three ofnclusions using photon midel
Einstein said that waves should be modelled as a stream of packers of photons that have discrete quanta of energy Rather than a fk ti our beam of energy that the wave model suggests
- this way first observation explained as Einstein explained each electron has a certain amount of energy needed to escape and can only transfer energy in one to one interactions. Thus only until a certain frequency is met can an electron be escaped, and since they are one to one, no matter how intense a lower frequency photon is, as electrons can’t store energy, they can’t escape
It explains why the effect id instaneotued, as we said they can’t store energy and as soon as the first one to one interaction that meets the threshold frewuency happens one photogeelctron escaped indsynsluslg
And increasing intensity just means increasing amount of photons, which will increase the s,Lunt of electrons released in one to one interactions.
The max KE is not affected by this. Einstein worked out a constant for Esch metal that shows the minimum amount of Ed’s to escape electrons as the work function, as some electrons deeper in may need more. Thus if frewuency they results in higher energy than the work function, by conservation if energy, the leftover is transferred into kinetic energy stored if the electrons . This means each electron has a max Ke it’s not guaranteed they are that because deeper electrons with anhigher thr hold frewuency might mean that they have loser ke but max ke is thst
And wave model is thst pose proprtionsl to intensity but this is incorrect as seen by gold leaf experiment
