Chapter 13 Flashcards
What did Planck suggest about wave nature
That it was a series of discrete quanta of energy, and Einstein called these packets Photons
This was going against what young proved again that wages were a continoud form
How to do an experiment to determine Planck constsnt using LED?
- LEDS emit a specific wavelength of light, but only once their THRESHOLD pd is met
- if we assume that at this threshold pd, ALL the energy from an electron is transferred to making the photon of light, then we can say EV = hf
- as e is constsnt , if we find the different Vs needed to first produce the photon , we can find Planck constsnt
1) establish a circuit of the LED, a safety reisdotr (always need one apparnelty) and a variable resistor connected to the LED, / varibale power supply .
2) place a black box around the LED with a window on your side
3) ensure diode is correct way
3) slowly increase the voltage until you FIRST SEE LIGJT . Record the voltsge at which the light is first emitted, this is the thrhold pd
4) repeat this experiment for different frequencies of light. You can determine this by checking the wvslenght on the manufacture label
5) now plot this voltsge against wavelength on a groan and use gradient to work out planks constsnt
Should get straight line through origin to show how v is proptinal to 1/lambda , and use gradient
As
This is not work function s differnt concept , need to assume
Assume that the energy of an electron is equal to energy of photon emitted wt threshold of (dodgy PAG)
Hat is the photoelectric effect
This is when em radiation is shone onto a metal ptoentisl of electrons being emitted
How ti use the gold leaf experiment to prove the photoelectric effect?
1) charge the metal plate with a negative charge by touching it with the negative electrode of a high voltage supply
As the charge spreads to the stem and leaf, due to both being negative, they REPEL EACH OTHER
2) now shine uv light onto the plate. Due to photoelectric effect electrons leave the plate and so it loses negative charge, and the resolution decreases.
3) this continues until no more repelled, perhaps if you leave it it will repel agsin as it becomes positive
( must start with negative charge)
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3 key observations from experiment
1) that electrons would only be emitted if the frequency of incident rwdisjton passes a certain frequency. This is the threshold frequency .
No matter how intense the radiation was, if it didn’t surpass the threshold frequency no electrons were emitted
2) as soon as the incident frewuency met theshold, the electrons emitted were instanoued
3) after the threshold frequency was met, increasing intensity still didn’t produce more energetic electrons , but rather MORE OF THEM
Why couldn’t these observations be explained by the wave nature of wave?
If wave nature was correct, then modelled as continous transfer of energy, no min frequency would have ti be met as eventually enough energy would be transferred for the electron to be emitted. However as this is not the case, the wave nature could not describe this phenomenon
Also, thr energy transfered by a wave continous nature is dependent on itd intensity, however increasing int sky had noneffect
Again how is wave model exo,sin energy transfer and why didn’t this explain photoelectric effect
1) modelled as proprtinsl to intensity, yet increasing intensity was I suffice to to cause phot effect
2: modelled as continous transfer of energy, so at low frequencies, eventually enough energy should’ve been transfered, yet not correvt
How does the photon model explain all the effects
Waves modelled as photons meant that energy is transferred in discrete quanta.
Einstein said that each electron in metal required a certain amount of energy needed to escape, and if it wasn’t met then no matter how many photons it wouldn’t escape
This is BEVAUSE photons are modelled as only being able to transfer energy in ONE TO ONE INTERACTIONS. energy can’t accumulate, thus the electron must have the exact energy needed, and thus a minimum frequency ,just be met or it will not happen at all
Also increasing the intensity now increased the. Number of photons. Thus as they are one. To one intersections, if the min frequency is met, this will increase the NUMBER KF ELECTRINS emitted not the nearby they had
It also explains why it’s instaneously emitted, because as soon as the correvt energy is met, then electrons will emit, due to them being ONE TO ONE INTERVATIOSN, and not rather the ability to collect energy
What is the work function of a metal
This is the MINIMUM AMOUNT OF ENERGY needed to eject an electron , and this is on the surface
Some photons could go and eject electrons depper in the atom if it had enough energy to. They need more energy to escape, but the MIN energy for an electron to escape in the metal is the work function
Any energy elft over will be transfered to KE of the electron
Therefore he = work function+ ke max
What about electrons moving up down energy levels?
They can’t unless the photom has EXACT ENERG
There can only be leftover enerby if the something after is moving. Such as electron emitted, or if electron bumps and moved after
Else energy must be exact as it can’t dissaper
Summary
Min energy needed to escape, thud electrons must be at a min frequency to cause the effect
As modelled a sphotjnnature, the interactions can only happen one to one. Therefore only when frewuency is met wil electrons emit and also emit dtairshnt away, electrons can’t store energy like continous model suggests
Increasing intensity increases one to one inter Sri one and thus emission rates
Increasing fewauemcy only way to increase max ke
Why is Einstein equation ke MAX
Because we said some electrons could be emitted deeper into the atom require more energy, but that means have less leftover energy
So the max is referring to electrons emitted on the SURFSCE OF THE ATOM , THOSE ELECTRONS WILL MOVE WITH KE MAX
Graph of ke mad vs frequency
Kemal = hf - work function
When frewuency is f., then no photons are emitted with Andy ke, thus x intercept (when no photons are emitted with ke) is when f= f0
The work function is on y intercept
So any other metal will have shape just translated across as h is the same
It clearly shows no electrons emitting ABEFORE THE WORK FUNCTION
How do electrons act as particles?
They have a mass and a charge, and can be deflected in electric and mand magnetic fields
