Threshold frequency(quantum phenomena)

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thefatone
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#1
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https://00c07169b8f01b5b433281e611ee...%20Physics.pdf

question 2c

i have E_{k\ max}= hf- \phi

and that's it i have no idea what to do from there

i also know that  \dfrac{1}{2} \phi=f
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thefatone
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help?
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samb1234
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#3
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(Original post by thefatone)
help?
Start by calculating the work function, and see if you can go from there (or there is a shortcut because of the values they have used)
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thefatone
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(Original post by samb1234)
Start by calculating the work function, and see if you can go from there (or there is a shortcut because of the values they have used)
so i did this

E_{k\ max}= hf- \phi

E_{k\ max}= \left( 6.63 \times 10^{-19} \times 5.6 \times 10^{14}\right) - \left( \dfrac{5.6 \times 10^{14}}{2}\right)
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samb1234
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(Original post by thefatone)
so i did this

E_{k\ max}= hf- \phi

E_{k\ max}= \left( 6.63 \times 10^{-19} \times 5.6 \times 10^{14}\right) - \left( \dfrac{5.6 \times 10^{14}}{2}\right)
Think about how the work function relates to the threshold frequency
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thefatone
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(Original post by samb1234)
Think about how the work function relates to the threshold frequency
work function is half the value of frequency isn't it?
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samb1234
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#7
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(Original post by thefatone)
work function is half the value of frequency isn't it?
no. E=hf
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thefatone
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#8
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(Original post by samb1234)
no. E=hf
but it asks for max kinetic energy....

and E=hf doesn't have  \phi in it :/
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Someboady
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#9
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(Original post by thefatone)
work function is half the value of frequency isn't it?
(Original post by thefatone)
but it asks for max kinetic energy....and E=hf doesn't have  \phi in it :/
Work function relates to the threshold frequency by
E (work function) = h (planck's constant) * f0 (Threshold Frequency)
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samb1234
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#10
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(Original post by thefatone)
but it asks for max kinetic energy....

and E=hf doesn't have  \phi in it :/
Work function is the minimum amount of energy required to liberate a photon, and we know that the threshold frequency is the minimum frequency required to liberate a photon. Therefore thew ork function =h x threshold frequency
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Someboady
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#11
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(Original post by thefatone)
but it asks for max kinetic energy....

and E=hf doesn't have  \phi in it :/
Ekmax = work function - hf (where f is threshold frequency)
thus for this question:
first calculate work function
workfunction = hf (where f is threshold frequency of 5.6 * 10^14)
thus workfunction is something you can calculate (my calculator is under my bed xD)
therefore
Ekmax = work function - (2*5.6 × 10^14) - since frequency is twice the threshold frequency
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The-Spartan
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#12
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(Original post by thefatone)
but it asks for max kinetic energy....

and E=hf doesn't have  \phi in it :/
You're pretty much there
E_k=hf-\phi
You know 2f=f_0 right? so f=2 \times 5.6 \times 10^{14}
E=hf is the best start, calculating the energy of the incoming photons.
E=6.63 \times 10^{-34} \times 2 \times 5.6 \times 10^{14}
\therefore E=7.426 \times 10^{-19} J. This is the total energy of the incoming photons yes?

Now not all this energy goes into the kinetic energy of the electron. Remember, some of it goes into liberating it. The energy needed for this is the work function energy.
E=hf_0 for the energy needed at the work function, we know \phi = hf_0 where f_0 is the threshold frequency.
\therefore \phi = 3.71\times 10^{-19}
I think this is where you are confused. Work function of a metal is the energy required to liberate an electron. Not the threshold frequency, which is just the minimum frequency of an incoming photon to have the energy equal to the work function.

Back to the first equation:
E_k=hf-\phi therefore
E_k=7.426 \times 10^{-19} - 3.71 \times 10^{-19} = 3.72 \times 10^{-19}
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samb1234
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#13
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(Original post by Someboady)
Ekmax = work function - hf (where f is threshold frequency)
thus for this question:
first calculate work function
workfunction = hf (where f is threshold frequency of 5.6 * 10^14)
thus workfunction is something you can calculate (my calculator is under my bed xD)
therefore
Ekmax = work function - (2*5.6 × 10^14) - since frequency is twice the threshold frequency
(Original post by The-Spartan)
You're pretty much there
E_k=hf-\phi
You know 2f=f_0 right? so f=2 \times 5.6 \times 10^{14}
E=hf is the best start, calculating the energy of the incoming photons.
E=6.63 \times 10^{-34} \times 2 \times 5.6 \times 10^{14}
\therefore E=7.426 \times 10^{-19} J. This is the total energy of the incoming photons yes?

Now not all this energy goes into the kinetic energy of the electron. Remember, some of it goes into liberating it. The energy needed for this is the work function energy.
E=hf_0 for the energy needed at the work function, we know \phi = hf_0 where f_0 is the threshold frequency.
\therefore \phi = 3.71\times 10^{-19}
I think this is where you are confused. Work function of a metal is the energy required to liberate an electron. Not the threshold frequency, which is just the minimum frequency of an incoming photon to have the energy equal to the work function.

Back to the first equation:
E_k=hf-\phi therefore
E_k=7.426 \times 10^{-19} - 3.71 \times 10^{-19} =<b> 3.72 \times 10^{-19}


On this particular question op you don't have to actually calculate the work function separately. Either way will work but algebraically if we know that f=2 f0 then we have Ek=2hf0 -hf0 =hf0, which is neater as it avoids any potential errors from rounding earlier on in the question (spartan's answer should really be 3.71, but by using rounded numbers earlier we end up with a slight rounding error, although you would still probably get the mark in the exam)
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The-Spartan
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#14
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#14
(Original post by samb1234)
[/b]

On this particular question op you don't have to actually calculate the work function separately. Either way will work but algebraically if we know that f=2 f0 then we have Ek=2hf0 -hf0 =hf0, which is neater as it avoids any potential errors from rounding earlier on in the question
True, but this only works in specific questions. I find it nicer to run through the set algorithm for kinetic energy shortcuts loose me marks all the time
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samb1234
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(Original post by The-Spartan)
True, but this only works in specific questions. I find it nicer to run through the set algorithm for kinetic energy shortcuts loose me marks all the time
The only reason i mentioned it for this particular question, is that your answer will end up rounding slightly wrong (as yours did) if you compute it and don't keep everything in as exact form as possible
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thefatone
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#16
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(Original post by Someboady)
Work function relates to the threshold frequency by
E (work function) = h (planck's constant) * f0 (Threshold Frequency)
(Original post by samb1234)
Work function is the minimum amount of energy required to liberate a photon, and we know that the threshold frequency is the minimum frequency required to liberate a photon. Therefore thew ork function =h x threshold frequency
i'm pretty sure E=hf
is
where
E= energy of photon in joules <--- how does that become the work function?? i don't understand
any formula working out and substitutions required pls i don't understand :/
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lerjj
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#17
(Original post by thefatone)
i'm pretty sure E=hf
is
where
E= energy of photon in joules <--- how does that become the work function?? i don't understand
any formula working out and substitutions required pls i don't understand :/
1)Energy is prportional to frequency
2)The work function is the energy needed to liberate a photon
3)If you shine a light of double threshold frequency, then half will be used to overcome the work function
So the remaining energy is the kinetic energy, which is just hf/2 of the incident light.
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Someboady
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#18
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#18
(Original post by thefatone)
i'm pretty sure E=hf
is
where
E= energy of photon in joules <--- how does that become the work function?? i don't understand
any formula working out and substitutions required pls i don't understand :/
Okay so basically... What is work function. What is threshold frequency
Threshold frequency is the minimum frequency of light required for electrons to be emitted from the metal.
Work function is the minimum amount of energy required for an electron to be emitted from the surface of a metal.
So if you link the two... The threshold frequency provides the work function right?
Thus the photon that is absorbed must have energy equal to the work function for the electron to be emitted. and we know work function and threshold frequency are linked
So we can say that the energy of the photon must have a minimum energy = h * threshold frequency i.e. E = hf. And we know that the minimum energy is the work function so:
work function = h*threshold frequency. This is a general rule. Learn this formula.
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thefatone
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#19
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#19
(Original post by Someboady)
Okay so basically... What is work function. What is threshold frequency
Threshold frequency is the minimum frequency of light required for electrons to be emitted from the metal.
Work function is the minimum amount of energy required for an electron to be emitted from the surface of a metal.
So if you link the two... The threshold frequency provides the work function right?
Thus the photon that is absorbed must have energy equal to the work function for the electron to be emitted. and we know work function and threshold frequency are linked
So we can say that the energy of the photon must have a minimum energy = h * threshold frequency i.e. E = hf. And we know that the minimum energy is the work function so:
work function = h*threshold frequency. This is a general rule. Learn this formula.
oh i see that's a formula i didn't learn thanks....
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thefatone
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#20
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#20
(Original post by The-Spartan)
You're pretty much there
E_k=hf-\phi
You know 2f=f_0 right? so f=2 \times 5.6 \times 10^{14}
E=hf is the best start, calculating the energy of the incoming photons.
E=6.63 \times 10^{-34} \times 2 \times 5.6 \times 10^{14}
\therefore E=7.426 \times 10^{-19} J. This is the total energy of the incoming photons yes?

Now not all this energy goes into the kinetic energy of the electron. Remember, some of it goes into liberating it. The energy needed for this is the work function energy.
E=hf_0 for the energy needed at the work function, we know \phi = hf_0 where f_0 is the threshold frequency.
\therefore \phi = 3.71\times 10^{-19}
I think this is where you are confused. Work function of a metal is the energy required to liberate an electron. Not the threshold frequency, which is just the minimum frequency of an incoming photon to have the energy equal to the work function.

Back to the first equation:
E_k=hf-\phi therefore
E_k=7.426 \times 10^{-19} - 3.71 \times 10^{-19} = 3.72 \times 10^{-19}
Thank you. i got it now
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