https://00c07169b8f01b5b433281e611ee...%20Physics.pdf
question 2c
i have
and that's it i have no idea what to do from there
i also know that
Threshold frequency(quantum phenomena)
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 08052016 16:52
Last edited by thefatone; 08052016 at 17:10. 
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 08052016 17:02
help?

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 08052016 17:09
(Original post by thefatone)
help? 
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 08052016 17:12
(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)

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 08052016 17:13

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 08052016 17:15
(Original post by samb1234)
Think about how the work function relates to the threshold frequency 
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 08052016 17:16
(Original post by thefatone)
work function is half the value of frequency isn't it? 
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 08052016 17:18
(Original post by samb1234)
no. E=hf
and E=hf doesn't have in it :/ 
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 08052016 17:19
(Original post by thefatone)
work function is half the value of frequency isn't it?
E (work function) = h (planck's constant) * f0 (Threshold Frequency) 
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 08052016 17:21

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 08052016 17:24
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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 08052016 17:45
You know right? so
is the best start, calculating the energy of the incoming photons.
. 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.
for the energy needed at the work function, we know where is the threshold frequency.
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:
therefore
Last edited by TheSpartan; 08052016 at 17:49. 
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 08052016 18:02
(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 TheSpartan)
You're pretty much there
You know right? so
is the best start, calculating the energy of the incoming photons.
. 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.
for the energy needed at the work function, we know where is the threshold frequency.
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:
therefore
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)Last edited by samb1234; 08052016 at 18:05. 
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 08052016 18:04
(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 
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 08052016 18:06
(Original post by TheSpartan)
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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 08052016 18:32
(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
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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 08052016 18:37
(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 :/
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 of the incident light. 
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 08052016 18:39
(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 :/
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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 08052016 18:53
(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. 
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 20
 08052016 18:55
(Original post by TheSpartan)
You're pretty much there
You know right? so
is the best start, calculating the energy of the incoming photons.
. 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.
for the energy needed at the work function, we know where is the threshold frequency.
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:
therefore
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Updated: May 8, 2016
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