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AS Physics- Photoelectric effect

Hello fellow physicists,
How can I determine the threshold frequency by experimentation?
Thank you
Well, you need to plot a graph of maximum kinetic energy of the electrons against the frequency of light. Plot a graph of f against Max KE.
Extrapolate back, to the highest frequency at which the max KE is 0, and this is your threshold frequency.
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PhRose
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thanks but I read about it somewhere and it said something about stopping potential and potential dividers and something about the electrons moving form negative to positive instead of positive to negative
I really didnt understand it so please explain it
thank so much for the above though
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PhRose
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anyone???
Original post by PhRose
thanks but I read about it somewhere and it said something about stopping potential and potential dividers and something about the electrons moving form negative to positive instead of positive to negative
I really didnt understand it so please explain it
thank so much for the above though


When electrons are moving through a potential, they gain or lose energy. When they go from negative to positive, they are attracted to the positive and repelled by the negative, so they accelerate, gaining kinetic energy. So, logically, when they go the other way (positive to negative) they lose kinetic energy.

So, what you can do is place the metal the electrons are coming off at a positive potential, and put a piece of metal at a negative potential a short distance away, the electrons will lose energy as they travel. This energy can be calculated as Q=V×eQ = V \times e with QQ being the energy lost, VV being the potential difference (voltage), and ee being the charge on an electron.

Now, if the electrons loose all their kinetic energy, they come to a halt, and will be accelerated the other way!

So, what you can do is use a potential divider to change the potential the electrons are moving through, and slowly increase the potential until the current produced is 0A. At the first instance that happens, the maximum kinetic energy can be calculated with V×eV \times e.

photo.GIF
(edited 10 years ago)
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PhRose
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thank you very much for helping but there's till something that I dont understand:
"So, logically, when they go the other way (negative to positive) they lose kinetic energy." I thought conventions current was form negative to positive and if they are moving to positive they should gain KE shouldn't they because they are attracted to it
Original post by PhRose
thank you very much for helping but there's till something that I dont understand:
"So, logically, when they go the other way (negative to positive) they lose kinetic energy." I thought conventions current was form negative to positive and if they are moving to positive they should gain KE shouldn't they because they are attracted to it


When talking about electrons, it's best to forget about conventional current (electron flow is in the opposite direction to conventional current).

Also, as I was typing quickly, I overlooked the fact I put the charges the wrong way round. I'll just change that now.
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PhRose
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Original post by ExcitinglyMundane
When talking about electrons, it's best to forget about conventional current (electron flow is in the opposite direction to conventional current).

Also, as I was typing quickly, I overlooked the fact I put the charges the wrong way round. I'll just change that now.


thank you sooo much :smile: you helped a lot but one last question; does the smae switch need to be made here of the positive and negative?
"So, what you can do is place the metal the electrons are coming off at a negative potential, and put a piece of metal at a positive potential a short distance away, the electrons will lose energy as they travel. This energy can be calculated as with being the energy lost, being the potential difference (voltage), and being the charge on an electron."
thanks again
Original post by PhRose
thank you sooo much :smile: you helped a lot but one last question; does the smae switch need to be made here of the positive and negative?
"So, what you can do is place the metal the electrons are coming off at a negative potential, and put a piece of metal at a positive potential a short distance away, the electrons will lose energy as they travel. This energy can be calculated as with being the energy lost, being the potential difference (voltage), and being the charge on an electron."
thanks again


Yes, it does! Clearly I was thinking of positrons when I typed it up :-P
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PhRose
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thank you so much :smile:
Original post by PhRose
thank you so much :smile:


You're welcome. My apologies for the added confusion.

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