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AS Physics Unit 1 Question (Help please)

I would appreciate it greatly if someone could explain the answer to this question for me -

A metal is illuminated with monochromatic light. Explain why the kinetic energy of the photoelectrons emitted has a range of values up to a certain maximum. (3 marks) (1 mark may be for written communication)

I have an idea of the answer but i dont understand why they would have an Ek(max) unless the frequency of the light also has a max. (which it may).

Any help would be appreciated - i know its a simple question but Physics is my weakest subject (Predicted C/B) and i struggle with it :frown:

Reply 1

gspot92

would some electrons have less e as they are all not on the surface of the metal and have to use some energy to get to the surface.

Reply 2

easty1050

I think its because when the light hits the surface, photons of energy are given to the electrons, and the energy of the photons vary, so not every electron would get hit by photons of the same energy levels, giving them a different amount of kinetic energy.

I think thats right but could be way off the mark, happy to be corrected! Phyysics is also my weakest subject, so goodluck!!

Reply 3

gspot92

easty1050

I think its because when the light hits the surface, photons of energy are given to the electrons, and the energy of the photons vary, so not every electron would get hit by photons of the same energy levels, giving them a different amount of kinetic energy.

I think thats right but could be way off the mark, happy to be corrected! Phyysics is also my weakest subject, so goodluck!!

i think my teacher said all photons have the same energy. they all have the same frequency, so their energy must be same??? :confused:

Reply 4

easty1050

gspot92

i think my teacher said all photons have the same energy. they all have the same frequency, so their energy must be same??? :confused:

Good point, cos energy is hf-work function, your ideas probably right then, electrons further down need more energy to get to the surface, then escape

Reply 5

AdotK

6

When you illuminate a metal with monochromatic light photons are absorbed by the electrons. The electrons exist in the form of delocalised electrons and this is the case for every metal. There is a certain maximum because if you imagine the electrons at the surface, the least amount of energy is required to release such electrons from the metal surface however electrons deeper within the metal will require more energy in order to escape the metallic structure. So the certain maximum Ek will be of those electrons at the surface and the energy required by the electrons to escape varies across a range.

I know this doesnt answer your question directly but thats what my physics teacher told me when i asked a similar question!

Reply 6

Mr. Tizzy XII

OP

12

Ah, it seems i did misinterpret the question, thank-you all for the help, i think i understand it now :smile:

Reply 7

temt91

The mark scheme for that question is (3 MAX):

incident photon energy is fixed [or photoelectron recieves a fixed amount of energy] (1 mark)
photon loses all its energy in a single interaction (1 mark)
electron can lose various amounts of energy to escape from the metal (1 mark)
electrons have a maximum energy = photon energy - work function (1 mark)

Reply 8

Mr. Tizzy XII

OP

12

Thank you Temt91, that clears up alot.

Reply 9

LivingLegend711

Original post by Mr. Tizzy XII

Thank you Temt91, that clears up alot.

i got the same question too.. thank you every one

Reply 10

emz26ace03

1

the answer is that some may need more energy as they are not on the surface, so they need the energy to get there.
and the maximum value is if there are no opposing particles(if it is travelling in a vacuum), so the kinetic energy varies for each of them and this is due to the presence of obstacles( opposing particles) they encounter when travelling.

(edited 6 years ago)

Reply 11

Tom-valley24

5

First of all it is important to appreciate that in the photoelectric effect one electron absorbs one photon (provided the frequency of the incident radiation is greater than the threshold frequency) and gains energy equal to hf.
An electron can be emitted from the surface of the metal if hf is equal to or greater than the work function of the metal. The work function is the minimum amount of energy needed for an electron to be emitted from the surface of the metal. So hf is equal or be greater than this critical value for the electron to have enough energy to be emitted from the surface of the metal. Intuitively the maximum amount of energy needed for the electron to be emitted from the metal can be substituted into the equation:
Ek max=hf - work function.
Since the incident radiation is monochromatic, so therefore all photons have the same frequency, each electron will gain the same energy hf. And since each electron has to overcome the work function, which is charaacteristic to each metal, the maximum amount of energy to electron can have is shown by the equation above.
It is possible for electrons to have smaller energys than this maximum because electrons deeper within the metal require more energy to be emitted from the surface of the metal, dependent on their relative psoitions in the metal.
Hope this helps.

AS Physics Unit 1 Question (Help please)

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