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AS Physics - Photoelectric Effect Help Please Watch

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1. 1) A modern 200 Watt sodium street lamp emits yellow light of wavelength 0.6 micrometers. Assuming it is 25 per cent efficient in converting electrical energy to light, calculate the number of photons of yellow light per second.

2) A certain metal plate shows photoelectric emission of electrons for radiation up to a wavelength of 600 nm. If light of wavelength 500 nm is used, what will the maximum velocity with which electrons are emitted?

These are the formula's I was given for question 2:
c = 3 X 10 8 m/s h = 6.6 X 10 -34 Js mass of electron = 9.1 X 10 -31 kg

Please can someone show me in steps how to do this question as I don't have a clue. Thanks.
2. 1st part: if you know the efficiency, you can calculate the output power. Then use the relation between energy and frequency (E = hf) to find the energy of one photon, and the definition of power (energy per unit time) to find the number of photons per unit time.

2nd part: if you now the wavelength at which photoemission stops, you can calculate the work function.
3. 1. Find total energy and divide by energy of one particle.
Use E = P x t and E = hf remembering lamp is 25% efficient
2. Use
4. (Original post by soup)
1. Find total energy and divide by energy of one particle.
Use E = P x t and E = hf remembering lamp is 25% efficient
2. Use
could you show me how to do question 1 please
5. (Original post by Luis456)
could you show me how to do question 1 please
Well 200W means 200J/s we have one second so 200J
But lamp is 25% efficient so it is 25% of 200 = 50J

Energy of one photon = hf = h x 0.6 mircometers.

No. of photons = total energy / energy of one
6. (Original post by soup)
Well 200W means 200J/s we have one second so 200J
But lamp is 25% efficient so it is 25% of 200 = 50J

Energy of one photon = hf = h x 0.6 mircometers.

No. of photons = total energy / energy of one
actually it's the wavelength that is 0.6 micrometres, not the frequency

you need c = f*lambda
7. (Original post by soup)
Well 200W means 200J/s we have one second so 200J
But lamp is 25% efficient so it is 25% of 200 = 50J

Energy of one photon = hf = h x 0.6 mircometers.

No. of photons = total energy / energy of one
Thanks
8. (Original post by laeti)
actually it's the wavelength that is 0.6 micrometres, not the frequency

you need c = f*lambda
ah good point - totally misread
9. (Original post by soup)
ah good point - totally misread
er so is the working out u did wrong then?
10. (Original post by Luis456)
er so is the working out u did wrong then?
Only for the energy of a photon which would now be = (hc)/0.6 micrometers

And you have to change final division aswell
11. (Original post by soup)
Only for the energy of a photon which would now be = (hc)/0.6 micrometers

And you have to change final division aswell
orite and could you show me in steps how to tackle the second question
12. (Original post by Luis456)
orite and could you show me in steps how to tackle the second question
We arnt going to do your homework for you
13. (Original post by hazbaz)
We arnt going to do your homework for you
Thats rude-We used to do the same thing:
14. What is the numerical value of Planck's Constant ?
15. (Original post by bijesh12)
What is the numerical value of Planck's Constant ?
If you look at the original post it is there

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