Hey there! Sign in to join this conversationNew here? Join for free

AS Physics - Photoelectric Effect Help Please Watch

Announcements
    • Thread Starter
    Offline

    0
    ReputationRep:
    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.
    Offline

    0
    ReputationRep:
    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.
    Offline

    2
    ReputationRep:
    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  E_k = hf - hf_0
    • Thread Starter
    Offline

    0
    ReputationRep:
    (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  E_k = hf - hf_0
    could you show me how to do question 1 please
    Offline

    2
    ReputationRep:
    (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
    Offline

    0
    ReputationRep:
    (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
    • Thread Starter
    Offline

    0
    ReputationRep:
    (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
    Offline

    2
    ReputationRep:
    (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
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by soup)
    ah good point - totally misread
    er so is the working out u did wrong then?
    Offline

    2
    ReputationRep:
    (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
    • Thread Starter
    Offline

    0
    ReputationRep:
    (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
    Offline

    0
    ReputationRep:
    (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
    Offline

    1
    ReputationRep:
    (Original post by hazbaz)
    We arnt going to do your homework for you
    Thats rude-We used to do the same thing:
    Offline

    12
    ReputationRep:
    What is the numerical value of Planck's Constant ?
    Offline

    2
    ReputationRep:
    (Original post by bijesh12)
    What is the numerical value of Planck's Constant ?
    If you look at the original post it is there
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    Will you be richer or poorer than your parents?
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

    Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

    Quick reply
    Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.