Photoelectric Effect? (1 Viewer)

[viviansidiss]

Firstly (this may be a stupid question) but is there a difference with intensity and frequency of light (say UV)? Also what is the relationship, if any between intensity, frequency, speed of electrons emitted and number of electrons emitted in the photoelectric effect?

 

[Pwnage101]

intensity and frequency are totally different properties.

The frequency is the number of waves per second, and that is how we classify the EM Spectrum - high frequency is Gamma ---> Radio waves have a low frequency.

Intensity refers to the amount of photons something emits.

Intensity and frequency have no relation - u can have high frequency, low intensity light or low frequency low intensity light, etc...

Now intensity efers to the number of photons of light there are.

Basically what u need to know is not matter how much frequency there is, if the frequency is below the threshold frequency no eleectrons will be emitted. However the more electrons with teh right frequency, the more electrons get emitted...

speed is related to frequency - the higehr the frequency the elctrons, the more energy it has - any excess energy that is not used up in removing the elctron will be transfered into Kinetic energy of the electron

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Lol its been 8 months since i have even thougth about this stuff, sorry if its not coherent

 

[viviansidiss]

Thanks for that very good reply =)

 

[k02033]

intensity and frequency are totally different properties.

The frequency is the number of waves per second, and that is how we classify the EM Spectrum - high frequency is Gamma ---> Radio waves have a low frequency.

Intensity refers to the amount of photons something emits.

Intensity and frequency have no relation - u can have high frequency, low intensity light or low frequency low intensity light, etc...

i am sry but that whole thing is wrong...

thats not true in a general, intensity is a measurment of how much energy the radiation delivers to the cathode per unit of time per unit of area of the cathode. as an equation you can think of intensity as I=nhf, where n is how many photons lands on the cathode per unit of time per unit of area. So there are 2 ways of increasing the radiation intensity, one is to increases its frequency or increase how many photons are emitted, or a combination of both.

but we all know that the kinetic energy of photoelectrons are only dependent on hf, ie the energy of each individual photon.

If i increase teh frequency of the radiation keeping n constant i can indeed increase intensity I=nhf

but this way of increasing intensity can actually increase photoelectron energy, since i am varying f.

so its only safe to say that changing the intensity of EMR while not varying its frequency will not change the photoelectron energy, but increase current

all these assumes f>fo obviously.

 

[viviansidiss]

So does increasing frequency make electrons emit faster? or more electrons are emitted at the same velocity?

 

[Tully B.]

Higher intensity light = larger number of photons = more electrons emitted when light strikes metal

Higher frequency light = higher energy photons = higher energy electrons emitted when light strikes metal (and yes, this higher energy comes in the form of kinetic energy, which means the electrons are faster.

Intensity and frequency are independent of each other.

 

[k02033]

from this, the emitted velocity of the electron is only dependent on the energy of each photon, E=hf. (so the key idea is that the ejection and initial kinetic energy of the photon is ONLY dependent on the energy of each individual photon that makes up the wave, and not dependent on entire wave as a whole)

so as long as you increase f (for f>fo) u will increase the initial ejection velocity of electrons.

now the trouble with intensity is that you can vary it in 2 ways. I=nhf. vary n or vary f

now if i increase intensity by increasing f keeping n constant, i will get the same amount of electrons flying off, but each now with more velocity,

now if i increase intensity by increasing n keeping f constant, i will get electrons flying off with same velocity, but more of them

 

[k02033]

Intensity and frequency are independent of each other.

why is everyone saying this , its not true

 

[Tully B.]

The frequency of light is independant of the intensity of light.
Maybe you thought we were talking about the intensity, or energy, of photons, which is reliant on the frequency of the light.

 

[k02033]

Intensity and frequency are independent of each other.

frequency of say red light is obviously independent of intensity that you measure of it.

but the other half of that statement is still wrong, intensity is a function of frequency

 

[Tully B.]

Could you show me this function? I wouldn't be surprised if they were teaching us something completely wrong; the HSC course simplifies the maths of physics to the point that they're almost lying to us.

 

[k02033]

read my 1st post in this thread

 

[Tully B.]

Alright, I read it. Makes sense. My bad.

 

[k02033]

also I=nhf, is only one function for intensity, it could be heaps depending how you as an experimentlist chose to meausure your n, i have choosen n to be the amount of photon arriving on my metal per unit area, per unit time.

but in the end, we have set intensity to hav units of energy per unit time per unit area

ie

 

[Tully B.]

I should've know you'd win; your custom title says "PHYSICS!". Mine just says Mr Pink.

 

[moo92]

We were just taught today that: if you have red and blue light and want to have the same energy per unit area you will need different numbers of photons since E(red) < E(blue).
So then to have the same intensity, you need more red photons then blue photons...if intensity is equal to W/m^2 (energy per unit area)?? but does that not contradict the definition of intensity being the number of photons emitted per unit area??

can somebody please help me...I am rather confused after listening to my teacher and now reading all these posts....

To have the same intensity of red light, do you need more photons emitted so it will have the same energy as blue light??
and how much of this crap is actually relevant to any questions they'd actually ask us in the HSC?
is it enough to say:
-For the same photocurrent to be produced you need the same number of photons as red and blue BUT the max KE of the photoelectrons emitted due to blue light will be higher as E = hf

 

[hamzie]

Just remember that intensity is related to the number of photons
and energy is related to frequency

So if you have 10 photons with a very low frequency.. this 1 photon with a much much higher frequency can still have more energy than it

the 10 photons however will have more intensity .. will last longer