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# Photoelectric Effect watch

1. Hi all,

Me and my buddy chum pal friend got a little stuck on this question. We understand that as the frequency of the source has increased, the kinetic energy increases, as the energy supplied by the radiation has increased. We can't, however, figure out why the number of photoelectrons emitted decreases, as the marking scheme says the answer is D.

Heck

2. Hey,
The SI unit of irradiance is Watts per metre squared - meaning that it’s proportional to power. If irradiance is constant, the amount of energy transferred per second (power) to the plate is constant - meaning the net amount of energy that electrons receive should stay the same. As a greater frequency increases the amount of energy individual photoelectrons have, the number of electrons emitted must be smaller for the net energy to remain the same.
Hope this helped. 👍
3. (Original post by DavMax15)
Hey,
The SI unit of irradiance is Watts per metre squared - meaning that it’s proportional to power. If irradiance is constant, the amount of energy transferred per second (power) to the plate is constant - meaning the net amount of energy that electrons receive should stay the same. As a greater frequency increases the amount of energy individual photoelectrons have, the number of electrons emitted must be smaller for the net energy to remain the same.
Hope this helped. 👍
This really helped, thank you so much!
4. (Original post by Heckinwhat)
This really helped, thank you so much!
Sure thing.
5. DavMax15 explains it in terms of total energy incident on the plate and conservation of energy which is perfectly valid.

However, you can also think about it in terms of photons.
If the frequency is increased, each photon has more energy since E = hf.
However since the total energy or irradiance is unchanged, there must therefore be fewer photons. Fewer photons equals fewer electrons beacuse as we know, 1 photon = 1 electron.

It's a very common misunderstanding to assume that increasing the frequency simply increases the ke of the electrons and to forget that the number of electrons will decrease.

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