Photoelectric Intensity Conundrum (1 Viewer)

[studybuddy09]

My teacher has drilled it into us that increase in intensity is an increase in number of photo electrons and a decrease in intensity is a decrease in number of photo electrons.

However I was going over some Success One and it says that if intensity remains the same and frequency increases then number of photo electrons decreases because there is the same energy per area but more energy per photon, so there are less photons.

I need help: can somebody explain this in Lehman's terms? I think i get it i just want to get it clear

and in an exam if i say that if intensity remains the same then number of photo electrons remains the same will i be penalized?

studybuddy09

 

[kaz1]

I learnt that increasing the intensity increases the current in Einstein's photoelectric effect experiment.

 

[Dumbledore]

My teacher has drilled it into us that increase in intensity is an increase in number of photo electrons and a decrease in intensity is a decrease in number of photo electrons.

However I was going over some Success One and it says that if intensity remains the same and frequency increases then number of photo electrons decreases because there is the same energy per area but more energy per photon, so there are less photons.

I need help: can somebody explain this in Lehman's terms? I think i get it i just want to get it clear

and in an exam if i say that if intensity remains the same then number of photo electrons remains the same will i be penalized?

studybuddy09

1.i think those parts are wrong, the quantity of photons is purely determined by its intensity as ur first statement said, if we assume this is true and by E=hf, the higher frequency will result in higher energy per photon(as said there 2), then the energy per unit area should also increase

3, i don't think there is anything wrong with that statement

 

[k02033]

hope this helps

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.

 

[k02033]

1.i think those parts are wrong, the quantity of photons is purely determined by its intensity as ur first statement said, if we assume this is true and by E=hf, the higher frequency will result in higher energy per photon(as said there 2), then the energy per unit area should also increase

3, i don't think there is anything wrong with that statement

those parts are right, intensity is dependent on 2 things
(1) number of photons arriving per unit time per unit area
(2) energy of each photon

and the 1st part of the statement requires the intensity to be constant. so concluding "then the energy per unit area should also increase" would be a contradiction.

 

[annabackwards]

The number of photos is determined by the intensity of light. That is, altering the intesity of light being emiited DOES NOT change the energy of each photon being emitted, merely the amount amount. The energy of photons only change when the frequency (and so wavelength) is altered.

For example, you have 2 lamps emitting light of different frequencies.
Since the number of photons being emmitted at any time is equal then the lights would have equal intensity. However, if one light was UV the other was Infrared then the photons being emitted would possess different amounts of energy - the UV light would have more energy than the Infrared because it has a higher frequence (E = hf).

So you can see, you can have different frequencies of light being emitted with EQUAL intensity, but the energy per phonon would be different.

 

[k02033]

For example, you have 2 lampes emitting light of different frequencies.
Since the number of photons being emmitted at any time is equal then the lights would have equal intensity.

not true, wow there were 2 previous threads on this topic.. and every student keeps saying this..
hsc murdered the definition of intensity

 

[k02033]

here is one of those threads..
http://community.boredofstudies.org/265/ideas-implementation/213303/photoelectric-effect.html

 

[annabackwards]

not true, wow there were 2 previous threads on this topic.. and every student keeps saying this..
hsc murdered the definition of intensity

Oh i see, it makes sense.

I guess every student keeps on thinking it because that's what BoS wants our teachers to teach us XD

 

[Dumbledore]

i hate it when they teach us the wrong things...

 

[youngminii]

Look, if you want the marks, you should just say what annabackwards said. Talking about all that extra shit that k02033 ALWAYS puts in won't score you magical bonus marks. It'll just waste time as it's beyond the scope of the syllabus, and the HSC markers probably won't even know it themselves. So imo listen to annabackwards unless you like being confused.

 

[annabackwards]

^
Damn BoS!

They're teaching us the wrong things about the photoelectric effect and the BCS theory... but meh let's just play their game for this year and pretend what they told us is true XD

 

[darkchild69]

Dont worry guys, even through first year uni physics you will still be mislead!

What you need to know is that 'intensity' of a light incident on a metal is referring to the number of photons which are passing a point each second.
The higher the intensity of a light source, the more photons of that frequency which are passing a point each second. So assuming that the frequency is greater than the threshold frequency, then due to more photons colliding with the metal surface each second, more photoelectrons will be emitted each second, which will result in an increase in current.


That's what BoS want you to know, lol...

 

[k02033]

Look, if you want the marks, you should just say what annabackwards said. Talking about all that extra shit that k02033 ALWAYS puts in won't score you magical bonus marks. It'll just waste time as it's beyond the scope of the syllabus, and the HSC markers probably won't even know it themselves. So imo listen to annabackwards unless you like being confused.

thats a really narrow minded view. you are in school to learn, not simply get some mark next to your name. And i dont think fully understanding photoelectric effect is out of the syllabus. And i am sure there are excellent students out there who, unlike you, actually want to learn real physics and understand it. And plus you are doing 1st year physics, and i wish you luck when you are struggling with wave mechanics. If you never plan to be confused by something, i dont know how you are going to survive uni, or in fact learn anything

notice that the previous 2 students were brave enough to take on the new
view on intensity and challenged themselves to a new theory, this is called learning. whereas you were the first to just sit back and complained

 

[Tully B.]

I got confused by this, especially since Success 1 contradicted what I thought we were meant to say, however my physics teacher said that one must take a pretty ignorant view of Physics to understand the HSC Physics course...

This would be the right answer:

Dont worry guys, even through first year uni physics you will still be mislead!

What you need to know is that 'intensity' of a light incident on a metal is referring to the number of photons which are passing a point each second.
The higher the intensity of a light source, the more photons of that frequency which are passing a point each second. So assuming that the frequency is greater than the threshold frequency, then due to more photons colliding with the metal surface each second, more photoelectrons will be emitted each second, which will result in an increase in current.


That's what BoS want you to know, lol...

thats a really narrow minded view. you are in school to learn, not simply get some mark next to your name. And i dont think fully understanding photoelectric effect is out of the syllabus. And i am sure there are excellent students out there who, unlike you, actually want to learn real physics and understand it. And plus you are doing 1st year physics, and i wish you luck when you are struggling with wave mechanics. If you never plan to be confused by something, i dont know how you are going to survive uni, or in fact learn anything

notice that the previous 2 students were brave enough to take on the new
view on intensity and challenged themselves to a new theory, this is called learning. whereas you were the first to just sit back and complained

HSC Physics is pretty shit. So, for now, most of us just want to get the marks, even if the consequence of this is ignorance to real physics. The maths of the course has deteriorated to the most basic shit imaginable, and without going quite intricately into proper Physics-based maths, it would be hard to understand the proper theory.

I've heard of people actually losing marks for not answering the question in line with the syllabus, even though they the write the "correct answer". It happened to my friend with BCS theory. He looked it up on wikipedia, then in the test he went on about phonons, when you don't really need to know what a phonon is in order to explain "HCS BCS".

 

[darkchild69]

HSC Physics is pretty shit. So, for now, most of us just want to get the marks, even if the consequence of this is ignorance to real physics. The maths of the course has deteriorated to the most basic shit imaginable, and without going quite intricately into proper Physics-based maths, it would be hard to understand the proper theory.

I've heard of people actually losing marks for not answering the question in line with the syllabus, even though they the write the "correct answer". It happened to my friend with BCS theory. He looked it up on wikipedia, then in the test he went on about phonons, when you don't really need to know what a phonon is in order to explain "HCS BCS".

True, HSC physics is extremely watered down and in some parts, just plain wrong. I tend to go a little beyond the syllabus, especially in Q2Q. But, i ensure that i tell my students that they wont be asked anything about it!

Delving a little deeper into the maths and using the maths to actually explain things (such as the black body curve) i think actually helps the understanding, as long as the students know what they need to know to get good marks and not go beyond it.

It really is a shame when students ask valid questions in class, but i have to say things like "Ok, this is the REAL answer to your question <blah, blah, blah> But, if you want to get full marks in the HSC, this SHOULD be the answer to your questions <bleh, bleh, bleh>

Sad.. but, life is full of misconceptions. BoS think they are making it better for understanding, but imo we are raising students who are mathematically illiterate and think Physics is all about impacts on society and if they choose to do a course in Physics and are confronted with Fourier Analysis, BVP and Matrices in the first semester, they will kinda be like wtf?

 

[k02033]

True, HSC physics is extremely watered down and in some parts, just plain wrong. I tend to go a little beyond the syllabus, especially in Q2Q. But, i ensure that i tell my students that they wont be asked anything about it!

Delving a little deeper into the maths and using the maths to actually explain things (such as the black body curve) i think actually helps the understanding, as long as the students know what they need to know to get good marks and not go beyond it.

It really is a shame when students ask valid questions in class, but i have to say things like "Ok, this is the REAL answer to your question <blah, blah, blah> But, if you want to get full marks in the HSC, this SHOULD be the answer to your questions <bleh, bleh, bleh>

Sad.. but, life is full of misconceptions. BoS think they are making it better for understanding, but imo we are raising students who are mathematically illiterate and think Physics is all about impacts on society and if they choose to do a course in Physics and are confronted with Fourier Analysis, BVP and Matrices in the first semester, they will kinda be like wtf?

you are actually in a position to fix this? right? do you know what happened to the old physics syllabus, and what does it take to bring it back?

 

[youngminii]

There's nothing wrong with knowing more than you need to. It's just that you need to know which part is an HSC answer and which part isn't. If you don't, you'll just regurgitate everything into an exam and waste time and risk the possibility of the marker only knowing HSC physics.

 

[darkchild69]

you are actually in a position to fix this? right? do you know what happened to the old physics syllabus, and what does it take to bring it back?

I wish there was something which could be done about it. The old physics syllabus whilst much more mathematically intense, concentrated much more on things which are now covered with engineering science, ie., rotational motion and the like.

I think that this syllabus is far more interesting, especially I2I and Q2Q, just the way in which some dot points are covered, it is just far too elementary. If they kept the syllabus the same, but just expanded on some of the points, it would be far better and would probably help some of the misconceptions and misunderstandings.

But, then again.. All of high school science essentially is a simplification. Even some university science is. I suppose it's just up to the individual if they want to extend themselves beyond "what is required." I encourage students to think about concepts and not just accept them because the syllabus says it is so, but i try not to focus too much on it in class, and when i do, i let them know that they don't need to know it.

I.e., I introduced the second focus section of Q2Q using Young's double slit experiment, which is nowhere in the syllabus, but has some interesting implications to the wave-particle duality of electrons

 

[youngminii]

I.e., I introduced the second focus section of Q2Q using Young's double slit experiment, which is nowhere in the syllabus, but has some interesting implications to the wave-particle duality of electrons

That's fine, my tutoring college always includes extra info and background information. Even Physics textbooks like Jacaranda give more info than required.
But what k02033 expects of us is to understand all the university style information on the dot point including all the mathematics and what not, and to use that information in a test. That's just ridiculous and time wasting.