Physics: Post your doubts here!

[nehaoscar]

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Hey,
Could anyone please provide me with the reasoning for the following questions of the photoelectric effect:
a) In graph 2 why does increase in frequency cause current to increase at a decreasing rate?
b)In graph 3 and 4 , what does negative stopping potential imply?Like what exactly does it mean?

Please help.

a - photoelectric current is the no. of photons hitting the metal causing emission - so it's dependent on intensity and not frequency (E=hf) so as f becomes greater than threshold f, current increases until some time after which it'll become constant as intensity will be constant
b - check here http://www.s-cool.co.uk/a-level/phy.../photoelectric-current-and-stopping-potential

 

[nehaoscar]

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Please help!

http://physics-ref.blogspot.in/2014/11/physics-9702-doubts-help-page-20.html
See 106

 

[Catherine_1]

http://physics-ref.blogspot.in/2014/11/physics-9702-doubts-help-page-20.html
See 106

Thank youuu!

 

[Catherine_1]

a - photoelectric current is the no. of photons hitting the metal causing emission - so it's dependent on intensity and not frequency (E=hf) so as f becomes greater than threshold f, current increases until some time after which it'll become constant as intensity will be constant
b - check here http://www.s-cool.co.uk/a-level/phy.../photoelectric-current-and-stopping-potential

Thank youuu once again! But I have a slight query if it depends on intensity then why does increase in frequency trigger initially an increase in current? Like, The way I am seeing: when we increase the frequency the electrons gain more ke so the flow faster and since current is the rate of flow of charged particles it increases.However,eventually what happens is that intensity affects the number of electrons emitted per unit time(I am right?:/ ) so since intensity is constant the current turns out to be constant as nos of electrons are same. Is my analysis correct? I am confused slightly?:/

Oh and yeah the following seems a pretty silly question but the contradictory nature is perplexing me. If you have an answer as to what would be ideal?

Somehow the answers are contradictory in nature.:/ Should the answer simply be a bright spot?

Thank you.

 

[nehaoscar]

Thank youuu once again! But I have a slight query if it depends on intensity then why does increase in frequency trigger initially an increase in current? Like, The way I am seeing: when we increase the frequency the electrons gain more ke so the flow faster and since current is the rate of flow of charged particles it increases.However,eventually what happens is that intensity affects the number of electrons emitted per unit time(I am right?:/ ) so since intensity is constant the current turns out to be constant as nos of electrons are same. Is my analysis correct? I am confused slightly?:/

Oh and yeah the following seems a pretty silly question but the contradictory nature is perplexing me. If you have an answer as to what would be ideal?

Somehow the answers are contradictory in nature.:/ Should the answer simply be a bright spot?

Thank you.

Yeah the way you're writing it seems right since eventually the intensity is constant so no. of e- emitted is constant so the current will be constant
so yep
And for the other question that you've posted I have no idea xP

 

[Catherine_1]

Yeah the way you're writing it seems right since eventually the intensity is constant so no. of e- emitted is constant so the current will be constant
so yep
And for the other question that you've posted I have no idea xP

Thank youuu. Cool, lol no problem.

 

[Catherine_1]

Okay so could anyone help me out with the three questions, my structured paper 4 is tomorrow, please help.

Ms




Part iii please


Ms. I am clueless why they have multiplied the transmitted and reflected intensities and also why squaring is taking place. :>




In the second part, how do we know the mass is same it's not mentioned as such?

Ms

 

[nehaoscar]

Okay so could anyone help me out with the two questions, my structured paper 4 is tomorrow, please help.

Ms

the error for each calibration is 2% so for it to be 10% it needs to decay by another 8%
S0:
0.92A = Ae^(-lamda*t) ---> i'm gonna use l for lamda
the A's cancel out
0.92 = e^(-lt)
now we know l = ln2/t1/2 ---> t1/2 is half life
so ln2/5.27*365 ---> since they want ans in days multiply t1/2 by 365 so l is in terms of days
so l = 3.6*10^-4
coming back to first eqn
0.92 = e^[(-3.6*10^-4)*t]
Rearrange to find t
so it gives 231 days rounded to 230

 

[nehaoscar]

In the second part, how do we know the mass is same it's not mentioned as such?

Ms

it says ions of the same isotopes and since e- have a v. small mass, it can be negligible so they'll have a same mass

 

[Catherine_1]

it says ions of the same isotopes and since e- have a v. small mass, it can be negligible so they'll have a same mass

So can we like,make this assumption whenever the same isotope is given?

the error for each calibration is 2% so for it to be 10% it needs to decay by another 8%
S0:
0.92A = Ae^(-lamda*t) ---> i'm gonna use l for lamda
the A's cancel out
0.92 = e^(-lt)
now we know l = ln2/t1/2 ---> t1/2 is half life
so ln2/5.27*365 ---> since they want ans in days multiply t1/2 by 365 so l is in terms of days
so l = 3.6*10^-4
coming back to first eqn
0.92 = e^[(-3.6*10^-4)*t]
Rearrange to find t
so it gives 231 days rounded to 230

Thankkkkk youuuu so much!!!!!! This question was quite different from the norm tho Btw if you get the other one too whenever do let me know, please.

 

[Unsa Khan]

Can someone please explain me the following objective of the Physics Syllabus of 2016 :
# Topic : Properties of waves

objective: Describe how wavelength affects diffraction at an edge

I can't find anything related to this in any of my physics textbooks

 

[nehaoscar]

So can we like,make this assumption whenever the same isotope is given?

yepp

 

[Catherine_1]

yepp

Thankssss

 

[nehaoscar]

Can someone please explain me the following objective of the Physics Syllabus of 2016 :
# Topic : Properties of waves

objective: Describe how wavelength affects diffraction at an edge

I can't find anything related to this in any of my physics textbooks

I can't actually find that objective in the syllabus but this is what it should be:
When a wave passes through a gap the diffraction effect is greatest when the width of the gap is about the same size as the wavelength of the wave

 

[Catherine_1]

b i) pleaseeeeeeeeeeeeeeeeeeeeee ?
I know the idea of interference and swamping but my doubt in there are specific way to determine what it should be, like could it have also been 10GHz or 13GHz too?

 

[nehaoscar]

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b i) pleaseeeeeeeeeeeeeeeeeeeeee ?
I know the idea of interference and swamping but my doubt in there are specific way to determine what it should be, like could it have also been 10GHz or 13GHz too?

it's always 14/11 or 6/4 or 30/20 so remember them!

 

[Catherine_1]

it's always 14/11 or 6/4 or 30/20 so remember them!

Oh that's so cool, thank youu so muchhhhh for letting me know! Btw did you notice that trend or your teacher told?

 

[nehaoscar]

Oh that's so cool, thank youu so muchhhhh for letting me know! Btw did you notice that trend or your teacher told?

teacher told xP

 

[Catherine_1]

teacher told xP

That's so nice I wish my teacher would also tell such tricks to get an easy 1 or 2 marks lol

 

[ThePacifics]

Hey all. I had this doubt. I want to give my Physics CIE AS level exams in 2016 november session. Can I give A levels in 2017 May June session?