A level OCR Physics A G482 waves and quantum physics help

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a123a
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Stonebridge
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(Original post by a123a)
I've posted this in the year 12 AS thread as well, but posting it here for extra help:

I'm on OCR Physics A.


I'm having a lot of trouble understanding some of the wave and quantum physics topics.


I'm reading the textbook and revision guide, and it's just not sinking in. And if I have a question it's not like I can ask the book.:rolleyes: My teachers are not really helping me as well...


The wave topic I'm fining difficult is the "stationary waves"; in particular demonstrating stationary waves in air columns and how to determine the speed of sound in air from the measurements on a stationary wave in a pipe closed at one end. Can someone please explain the steps you have to do as well as describe them.
It's very difficult to help when you ask a very general question. We can't really rewrite your text book. We don't know what you already know and understand, and what it is that is causing the confusion.
There's no point in us writing a long description of how to do this experiment (you will find it on a number of tutorial sites if you bother to Google them) if it's just one concept that's causing the problem.
For example. Can we assume you know what wavelength and frequency are, and understand the formula v = fλ
You say you can't ask the book a question.
So ask that question here? What is the sticking point?

To focus your reply, can I just say that, if you understand the formula v = fλ then the method you are asking about simply uses this to calculate v. It does it by knowing the frequency of the wave (via a tuning fork or signal generator of known value) and finding the wavelength from the length of the pipe.
Which part of this don't you understand?
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a123a
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My point of this was that if someone replied with information that they knew of I could broaden my understanding of the topic; if I didn't understand anything from their reply I could ask questions until I understood. I can't do this with a book nor with a video on google. I asked for the description so that would be the basis of a discussion.

What I don't get (with the experiment for determining sound), is why does it have to be a closed pipe? What does finding the max resonance mean? And how, by finding this, does it give us the distance between a node and antinode? I understand once you've got the distance you times it by 4 to get the wavelength; then workout velocity with the equations you've stated-yes I know that equation
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a123a
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(Original post by Stonebridge)
It's very difficult to help when you ask a very general question. We can't really rewrite your text book. We don't know what you already know and understand, and what it is that is causing the confusion.
There's no point in us writing a long description of how to do this experiment (you will find it on a number of tutorial sites if you bother to Google them) if it's just one concept that's causing the problem.
For example. Can we assume you know what wavelength and frequency are, and understand the formula v = fλ
You say you can't ask the book a question.
So ask that question here? What is the sticking point?

To focus your reply, can I just say that, if you understand the formula v = fλ then the method you are asking about simply uses this to calculate v. It does it by knowing the frequency of the wave (via a tuning fork or signal generator of known value) and finding the wavelength from the length of the pipe.
Which part of this don't you understand?

Sorry look at the post above.
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Stonebridge
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(Original post by a123a)
My point of this was that if someone replied with information that they knew of I could broaden my understanding of the topic; if I didn't understand anything from their reply I could ask questions until I understood. I can't do this with a book nor with a video on google. I asked for the description so that would be the basis of a discussion.
Well, as I say, that's not how it works on here.

What I don't get (with the experiment for determining sound), is why does it have to be a closed pipe?
It doesn't

What does finding the max resonance mean?
I've never heard of "max(imum) resonance".
Resonance occurs when the driving force (here the tuning fork or sig gen speaker) has the same frequency as the natural frequency of vibration of the air in the pipe. Then you get waves with maximum amplitude from the pipe.

And how, by finding this, does it give us the distance between a node and antinode?
Standing waves form when you have two waves with the same frequency travelling in opposite directions such that they interfere. In a pipe you get the wave emitted by the fork/speaker travelling one way and the same wave reflected from the other end (open or closed) travelling the other way. The interference pattern produced (standing wave) is characterised by places of maximum displacement/amplitude (antinodes) and places of minimum displacement (nodes). You can get this by considering superposition of waves. (Have you done this?)
The standing wave that forms in the pipe can have various combinations of node and antinode*. In the case of a pipe closed at one end it's nice and simple. There must be a node at the closed end. The simplest case for a closed pipe has that node at the closed and and the 1st antinode at the open end. This means a quarter wavelength is enclosed in the pipe.

I understand once you've got the distance you times it by 4 to get the wavelength; then workout velocity with the equations you've stated-yes I know that equation
Yes.
Given this information, multiplying the pipe length by 4 gives you the wavelength of the wave. As you have f you can now find v. This is the case for the simplest (fundamental) mode of vibration of the standing wave. There are other possible "modes".

*
http://www.s-cool.co.uk/a-level/phys...waves-in-pipes
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Basit2010
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I did this last year and got 124/150 (a) - Honestly was in the exact same position as you with Physics it is incredibly difficult if your not in the right mind frame or interested in circuits! I used the CGP revision guide to help me as I felt it explained it really well. Also just remember that the grade boundaries for this exam is very low, I remember doing this paper and I skipped a few pages (20 marks or so) on the waves as I didn't know how to answer them but made up for it in the calculations of photons etc.
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a123a
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(Original post by Stonebridge)
Well, as I say, that's not how it works on here.



It doesn't


I've never heard of "max(imum) resonance".
Resonance occurs when the driving force (here the tuning fork or sig gen speaker) has the same frequency as the natural frequency of vibration of the air in the pipe. Then you get waves with maximum amplitude from the pipe.


Standing waves form when you have two waves with the same frequency travelling in opposite directions such that they interfere. In a pipe you get the wave emitted by the fork/speaker travelling one way and the same wave reflected from the other end (open or closed) travelling the other way. The interference pattern produced (standing wave) is characterised by places of maximum displacement/amplitude (antinodes) and places of minimum displacement (nodes). You can get this by considering superposition of waves. (Have you done this?)
The standing wave that forms in the pipe can have various combinations of node and antinode*. In the case of a pipe closed at one end it's nice and simple. There must be a node at the closed end. The simplest case for a closed pipe has that node at the closed and and the 1st antinode at the open end. This means a quarter wavelength is enclosed in the pipe.



Yes.
Given this information, multiplying the pipe length by 4 gives you the wavelength of the wave. As you have f you can now find v. This is the case for the simplest (fundamental) mode of vibration of the standing wave. There are other possible "modes".

*
http://www.s-cool.co.uk/a-level/phys...waves-in-pipes

Ok thanks for this reply. Just wanted to ask about the other possible "modes". So if it was the second harmonic I would times the distance by 4 then divide by 3? But when conducting the experiment how do you know if the wave is fundamental or another? Aren't you just moving the fork up and down to hear the loudest sound? And that's the point of resonance? So I get that you times by 4 for the fundamental but how do you know it's a fundamental (without a diagram showing the wave, so like when you doing the practical)?

I have done superposition.
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a123a
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(Original post by Basit2010)
I did this last year and got 124/150 (a) - Honestly was in the exact same position as you with Physics it is incredibly difficult if your not in the right mind frame or interested in circuits! I used the CGP revision guide to help me as I felt it explained it really well. Also just remember that the grade boundaries for this exam is very low, I remember doing this paper and I skipped a few pages (20 marks or so) on the waves as I didn't know how to answer them but made up for it in the calculations of photons etc.
Thank you! Well done for your mark! I have the CGP book as well but still didn't understand this bit and the quantum Physics: determining Planck constant bit. But it's starting to sink in... I'm just writing my notes for G482 so I can start going through them. Have you got any tips for this exam and generally for OCR A physics? Is past papers key? And would you say all the topics are covered in the revision guide and CGP book for what comes up in the exam?

thank you!
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Basit2010
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(Original post by a123a)
Thank you! Well done for your mark! I have the CGP book as well but still didn't understand this bit and the quantum Physics: determining Planck constant bit. But it's starting to sink in... I'm just writing my notes for G482 so I can start going through them. Have you got any tips for this exam and generally for OCR A physics? Is past papers key? And would you say all the topics are covered in the revision guide and CGP book for what comes up in the exam?

thank you!
What I did for my AS levels was memorize the CGP revision guides and do the past papers the practice exam technique. The CGP revision guides do include everything you need to know for the spec but it may be a good idea to cross reference with the bigger textbooks. The only tip I can give is make sure you nail the calculation questions as the long worded ones may be difficult to score marks in because if you miss out a key word you often loose the mark. Also just make sure you remember all the experiments and how to set them up as this can be a good 5 mark question!

And just remember, low boundaries (was even 56/100 for an A in 2009 I think). It really helped calm down my nerves..
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Stonebridge
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(Original post by a123a)
Ok thanks for this reply. Just wanted to ask about the other possible "modes". So if it was the second harmonic I would times the distance by 4 then divide by 3?
Yes exactly. The next possible mode after the fundamental in a pipe closed at one end will still have to have a node at the closed end and an antinode at the open end. So it looks like this
<><
which corresponds to 3/4 of a wave in the tube.
So the wavelength of this wave is 4/3 times the length of the tube.
But when conducting the experiment how do you know if the wave is fundamental or another?
The fundamental is always the lowest note (lowest frequency) you can get out of the pipe.

Aren't you just moving the fork up and down to hear the loudest sound? And that's the point of resonance?
There are a number of variations in the way you can do this experiment. The classic method, the Resonance Tube method, involved using a tuning fork of fixed frequency. The tube is held vertically and gradually filled with water until you get resonance.
There's a description here http://hyperphysics.phy-astr.gsu.edu.../restube2.html
There are also 3 or 4 vids on YouTube.

Another method is to vary the frequency of the sound output from a signal generator, holding the speaker near the end of the tube, until you get resonance. The fundamental is the lowest frequency for which this happens.
If you are doing this practically you need to say what equipment you have and which method is expected.
If you are describing an experiment for theory, then it doesn't matter which. There are plenty of descriptions available in books and on the web.
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Vorsah
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(Original post by a123a)
I've posted this in the year 12 AS thread as well, but posting it here for extra help:

I'm on OCR Physics A.


I'm having a lot of trouble understanding some of the wave and quantum physics topics.


I'm reading the textbook and revision guide, and it's just not sinking in. And if I have a question it's not like I can ask the book.:rolleyes: My teachers are not really helping me as well...


The wave topic I'm fining difficult is the "stationary waves"; in particular demonstrating stationary waves in air columns and how to determine the speed of sound in air from the measurements on a stationary wave in a pipe closed at one end. Can someone please explain the steps you have to do as well as describe them.
The quantum physics topic I'm fining difficult to understand is how LEDs can be used to estimate the Planck constant h using the equation eV=(hc)/λ. Like why do you find the voltage threshold? What's that got to do with it and how do you even do the experiment? And how does plotting a graph of V against 1/λ mean the gradient is (hc)/e?


Can anyone help? It's really frustrating that I'm not understanding these topics :/
Check out drphysicsa on YouTube, his videos are pretty good
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a123a
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(Original post by Basit2010)
What I did for my AS levels was memorize the CGP revision guides and do the past papers the practice exam technique. The CGP revision guides do include everything you need to know for the spec but it may be a good idea to cross reference with the bigger textbooks. The only tip I can give is make sure you nail the calculation questions as the long worded ones may be difficult to score marks in because if you miss out a key word you often loose the mark. Also just make sure you remember all the experiments and how to set them up as this can be a good 5 mark question!

And just remember, low boundaries (was even 56/100 for an A in 2009 I think). It really helped calm down my nerves..
Thank you. That's reassuring. Some calculations are ok but I do find some calculations really hard as there isn't really a set method; just have to use the units you have got to get the possible outcome.

(Original post by Stonebridge)
Yes exactly. The next possible mode after the fundamental in a pipe closed at one end will still have to have a node at the closed end and an antinode at the open end. So it looks like this
<><
which corresponds to 3/4 of a wave in the tube.
So the wavelength of this wave is 4/3 times the length of the tube.

The fundamental is always the lowest note (lowest frequency) you can get out of the pipe.


There are a number of variations in the way you can do this experiment. The classic method, the Resonance Tube method, involved using a tuning fork of fixed frequency. The tube is held vertically and gradually filled with water until you get resonance.
There's a description here http://hyperphysics.phy-astr.gsu.edu.../restube2.html
There are also 3 or 4 vids on YouTube.

Another method is to vary the frequency of the sound output from a signal generator, holding the speaker near the end of the tube, until you get resonance. The fundamental is the lowest frequency for which this happens.
If you are doing this practically you need to say what equipment you have and which method is expected.
If you are describing an experiment for theory, then it doesn't matter which. There are plenty of descriptions available in books and on the web.
Thank you for your help you've explained what I didn't understand. Yep I've checked the link, and will look at the videos just now. I'll make sure next time my opening post is a bit more precise.
Thank you again.
(Original post by Vorsah)
Check out drphysicsa on YouTube, his videos are pretty good
yep will do, thanks!
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_jessmc
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If you go on the last question of the 2013 january paper (7) it explains it in the answers. You find the threshold voltage (energy required to only just emit photons for that particular wavelength). As you already know the wavelength, you plot a graph of V and 1/λ and then the gradient is hc/e so you rearrange to find h
I hope this helps sorry if my wording is a bit off
It also gives you the method of finding it as well
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