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March 29, 2024, 08:34:19 pm

Author Topic: HSC Physics Question Thread  (Read 1030705 times)  Share 

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jamonwindeyer

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Re: Physics Question Thread
« Reply #885 on: August 29, 2016, 09:39:48 pm »
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To measure the diffraction, we used a ruler to measure the distance between the minima.
I'll link a video which shows how the experiment was completed.
https://www.youtube.com/watch?v=kpsN78mQ6YY

Oh that's really cool!

Okay, well definitely:
- Inaccuracy in wavelength of laser (often varies by a small percentage, to my knowledge)
- Inaccuracy of measuring tool (EG - if using a ruler, it is only accurate to nearest millimetre, even if used perfectly)

And perhaps some others! Anyone want to tag in?

jakesilove

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Re: Physics Question Thread
« Reply #886 on: August 29, 2016, 09:45:23 pm »
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Hey guys. So this is a question from 2014 hsc that's kind of confusing me:
A photon is incident on a hydrogen atom in the ground state. Explain, using de Broglie’s hypothesis, why the photon is not absorbed by the hydrogen atom.

So I know de Broglie proposed that electrons were standing waves to explain stability of electron orbits. These are quantised as the electron orbit must have a circumference equal to the multiple of the wavelength of the electron (2.pi.r = n.lambda) to allow for a standing wave (otherwise the beginning and end points are out of phase, leading to destructive interference and the wave losing energy). Therefore, the electrons only absorb photons if its energy (e=hc/lamda) allowed it to jump to one of these fixed energy states. But there are no numbers in the question so I'm not sure what the explanation is as to why the photon won't be absorbed by the electron in the hydrogen atom and the bos sample answer didn't rlly specify this. Thanks  :)

Hey! I would say exactly what you've written there; that only a very specific TYPE of photon could be absorbed by the electrons, for all the reasons outlined above. I would definitely give you full marks; you've explained the concepts, included relevant formulas, and done it all succinctly. Well done; the question is certainly worded badly, but I don't see any other way of answering it.
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jakesilove

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Re: Physics Question Thread
« Reply #887 on: August 29, 2016, 09:49:28 pm »
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To measure the diffraction, we used a ruler to measure the distance between the minima.
I'll link a video which shows how the experiment was completed.
https://www.youtube.com/watch?v=kpsN78mQ6YY

Lol this experiment is hilarious. There should definitely be an error in wavelength (usually in the order of 5nm). The biggest error is using a ruler, as Jamon pointed out. If you want to go into a bit more depth, you could discuss the approximation used (what they did is that they approximated sin(theta)=tan(theta). Draw a diagram to understand what I mean!). If you don't quite understand, happy to draw up a diagram. This approximation is good we D is much bigger than d, and in this case it sort of is, but still; as with any approximation, there will be an associated error.

Jake
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jakesilove

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Re: Physics Question Thread
« Reply #888 on: August 29, 2016, 09:51:56 pm »
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Lol this experiment is hilarious. There should definitely be an error in wavelength (usually in the order of 5nm). The biggest error is using a ruler, as Jamon pointed out. If you want to go into a bit more depth, you could discuss the approximation used (what they did is that they approximated sin(theta)=tan(theta). Draw a diagram to understand what I mean!). If you don't quite understand, happy to draw up a diagram. This approximation is good we D is much bigger than d, and in this case it sort of is, but still; as with any approximation, there will be an associated error.

Jake

I also don't mean to be demeaning about the experiment in any way; it's just similar to Young's single and double slit experiment, and it's like they need to find a legit application to explain what's going on the students (by measuring a human hair), rather than just explaining the theory to you, which you could definitely understand.
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Jakeybaby

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Re: Physics Question Thread
« Reply #889 on: August 29, 2016, 09:56:18 pm »
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I also don't mean to be demeaning about the experiment in any way; it's just similar to Young's single and double slit experiment, and it's like they need to find a legit application to explain what's going on the students (by measuring a human hair), rather than just explaining the theory to you, which you could definitely understand.
Instead of using the equation which they used, we were instructed to use the following equation:

Bandwidth = (Wavelength * L) / a
Where L: Distance from hair to screen
a: width of hair sample
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Re: Physics Question Thread
« Reply #890 on: August 29, 2016, 10:01:43 pm »
+1
Hey! I would say exactly what you've written there; that only a very specific TYPE of photon could be absorbed by the electrons, for all the reasons outlined above. I would definitely give you full marks; you've explained the concepts, included relevant formulas, and done it all succinctly. Well done; the question is certainly worded badly, but I don't see any other way of answering it.

Lol that explains a lot. Thought there was some weird fact about the ground state or something. Thank you rlly helped clear things ;D

jakesilove

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Re: Physics Question Thread
« Reply #891 on: August 29, 2016, 10:17:41 pm »
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Instead of using the equation which they used, we were instructed to use the following equation:

Bandwidth = (Wavelength * L) / a
Where L: Distance from hair to screen
a: width of hair sample

I would probably just focus on the errors in those variables then; Wavelength, and use of a ruler. I can't imagine you need much more depth than that!
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Jakeybaby

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Re: Physics Question Thread
« Reply #892 on: August 29, 2016, 10:31:42 pm »
+1
Thankyou to both of you! Really appreciate your help! :)
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Neutron

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Re: Physics Question Thread
« Reply #893 on: August 31, 2016, 10:50:46 pm »
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I'm back! Still drained from trials though rip it's been 2 weeks :/ Anyhow, first off, I just wanted to thank all of you for helping me throughout the year! This forum has been so so amazing and ahh I can't believe only the actual hsc is left (yay :/) heh okay so, with p-type semiconductors, you know how the primary mobile charge carriers are holes in the valence band? In this case, does it not matter that the free charge carriers are not in the conduction band? Cause I'm doing an HSC question and the solutions said "These extra holes lower the energy required for charge to be mobilised to the conduction band and move as current". I'm just a bit confused cause I thought the acceptor level formed was so that the electrons could easily jump from the valence band into the acceptor level, meaning it left behind more holes in the valence? Nothing to do with the conduction band? Thank you y'alllll

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Re: Physics Question Thread
« Reply #894 on: August 31, 2016, 11:06:17 pm »
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I'm back! Still drained from trials though rip it's been 2 weeks :/ Anyhow, first off, I just wanted to thank all of you for helping me throughout the year! This forum has been so so amazing and ahh I can't believe only the actual hsc is left (yay :/) heh okay so, with p-type semiconductors, you know how the primary mobile charge carriers are holes in the valence band? In this case, does it not matter that the free charge carriers are not in the conduction band? Cause I'm doing an HSC question and the solutions said "These extra holes lower the energy required for charge to be mobilised to the conduction band and move as current". I'm just a bit confused cause I thought the acceptor level formed was so that the electrons could easily jump from the valence band into the acceptor level, meaning it left behind more holes in the valence? Nothing to do with the conduction band? Thank you y'alllll

Neutron

Hey Neutron! ;D welcome back, thank you so much for your kind words :)

That wording does seem a tad strange to me, is it from a sample solution from BOSTES or? :)

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Re: Physics Question Thread
« Reply #895 on: September 01, 2016, 01:55:17 pm »
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Hey Neutron! ;D welcome back, thank you so much for your kind words :)

That wording does seem a tad strange to me, is it from a sample solution from BOSTES or? :)


Yeah it was, from the success one books :O Are we not supposed to follow those??

jakesilove

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Re: Physics Question Thread
« Reply #896 on: September 01, 2016, 02:26:02 pm »
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Yeah it was, from the success one books :O Are we not supposed to follow those??

Basically, the answer given above is correct, and so is your understanding of it. The acceptor/donor bands are just representations of the fact that it is easier for electrons to be mobilised to form current. However you want to say that is up to you, because in reality the reasons ISN'T taught at an HSC level. So, saying anything along the lines of 'doping the semi-conductor increases the ease with which current can flow. This is because of excess electrons/holes etc. etc.' will definitely get you the marks!
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Re: Physics Question Thread
« Reply #897 on: September 01, 2016, 06:37:55 pm »
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Yeah it was, from the success one books :O Are we not supposed to follow those??

So the Success One answers are correct (they are BOSTES, after all), but they aren't always 100% clear. I don't like the way that answer was worded, for example ;)

But here is an explanation: Remember that as Jake said, the valence and conduction bands are representations/models of reality. The conduction band represents a set of energies that an electron/hole must attain to break free from a covalent bond and travel freely (within reason) through the lattice. The conduction band isn't actually 'above' the valence band; that is just the way we represent it. There is an energy level separating them, not physical space :)

Essentially, if an electron is moving through the atom, then it is in the conduction band (for our understanding, at least). It must be, because if it wasn't, it would still be forming part of a covalent bond. The acceptor impurity atoms that are introduced generate additional holes in the lattice; an adjacent electron may therefore jump to fill the hole. It had the energy to break its covalent bond to do so, and therefore, in the conduction band :)

Kinda technical, but does that make sense? :)


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Re: Physics Question Thread
« Reply #898 on: September 06, 2016, 02:31:08 pm »
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The captain of a fishing boat uses an echo sounder to determine the depth of a school of fish below the boat. The captain finds that the reflected waves return after o.15s and o.20s. The captain believes the first reflectin to be from the fish while the seconds is from the ocean floor. If the speed of sound in seawater is 1440m/s, determine the depth of the sea floor. (3 marks)

can anyone tell me the expected way of doing a question like this for year 11 physics? i looked it up and found ways which correlate to actual echo sounders, but realise i've never done this stuff in class.

any help is greatly appreciated!

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Re: Physics Question Thread
« Reply #899 on: September 06, 2016, 02:39:08 pm »
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The captain of a fishing boat uses an echo sounder to determine the depth of a school of fish below the boat. The captain finds that the reflected waves return after o.15s and o.20s. The captain believes the first reflectin to be from the fish while the seconds is from the ocean floor. If the speed of sound in seawater is 1440m/s, determine the depth of the sea floor. (3 marks)

can anyone tell me the expected way of doing a question like this for year 11 physics? i looked it up and found ways which correlate to actual echo sounders, but realise i've never done this stuff in class.

any help is greatly appreciated!

Hey conic! You'd just be using \(\text{Speed}=\frac{\text{Distance}}{\text{Time}}\) formulae. Since the time taken to return to the boat is 0.20 seconds, and the speed of sounds is 1440m/s:



However, note that this is the distance travelled to the floor and back, so the actual answer is: