ERRATA - Q20 the answer is D not C. I inadvertently forgot the aspect of the time light travels from very distance galaxies . Hence they appear to have a lower acceleration since their light left a long time ago. Hence the gradient curves to a smaller slope
Wow! Thank you so much Paul, you're an insipration to every physics student in the state! I got what I felt were some of the hardest ones. The only answer I got wronng was number 2 because I got step up/down transformers mixed up... stings a little, but I'm pretty happy
man this is getting tedious. f = ma on a Band 5-6 Q18. Sometimes it can be so simple. It's what the examiners want though - dont forget your roots! My students over the last 8 years have benefited greatly thanks 2 u - albiet earlier cohorts were not the best times. We're having a great time now. Cheers
question c was so annoying, because on the paper it was the light lines that were shaded in. So I wasn't sure whether the light beam was the gap in between the dark bits (i.e what was light) or the dark lines (which had the shape of the light beam)
Hi Paul. Regarding the electron in question 6 option D, we know that in a circular path at constant speed the electron is not losing any energy. Where does the energy for the EM waves come from? I guess we have to assume there is an external energy source being used to push that electron?
Actually that is incorrect. An electron travelling in a circular orbit is always accelerating. According to Maxwell's theories, EMR is generated whenever an electron(s) accelerate. So therefore electrons MUST emit EMR, at least in classical physics . That means it SHOULD spiral in to the nucleus. Since that doesn't happen, (neither the EMR emitted nor the spiral of death), Its the reason there had had to be a reassessment as to why electrons sty in stable orbits.
Sorry Paul, I'm not quite following this. Can you explain further? What part is incorrect? According to classical physics, indeed the electron should spiral in. However, option D says it is not spiralling in; it is travelling in a circular orbit (no loss of electrical potential energy) and at a constant speed (no loss of kinetic energy). That is not possible in classical physics (hence why I queried a possible energy source to permit the classical case without contravening the Law of Conservation of Energy). As per elsewhere in the syllabus, no work is being done in uniform circular motion. I see "constant speed / circular path" and I immediately think "Bohr!"; option B, though, also implies Bohr, as you indicated. In that sense, what distinguishes options B and D?@@PhysicsHigh
I think you may be over thinking this. This question is simply asking, according to Maxwell, what must be occurring if EMR is produced? The answer is that it is accelerating. The student then needs to work out which scenario is one that is accelerating , and that is D. Remember of course that Maxwell had no concept of the electron in orbit. It has nothing to do with the Bohr model of the atom which came later This is based on classical physics. Of course, as you surmise that cant be possible in reality. The electron IS in a stable orbit and yet is NOT radiating EMR. But that contradicts Maxwell's theories. Alternatively, the electron IS radiating EMR but that contradicts LCE This is of course where Bohr comes in . But as I said, that is not the intent of the question. Does that help?
@@PhysicsHigh I do like your wording: "according to Maxwell, what must be occurring if EMR is produced". Indeed, as the other options were no good, then by elimination we arrive at option D and are forced to conclude that this option can squeeze into acceptability by tightly constraining ourselves to Maxwell without consideration of energy. I think the question should have been written as you actually worded it: ", an electron would produce an electromagnetic wave when it is..." Knowing the energy issue, that is why I thought I would ask you the question about a possible source of energy. As the Law of Conservation of Energy gets applied multiple times throughout the syllabus and also appears in novel situations that the HSC exam sometimes springs (which I think is a good test of students' understanding), it is always a good thing to think about. Therefore, on seeing that D had to be the correct option, I started considering what might be a source of energy to make it possible. At the subatomic scale, Bohr concluding what he did, it would seem that we can't find a source there. My original question to you was aimed more at considering larger scale possibilities such as a wire loop: would anything be able to drive electrons around it at a constant speed, supplying energy and enabling the accelerating electrons to produce EM waves? I like exploring such links to other parts of physics with students as I think it's great for the brain. As regards "overthinking", I've been tempted to say that to students a couple of times over the last couple of decades, but I've always bitten my tongue. I don't think there is overthinking. Thinking more deeply about something is how new understanding emerges. It fosters imagination. It produces novel hypotheses. Underthinking is a much riskier prospect in life!
ERRATA - Q20 the answer is D not C. I inadvertently forgot the aspect of the time light travels from very distance galaxies . Hence they appear to have a lower acceleration since their light left a long time ago. Hence the gradient curves to a smaller slope
Wow! Thank you so much Paul, you're an insipration to every physics student in the state! I got what I felt were some of the hardest ones. The only answer I got wronng was number 2 because I got step up/down transformers mixed up... stings a little, but I'm pretty happy
man this is getting tedious. f = ma on a Band 5-6 Q18. Sometimes it can be so simple. It's what the examiners want though - dont forget your roots! My students over the last 8 years have benefited greatly thanks 2 u - albiet earlier cohorts were not the best times. We're having a great time now. Cheers
Paul will you be doing the extended response questions too?
I will but I will delay until the marking guidelines have been published.
Neat trick from NESA in q3, making the bright fringes dark and the dark fringes white. 😂
question c was so annoying, because on the paper it was the light lines that were shaded in. So I wasn't sure whether the light beam was the gap in between the dark bits (i.e what was light) or the dark lines (which had the shape of the light beam)
Hi Paul. Regarding the electron in question 6 option D, we know that in a circular path at constant speed the electron is not losing any energy. Where does the energy for the EM waves come from? I guess we have to assume there is an external energy source being used to push that electron?
Actually that is incorrect. An electron travelling in a circular orbit is always accelerating. According to Maxwell's theories, EMR is generated whenever an electron(s) accelerate. So therefore electrons MUST emit EMR, at least in classical physics . That means it SHOULD spiral in to the nucleus. Since that doesn't happen, (neither the EMR emitted nor the spiral of death), Its the reason there had had to be a reassessment as to why electrons sty in stable orbits.
Sorry Paul, I'm not quite following this. Can you explain further?
What part is incorrect? According to classical physics, indeed the electron should spiral in. However, option D says it is not spiralling in; it is travelling in a circular orbit (no loss of electrical potential energy) and at a constant speed (no loss of kinetic energy). That is not possible in classical physics (hence why I queried a possible energy source to permit the classical case without contravening the Law of Conservation of Energy). As per elsewhere in the syllabus, no work is being done in uniform circular motion. I see "constant speed / circular path" and I immediately think "Bohr!"; option B, though, also implies Bohr, as you indicated. In that sense, what distinguishes options B and D?@@PhysicsHigh
I think you may be over thinking this. This question is simply asking, according to Maxwell, what must be occurring if EMR is produced? The answer is that it is accelerating. The student then needs to work out which scenario is one that is accelerating , and that is D. Remember of course that Maxwell had no concept of the electron in orbit. It has nothing to do with the Bohr model of the atom which came later
This is based on classical physics.
Of course, as you surmise that cant be possible in reality. The electron IS in a stable orbit and yet is NOT radiating EMR. But that contradicts Maxwell's theories.
Alternatively, the electron IS radiating EMR but that contradicts LCE
This is of course where Bohr comes in . But as I said, that is not the intent of the question.
Does that help?
@@PhysicsHigh I do like your wording: "according to Maxwell, what must be occurring if EMR is produced". Indeed, as the other options were no good, then by elimination we arrive at option D and are forced to conclude that this option can squeeze into acceptability by tightly constraining ourselves to Maxwell without consideration of energy. I think the question should have been written as you actually worded it: ", an electron would produce an electromagnetic wave when it is..."
Knowing the energy issue, that is why I thought I would ask you the question about a possible source of energy. As the Law of Conservation of Energy gets applied multiple times throughout the syllabus and also appears in novel situations that the HSC exam sometimes springs (which I think is a good test of students' understanding), it is always a good thing to think about. Therefore, on seeing that D had to be the correct option, I started considering what might be a source of energy to make it possible. At the subatomic scale, Bohr concluding what he did, it would seem that we can't find a source there. My original question to you was aimed more at considering larger scale possibilities such as a wire loop: would anything be able to drive electrons around it at a constant speed, supplying energy and enabling the accelerating electrons to produce EM waves? I like exploring such links to other parts of physics with students as I think it's great for the brain.
As regards "overthinking", I've been tempted to say that to students a couple of times over the last couple of decades, but I've always bitten my tongue. I don't think there is overthinking. Thinking more deeply about something is how new understanding emerges. It fosters imagination. It produces novel hypotheses. Underthinking is a much riskier prospect in life!
Your answer to question 20 is incorrect mate :)
What would the correct answer be then? Nothing else makes any sense.
Thanks - I realised that after I published , after some discussion with colleagues. I will post an errata in the description and a pinned comment
@@witchofthescarlethaze3504 Actually D makes sense as a result, see my pinned comment
@@PhysicsHighIn my opinion it is quite the counterintuitive and yet beautiful answer