HSC Physics Question Thread

[RuiAce]

Hey emm!

Sure thing! Firstly let's just recall Max Planck's first postulate about the quantisation of energy levels in an atom. He stated that energies exist in discrete amounts, not just any random amount, and that in order for an electron to move from one energy level to another, a well-defined amount of energy has to be absorbed or emitted by the electron. Now let's think about the blackbody radiation curve. We know that ultraviolet radiations, X-rays and gamma rays all have low wavelength and hence high frequency, as described by the blackbody curve. According to Planck's equation E=hf, when wavelength becomes increasingly small, frequency would become increasingly large and hence energy would also becoming infinitely large. Such phenomenal energy change would involve a significantly large leap from one energy level to another. However, because such change in energy level is impossible in any atom, high frequency radiations such as UV cannot be emitted and thus for X-rays, UV rays and gamma rays, the intensity is 0. This hence provides an explanation for ultraviolet catastrophe.

This last bit sounds a bit out of place. Aren't we looking for how the UV catastrophe is resolved?

Physics in focus definition here.

When a black body is heated to some temperature
in a vacuum, for example, by electric heating, it starts to emit radiation perfectly, known as black body radiation. This radiation can cover the entire range of the EMR spectrum with the intensity varying with the wavelength. If the individual wavelengths of this radiation are detected and the corresponding intensities measured experimentally, the data can then be plotted as ‘intensity’ versus
‘wavelength’, to produce a black body radiation curve.

The UV catastrophe arises when we apply classical physics calculations. If we use this, we deduce that the intensity of the EMR emitted should approach infinity as we reach the UV end of the EMS. But experimentally this was not true; for a certain temperature given, there would be a corresponding EMR wavelength that was emitted the most (graphically, this means that the blackbody radiation curve had a maximum point).

E=hf, a fundamental equation of quantum physics, was introduced to resolve this dilemma, not to explain it.

Hi! Could you please explain how Planck's Law solved the Ultraviolet Catastrophe issue?

Now, I am not sure as to what exactly you mean by Planck's Law. Planck's law is described using much more complicated equations in Wikipedia and is far out of the scope of the syllabus. https://en.wikipedia.org/wiki/Planck%27s_law

I do not recall E=hf being given a name in the course, but Wikipedia also states that E=hf is called the "Planck-Einstein Relation"

[Happy Physics Land]

This last bit sounds a bit out of place. Aren't we looking for how the UV catastrophe is resolved?

Physics in focus definition here.

When a black body is heated to some temperature
in a vacuum, for example, by electric heating, it starts to emit radiation perfectly, known as black body radiation. This radiation can cover the entire range of the EMR spectrum with the intensity varying with the wavelength. If the individual wavelengths of this radiation are detected and the corresponding intensities measured experimentally, the data can then be plotted as ‘intensity’ versus
‘wavelength’, to produce a black body radiation curve.

The UV catastrophe arises when we apply classical physics calculations. If we use this, we deduce that the intensity of the EMR emitted should approach infinity as we reach the UV end of the EMS. But experimentally this was not true; for a certain temperature given, there would be a corresponding EMR wavelength that was emitted the most (graphically, this means that the blackbody radiation curve had a maximum point).

But classical physics couldnt explain the ultraviolet catastrophe and it was only explained through Planck's equation E=hf, implying that there is 0 intensity for high frequency radiations because none of these radiations can be released due to a significant change in energy levels being impossible in any atom.

[RuiAce]

But classical physics couldnt explain the ultraviolet catastrophe and it was only explained through Planck's equation E=hf, implying that there is 0 intensity for high frequency radiations because none of these radiations can be released due to a significant change in energy levels being impossible in any atom.

UV catastrophe was never real. It was a problem resulted from classical physics as you stated.

It's not really "explained" per se. More like corrected or justified.

[smiley2101]

Thank you so much! You guys are lifesavers! Just another question- I don't really understand the black body radiation curve. How was the UV catastrophe line calculated in the first place (meaning why did scientist think this was what happened) and why is the actual graph a bell curve this topic is so hard

[RuiAce]

Thank you so much! You guys are lifesavers! Just another question- I don't really understand the black body radiation curve. How was the UV catastrophe line calculated in the first place (meaning why did scientist think this was what happened) and why is the actual graph a bell curve this topic is so hard

The actual graph is out of genuine experimentation. It just happened to be verified by Planck's idea that energy was quantised.

Classical calculations aren't really important but they basically predicted that smaller wave length would always mean intensity of EMR released is greater.

[smiley2101]

Thanks rui! Also I'm finding it hard to see the connection with why the syllabus is asking us to look at cathode Ray's, black body and the photoelectric effect? Is there a connection?

[RuiAce]

Thanks rui! Also I'm finding it hard to see the connection with why the syllabus is asking us to look at cathode Ray's, black body and the photoelectric effect? Is there a connection?

Not really.

The topic is "From Ideas to Implementation". But in reality whilst they are all ideas it's really just four cleverly chosen ideas for the course in my opinion.

[soldmychildforyeezys]

Hi,

I'm having trouble determining the change in flux just by analysing a simple scenario by eye.

For example, a bar magnet that passes through a coil moving right. When it exits the coil, I know the magnetic flux is decreasing. But is it decreasing to the left or to the right? Does this tell you the direction of the change in flux as well?
Even though the bar magnet is moving to the right, it doesn't mean the flux is decreasing in the same direction correct?

This problem makes it hard for me to answer worded problems. I also know that the magnetic field direction would be decreasing, but not sure about the direction. Hence the magnetic flux has to decrease, again not sure about the direction. Hence can't find the direction of the induced magnetic field, there can't find direction of induced current.

Sorry, this is a lot longer than it had to be. Probably because of how confused I am rip.

[jamonwindeyer]

Not really.

The topic is "From Ideas to Implementation". But in reality whilst they are all ideas it's really just four cleverly chosen ideas for the course in my opinion.

BOSTES perception, they are all advances in physics that resulted in useful applications for society 

[jamonwindeyer]

The actual graph is out of genuine experimentation. It just happened to be verified by Planck's idea that energy was quantised.

Classical calculations aren't really important but they basically predicted that smaller wave length would always mean intensity of EMR released is greater.

It isn't exactly necessary knowledge specifically, but the classical law which predicted the incorrect 'Catastrophe Curve' was Rayleigh Jeans Law, which was based on classical understanding of the propagation of waves in space 

[jamonwindeyer]

Hi,

I'm having trouble determining the change in flux just by analysing a simple scenario by eye.

For example, a bar magnet that passes through a coil moving right. When it exits the coil, I know the magnetic flux is decreasing. But is it decreasing to the left or to the right? Does this tell you the direction of the change in flux as well?
Even though the bar magnet is moving to the right, it doesn't mean the flux is decreasing in the same direction correct?

This problem makes it hard for me to answer worded problems. I also know that the magnetic field direction would be decreasing, but not sure about the direction. Hence the magnetic flux has to decrease, again not sure about the direction. Hence can't find the direction of the induced magnetic field, there can't find direction of induced current.

Sorry, this is a lot longer than it had to be. Probably because of how confused I am rip.

Hey there! It sounds like you've got a lot running through your head, but I think the focus of your question is the end sentence:

...direction of induced current.

That's the big thing that these sort of questions require. Direction of change of magnetic flux isn't really a thing, and the change in magnitude of the flux is something it sounds like you have a grasp of. Let me try and offer a solution to finding the direction of induced current, based on a more practical application based around Lenz's Law.

Consider the scenario you proposed, a bar magnet moving away from a coil. Let's assume the north pole is pointed towards the coil, and that it is moving away on the right side (that is, moving away to the right).

What we have here is a north pole moving away from the coil. Lenz's Law states that induced currents will oppose the change that created them. So, since the north pole is moving away, an induced current will act to bring the north pole back towards the coil.

What this means is that the induced current will set up a South pole on the right hand side of the coil, nearest the magnet, to pull the north pole back. Therefore, the north pole is on the left. We can now use the Right Hand Grip Rule to find the direction of current. The way it works is simple; take your right hand and give a thumbs up to yourself (yay Physics). The thumb points in the direction of the north pole. The fingers wrap around in the direction of current.

Using this analysis, we can find the direction of induced current in, essentially, two quick steps.

I hope this helps with the confusion. Again, the direction of change of magnetic flux isn't really a question that gets asked in the HSC. A little too ambiguous. Change in magnitude is asked, but this is okay, since if the magnet is moving away from the coil then the magnitude is decreasing. I mean, you could interpret it is something similar to: a magnetic flux to the left, decreasing, but yeah. Not really a thing to my knowledge 

[soldmychildforyeezys]

Thanks man!

To clarify one last thing, is there a change in magnetic field when the magnet leaving further away from the coil in your example? I think I was confusing myself with thinking that the magnetic field will decrease when it moves further away, thus the flux would decrease. Therefore the induced current would create an induced flux to reinforce this decreasing flux.

[jakesilove]

Thanks man!

To clarify one last thing, is there a change in magnetic field when the magnet leaving further away from the coil in your example? I think I was confusing myself with thinking that the magnetic field will decrease when it moves further away, thus the flux would decrease. Therefore the induced current would create an induced flux to reinforce this decreasing flux.

Whilst I'll let Jamon answer your question, just wanted to jump in and say that I love you username. Bit too real though aha.

Jake

[jamonwindeyer]

Thanks man!

To clarify one last thing, is there a change in magnetic field when the magnet leaving further away from the coil in your example? I think I was confusing myself with thinking that the magnetic field will decrease when it moves further away, thus the flux would decrease. Therefore the induced current would create an induced flux to reinforce this decreasing flux.

No problem! Sorry if I've misinterpreted, is the question whether a change in magnetic flux occurs for a magnet moving away from the coil? If so, then yes absolutely! The magnitude of magnetic flux through the coil (and thus, magnitude of magnetic field strength) will decrease. Sorry for confusion, is that what you mean? 

[soldmychildforyeezys]

No problem! Sorry if I've misinterpreted, is the question whether a change in magnetic flux occurs for a magnet moving away from the coil? If so, then yes absolutely! The magnitude of magnetic flux through the coil (and thus, magnitude of magnetic field strength) will decrease. Sorry for confusion, is that what you mean? 

No worries, it was probably me being a bit confusing.

Last thing, so for a bar magnet moving through the coil. For your example, if the poles were reversed in the bar magnet. This would cause the induced magnetic field to be in the opposite direction, correct?

I think I was confusing myself by taking the direction of the moving bar magnet as the direction it was decreasing, opposed to looking at the actual magnetic field line directions.