HSC Physics Question Thread

[beau77bro]

ohhhhh ok i see i see. i just watched this video, https://www.youtube.com/watch?v=MD_xiokVXwI and is this relevant or just like too far. would i explain it by saying, the issue becomes controlling electron spins, directing them so that they function accurately as a single integrated circuit? or is this just too far, and too different to what we just spoke about? im sorry to keep badgering you with these question, i am extremely interested, you are very patient ahhahah.

[jakesilove]

ohhhhh ok i see i see. i just watched this video, https://www.youtube.com/watch?v=MD_xiokVXwI and is this relevant or just like too far. would i explain it by saying, the issue becomes controlling electron spins, directing them so that they function accurately as a single integrated circuit? or is this just too far, and too different to what we just spoke about? im sorry to keep badgering you with these question, i am extremely interested, you are very patient ahhahah.

Probably a bit far

[Bubbly_bluey]

Hey! Essentially, it comes down to the fact that energy can be emitted/absorbed as discrete packets, rather than as waves. Planck solved the ultraviolet catastrophe by proposing that light (and thus energy) could come in phonons. This completely revolutionised our understanding of electromagnetic radiation, and a few years later Einstein would come and formalise the theory (think all-or-nothing principle, etc.). Additionally, Planck introduced the formula E=hf, from which we could accurately determine the energy/frequency of electromagnetic radiation. However, his calculation of the constant, h, was pretty far off (this would be refined later).

This is a broad overview of the topic area; basically, look at how Planck solved the Black Body problem. If you have any more specific questions, or want any clarification, let me know!

Thank you! But I'm still not sure how Planck had revolutionised our understanding of electromagnetic radiation. What was the scientist's understanding of it before Planck's idea came along? Was it just how they thought that energy emitted from the black body would be a continuous wave by classical physics?

[jamonwindeyer]

Thank you! But I'm still not sure how Planck had revolutionised our understanding of electromagnetic radiation. What was the scientist's understanding of it before Planck's idea came along? Was it just how they thought that energy emitted from the black body would be a continuous wave by classical physics?

The Classical Law that dictated the shape of a black body radiation curve is called Rayleigh Jean's Law, but you don't need to know that. The specifics of the classical theory aren't assessable

[katnisschung]

cries are they allowed to explicitly ask u what the meissner effect like its not in the syllabus
but i had like 1 point on it on my notes to explain the phenomenon for the magnet hover above
the superconductor like they actually had a 4 marker "explain the meissner effect" RIP PHYSICS MARK
RIP HSC RIP ATAR

[bsdfjnlkasn]

Hey there,

I'm just a bit confused about p-type conductors. Does any conduction occur in the conduction band within p-type conductors as all i'm gathering is that conduction only occurs in the dopant/acceptor level. This acceptor level accepts electrons from the valence band (located just below it) - so what's the relevance of the conduction band? How does this new acceptor band just form? Does electron-hole conduction occur here? Is this where electron-hole conduction occurs in intrinsic conduction? Because I thought that intrinsic conduction occurred between the conduction and valence bands. So why does a whole new band form in this case of extrinsic/doped semiconductors? Furthermore, I'm just confused about how we can consider there being a valence band in semiconductors when we always discuss things in terms of the lattice and how electrons are shared intermolecularly. How can these interactions form a broader band? I'm not too sure about this whole band and atomic level business works as I don't see how they're related and how the two explanations are equally valid as one another - if in fact not related at all. In terms of considering the valence band for semiconductors, how is the band formed and is there an analogy or something that can help me picture it? In exams, should we discuss band theory or the intermolecular electron exchanges (particularly for positive holes in p-type conductors).

I know that's a heaps and heaps of questions, but I hope they're easy to address/understand. I'm still getting my head around this whole semiconductor business (we're being assessed on it as a part of a research task, so haven't actually been taught it). As always, any help would be super super appreciated.

Thank you!!

[jamonwindeyer]

cries are they allowed to explicitly ask u what the meissner effect like its not in the syllabus
but i had like 1 point on it on my notes to explain the phenomenon for the magnet hover above
the superconductor like they actually had a 4 marker "explain the meissner effect" RIP PHYSICS MARK
RIP HSC RIP ATAR

Ouch! Really all they can ask you to explain is that it is the exclusion of magnetic flux from a superconductor below critical temperature - The exact workings of it for 4 marks certainly seems a bit much!! Don't worry though, it's not going to impact you in the long run!

[bsdfjnlkasn]

Hey I'm back again,

Sorry if this is a really stupid question but what's the difference between a diode, resistor and transistor? All i'm seeing is these terms thrown around but not many definitions - it will really help me piece together the content haha

Thank you!!

[jakesilove]

Hey I'm back again,

Sorry if this is a really stupid question but what's the difference between a diode, resistor and transistor? All i'm seeing is these terms thrown around but not many definitions - it will really help me piece together the content haha

Thank you!!

Hey! This is definitely not necessary to understand in the course, but it's still easy to give a quick overview.

A diode is a semiconductor device that essentially 'rectifies' the current. In english, it ensures that current can only travel in one direction, not two.

Transistors are semiconductor devices that are used as 'switches' or 'amplifiers' of current. That's exactly what it sounds like.

A resistor is just anything with a high enough resistance to ensure the system doesn't short circuit. Nothing semiconductor related here.

All of these devices have different physical make-ups in terms of Semiconductors, but actually the course doesn't require you to have any sort of understanding of the physical build of transistors! Only society and the environment, and the fact that semiconductors led to transistors

[bsdfjnlkasn]

Hey! This is definitely not necessary to understand in the course, but it's still easy to give a quick overview.

A diode is a semiconductor device that essentially 'rectifies' the current. In english, it ensures that current can only travel in one direction, not two.

Transistors are semiconductor devices that are used as 'switches' or 'amplifiers' of current. That's exactly what it sounds like.

A resistor is just anything with a high enough resistance to ensure the system doesn't short circuit. Nothing semiconductor related here.

All of these devices have different physical make-ups in terms of Semiconductors, but actually the course doesn't require you to have any sort of understanding of the physical build of transistors! Only society and the environment, and the fact that semiconductors led to transistors

Hey thanks so much Jake! Do you think you could possibly help me out with the questions I posted last night? There's a lot so don't worry if you don't have time


EDIT: I'm just a bit confused here about the specifics of the depletion zone when a p-n junction is established. Is it the same as the potential barrier? If not, when does the depletion zone form? Or is it like the potential barrier prevents charges from moving across the junction which then results in a depletion zone?

I also read in my textbook that after the potential difference is established across a p-n junction (after charges drift across junction to opposite ends) that the semiconductor becomes electrically charged. Is this right? Because for a substance to become charged, shouldn't charges be removed/added, not moved? Further, could I get an explanation of the equilibrium business that occurs just after the p-n junction is formed as i've read two different things. I've read that some electrons move back to the n-type and the holes to the p-type and this will continue to happen until the depletion region no longer has any charges passing through. I've also read that charges will diffuse across the p-n junction until the number of electrons (which are now on the p-type’s boundary) have accumulated a large enough electrical charge to repel/prevent any more charge carriers from cross over the junction. Which is correct and why?

Finally is the following note correct? If a voltage is applied to the p-n junction, it will act as a diode, allowing current to flow from p --> n
Are we talking about an external voltage and when there is forward bias?


Thanks again

[jamonwindeyer]

I'm just a bit confused here about the specifics of the depletion zone when a p-n junction is established. Is it the same as the potential barrier? If not, when does the depletion zone form? Or is it like the potential barrier prevents charges from moving across the junction which then results in a depletion zone?

The depletion zone is the area within which charge carriers have all recombined with atoms (charge carriers have been depleted) - This indeed sets up a potential difference which eventually stops more charge carriers from moving and depleting! When this happens, we call the balance equilibrium so I suppose it is the depletion region that sets up the potential barrier

I also read in my textbook that after the potential difference is established across a p-n junction (after charges drift across junction to opposite ends) that the semiconductor becomes electrically charged. Is this right? Because for a substance to become charged, shouldn't charges be removed/added, not moved?

Before the N and P type semiconductors are brought together, they aren't charged. P and N type semiconductors might have additional electrons/holes, but the charge is still neutral (the protons in the atoms balance everything out). Now, when we bring the two together, we've got negative and positive charges moving around. They were neutral to begin - So once everything has moved, their must now be regions of charge. Specifically, the electrons that drift to the P-type semiconductor make it negatively charged. The holes that drift to the N-type semiconductor make it positively charged.

Further, could I get an explanation of the equilibrium business that occurs just after the p-n junction is formed as i've read two different things. I've read that some electrons move back to the n-type and the holes to the p-type and this will continue to happen until the depletion region no longer has any charges passing through. I've also read that charges will diffuse across the p-n junction until the number of electrons (which are now on the p-type’s boundary) have accumulated a large enough electrical charge to repel/prevent any more charge carriers from cross over the junction. Which is correct and why?

I'd say the second is correct more than the first, because electrons move to the P-type, not the N-type. Does a better job explaining it. But it sounds like both are trying to say the same thing

Finally is the following note correct? If a voltage is applied to the p-n junction, it will act as a diode, allowing current to flow from p --> n
Are we talking about an external voltage and when there is forward bias?

Yep - The junction itself is a diode, and yes, applying an external voltage to forward bias it will allow current to flow!

[katnisschung]

why does a charge that enters a magnetic field at a right angle start to move in a circular path?

[jamonwindeyer]

why does a charge that enters a magnetic field at a right angle start to move in a circular path?

Hey! Because by the right hand grip rule, the force on the charged particle is perpendicular to its motion. A force perpendicular to motion is what we get for uniform circular motion - Compare it to the direction of the force of gravity in an orbit

[kiwiberry]

Hey guys, I'm kinda confused about how solar cells work. I get that at the p-n junction, electrons flow from n to p-type, setting up a potential difference which eventually prevents further flow of charges, forming the depletion zone. Does light then strike the electrons on the surface of the p-type or those in the n-type? I'm getting conflicting explanations from different sources and I'm really confused. Also, is only the n-type exposed to sunlight? If someone could give me a brief run down on how solar cells work that would be great 😅

[jamonwindeyer]

Hey guys, I'm kinda confused about how solar cells work. I get that at the p-n junction, electrons flow from n to p-type, setting up a potential difference which eventually prevents further flow of charges, forming the depletion zone. Does light then strike the electrons on the surface of the p-type or those in the n-type? I'm getting conflicting explanations from different sources and I'm really confused. Also, is only the n-type exposed to sunlight? If someone could give me a brief run down on how solar cells work that would be great 😅

Hey! So the electric field/potential difference set up by the depletion zone is directed from the N type to the P type, due to the excess positive charge in the N-type and the excess negative charge in the P-type. This sets up an electric field which pushes positive charges/holes towards the P-type, and negative charges/electrons towards the N-type. It doesn't really matter where it happens (just imagine it happening somewhere in the depletion zone), but when a photon of appropriate frequency strikes the diode, it frees an electron (thus also forming a hole). This is called an electron-hole pair. The electric field pushes electrons into the N-type, and holes towards the P-type - This constitutes the flow of current.

This video does a great job of explaining it

This is a highly simplified version of what actually happens, mind you. Solar cells aren't actually just sticking a P and an N type semiconductor together and shining a light on it. But this is a perfectly acceptable description for HSC Physics