HSC Physics Question Thread

[S200]

Why are superconductors used on a maglev train?

Heat dissapation?
Imagine how hot the electromagnets would get!
I know that the Synchrotron  uses water-cooled copper pipes in most of their magnets, but even they still need superconductors for some operations...

[Jane20]

Hi guys
Can anyone please help me with this question!

How do you determine the force between 2 parallel current carrying conductors Experimentally ?

Thank you

[blasonduo]

Why are superconductors used on a maglev train?

Ordinary trains need to use wheels and thus are susceptible to static friction. The friction removes energy from the system which will eventually cause the train to stop. As maglev trains do not, they require less energy to run, making it both more energy efficient and faster. It also removes the constant screeching from the tracks and reduced noise pollution for nearby residents.

Hope this helps!

[dermite]

Hi guys
Can anyone please help me with this question!

How do you determine the force between 2 parallel current carrying conductors Experimentally ?

Thank you

use
F = Kl I₁I₂/d²

[jamonwindeyer]

Hi guys
Can anyone please help me with this question!

How do you determine the force between 2 parallel current carrying conductors Experimentally ?

Thank you

Hey! To expand on dermite's answer, if you pass a known current through two wires of known length, a known distance apart, then the force is the only unknown in the formula! That can be used for verification. You could also place one wire on a scale, and measure how its weight changes when another wire carrying the same current is hung just above the first wire! This second method is a direct measurement and so probably more like what they are after, potentially?

[Jane20]

Hey! To expand on dermite's answer, if you pass a known current through two wires of known length, a known distance apart, then the force is the only unknown in the formula! That can be used for verification. You could also place one wire on a scale, and measure how its weight changes when another wire carrying the same current is hung just above the first wire! This second method is a direct measurement and so probably more like what they are after, potentially?

I think the 2nd method would work since the question asks for measurement from experiment.
Thank you !

[Jasoon]

Hi All,
Could you please help me with this dilemma. So I've researched some stuff on induction cooktops and in some notes they say the heat is generated more efficiently due to a higher resistance in the pot because of resistive heating between atoms and eddy currents? But in the 2012 HSC for multiple choice (question 19) they say the heat is better generated due to a lower resistance as P=I^2R and if R is small then I will be large as V=IR and thus power will be high leading to a greater heat?

[envisagator]

Hi All,
Could you please help me with this dilemma. So I've researched some stuff on induction cooktops and in some notes they say the heat is generated more efficiently due to a higher resistance in the pot because of resistive heating between atoms and eddy currents? But in the 2012 HSC for multiple choice (question 19) they say the heat is better generated due to a lower resistance as P=I^2R and if R is small then I will be large as V=IR and thus power will be high leading to a greater heat?

Welcome to the forums Jasoon!!!!

So the first part of what you said is correct, that is, heat is generated more efficiently with increased resistive heating from eddy currents. The tricky part of this MC is to consider: V=IR, where 'I' is eddy current which you want to make very large so that heat generated by resistive heating is high. But if the resistance is high then the current will be low (inversely proportional), therefore, the resistance must be low for the current to be high to create a more efficient cooktop. Hence, answer is C

Hope this helps

[jamonwindeyer]

Hey!! To expand on the answer above, it is really important to remember that induction produces a voltage, not a current. So when we are using a cooktop, we only have a set potential difference to work with. So, if we want lots of eddy currents, we need a fairly low resistance. And we want lots of eddy currents because the heating effects come from \(P=I^2R\) - The current is squared, so it being larger is more important!

[Fergus6748]

Hey guys, I'm having a bit of trouble with this, how would you approach answering this question:

"Discuss the relationship between thought and reality"

Thanks a bunch

[jamonwindeyer]

Hey guys, I'm having a bit of trouble with this, how would you approach answering this question:

"Discuss the relationship between thought and reality"

Thanks a bunch

Hey! This is the Physics Q+A not Philosophy Q+A

... Kidding, what a weird question! I think it is a play at the Einstein's thought experiment stuff. My response would be:

- Thought experiments can be used to formulate aspects of our reality long before experimental evidence exists.
- Rattle off a bunch of evidence based on Einstein's thought experiments, and the later experimental proof of those theories
- Conclude

[S200]

Hey! This is the Physics Q+A not Philosophy Q+A

... Kidding, what a weird question! I think it is a play at the Einstein's thought experiment stuff. My response would be:

- Thought experiments can be used to formulate aspects of our reality long before experimental evidence exists.
- Rattle off a bunch of evidence based on Einstein's thought experiments, and the later experimental proof of those theories
- Conclude

Further on this, I would particularly use the trains TE that kinda well describes length contraction and everything else.

[Jasoon]

Hey, envisagator and Jamon
Thanks for your reply. I think the actual resistance that's used is a moderate resistance but I can nowsee why a low resistance is preferred over a high.

I also have another question  . Does the meisnner effect involve any eddy currents (creating a magnetic field that opposes the external magnetic field) at all, and how the magnet levitates? Or is it just the total exclusion of a magnetic field and no eddy currents are induced from an emf, because to have eddy currents there must be a change in magnetic flux, and if the magnet is levitating there isn't any. I've read up on some theory and experiments like quantum pinning (which exceeds the HSC I believe) and how if a magnetic was placed on a superconductor that was cooled down to it's critical temp it would levitate, showing that although there is no change in magnetic flux the magnet still levitates.

Thanks

[jamonwindeyer]

Hey, envisagator and Jamon
Thanks for your reply. I think the actual resistance that's used is a moderate resistance but I can nowsee why a low resistance is preferred over a high.

I also have another question  . Does the meisnner effect involve any eddy currents (creating a magnetic field that opposes the external magnetic field) at all, and how the magnet levitates? Or is it just the total exclusion of a magnetic field and no eddy currents are induced from an emf, because to have eddy currents there must be a change in magnetic flux, and if the magnet is levitating there isn't any. I've read up on some theory and experiments like quantum pinning (which exceeds the HSC I believe) and how if a magnetic was placed on a superconductor that was cooled down to it's critical temp it would levitate, showing that although there is no change in magnetic flux the magnet still levitates.

Thanks

Very welcome!

So as you say it is a totally separate effect, nothing to do with eddy currents. You don't need to know why the flux is excluded, just that it happens, and that we call it Meisner Effect and yep, all that quantum pinning stuff is beyond the course as well, the syllabus requirements here are actually pretty simplistic

[Jane20]

Hi guys can someone pls help me with this question ?

Given an electron with its quantum wavelength is 4.75x10^(-9). Calculate its velocity.
For this i use de Broglie's equation
But then part b) it says
Find the quantum frequency of the electron.
For this question, why do we have to use light velocity c=fx(wavelength) instead of the actual velocity of the electron (calculated from part a) ?

Thank you