HSC Physics Question Thread

[jamonwindeyer]

Hi! For Question 9, the answers say C - how is this true, because shouldn't there still be eddy currents generated in R but just broken - so R will still experience some resistance though just not as much as Q (I've probably asked about this before earlier in the year but I forgot  )

Regarding Q8, how can you tell whether the source of power is AC/DC - this one said that it was DC (is there a different symbol used to show AC power sources?)

And lastly, I don't understand the answers for Q15; shouldn't electrons/ holes move in opposite directions? - The Answer is C.

Thanks in advance~ 

Hey! For Q9, the eddy currents would be there, but the effect is thought to be negligible. Thus you can ignore them in this circumstance

AC sources normally have a circle with a squiggly line; DC sources are almost always two straight lines in HSC Physics, one longer than the other, with that longer line representing the positive terminal. Otherwise, you look for a '+' or a '-', or both, to indicate the DC source and its direction

[FallonXay]

Hi for your last question- the electrons and holes move in opposite directions in an electric field- not necessarily in a magnetic field though (though they can). If you use your right hand palm rule for the conventional current (movement of positive charge) youll find there is a downwards force on the 'holes'. Now electron flow is in the OPPOSITE direction to conventional current. So when you use your left hand palm rule (or whatever youve been taught to use for the flow of a negative charge) you point ur thumb in the OPPOSITE direction to the conventional current and youll find that the electrons ALSO experience a downwards force.
Thus both holes and electrons move to the bottom of the rod ie. the same side of the rod.
Therefore C. hope this helps

Thanks - the downward force on the holes makes sense
But with the (negative) electrons, I always thought that you still use the right hand palm rule but the thumb just points in the opposite direction to the conventional current - thus giving an upward force? Sorry, I don't quite understand this part, would you be able to elaborate? Thanks a ton!!!

[jamonwindeyer]

Silly question, but when using a vector diagram to show the net resultant force should you put in lengths of each side or angles or not??

Hey hey! Not a silly question at all. If it requests it and/or those sides/angles are self apparent or easily found, sure! But if not don't worry. Essentially, don't go into much detail unless you have to. Usually vector diagrams are just testing how well you understand the different forces at play and what direction they act in

[jamonwindeyer]

Thanks - the downward force on the holes makes sense
But with the (negative) electrons, I always thought that you still use the right hand palm rule but the thumb just points in the opposite direction to the conventional current - thus giving an upward force? Sorry, I don't quite understand this part, would you be able to elaborate? Thanks a ton!!!

This is true! With electrons you do point the thumb in the opposite direction, but the issue is that we have TWO things that have swapped. The electron charge is opposite in sign to the holes, AND they are moving in the opposite direction. These two things together mean that the forces actually both end up in the same direction. So you point your thumb in the opposite direction for the charge, and then again because the electrons are moving in the opposite direction. So really, it doesn't switch at all, hence the same force direction

[maksim]

Hi, I was just wondering what the expected and maximum g-forces would be for an astronaut during a rocket launch.

[Goodwil]

Why is the answer A and not C here?

[imtrying]

Could someone explain how angles are measured for force vs torque because this always confuses me

[jakesilove]

Hi, I was just wondering what the expected and maximum g-forces would be for an astronaut during a rocket launch.

Hey! Check out the table at the bottom of this page! Although you definitely don't need to know this

[jakesilove]

Why is the answer A and not C here?

Hey! Check out my answer here

[jakesilove]

Could someone explain how angles are measured for force vs torque because this always confuses me

Always confused me as well! For force, we're generally talking about a wire, right? So the angle there is easy; just imagine what the field lines look like, and then see how those lines 'interact' with the wire. Usually, it will just be perpendicular, or zero! Simply draw the field lines, and stick the wire through it, and look at the angle made between them

For torque, we care about net rotation. So, what's actually rotating? The loop! It wouldn't really make a huge amount of sense to talk about the torque on a single wire, as that wire isn't turning. So, if we care about the loop as a whole, then the 'plane' we look to is the plane of the loop. Imagine a the loop as a surface, and then look at how the field lines interact with that surface. When the loop is at it's starting point, it's like the field lines are running in the same plane as the loop. When the loop is perpendicular to it's starting position, the field lines are perpendicular to the plane of the loop! So, that's the angle we're talking about. It's seriously confusing. I'm still confused.

[imtrying]

Always confused me as well! For force, we're generally talking about a wire, right? So the angle there is easy; just imagine what the field lines look like, and then see how those lines 'interact' with the wire. Usually, it will just be perpendicular, or zero! Simply draw the field lines, and stick the wire through it, and look at the angle made between them

For torque, we care about net rotation. So, what's actually rotating? The loop! It wouldn't really make a huge amount of sense to talk about the torque on a single wire, as that wire isn't turning. So, if we care about the loop as a whole, then the 'plane' we look to is the plane of the loop. Imagine a the loop as a surface, and then look at how the field lines interact with that surface. When the loop is at it's starting point, it's like the field lines are running in the same plane as the loop. When the loop is perpendicular to it's starting position, the field lines are perpendicular to the plane of the loop! So, that's the angle we're talking about. It's seriously confusing. I'm still confused.

Thank you, less confused now:)

Alas, the confusion continues...how do I do question 18 of this paper https://www.boardofstudies.nsw.edu.au/hsc_exams/2014/pdf_doc/2014-hsc-physics.pdf (sorry i couldn't upload a photo here's the link to the 2014 HSC)

[JemexR]

Hi, could someone explain the differences between the four options for the HSC 2010 Q20 Multiple choice? I can understand how to identify which way the wire would turn given both the direction of the current and the direction of the magnetic field, but I cannot differentiate at this point how the diagrams differ.  THanks

https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2010exams/pdf_doc/2010-hsc-exam-physics.pdf

[JemexR]

Alas, the confusion continues...how do I do question 18 of this paper https://www.boardofstudies.nsw.edu.au/hsc_exams/2014/pdf_doc/2014-hsc-physics.pdf (sorry i couldn't upload a photo here's the link to the 2014 HSC)

Given that cathode rays are electrons, you have to use whichever hand rule you were taught for the electron (motors), from this, you can deduce that the electron will therefore move "down" due to the given magnetic field. Therefore, in order to counteract this movement, the electric field's direction must be "down" as well, given that the electron will be attracted to the positive terminal.

[imtrying]

Given that cathode rays are electrons, you have to use whichever hand rule you were taught for the electron (motors), from this, you can deduce that the electron will therefore move "down" due to the given magnetic field. Therefore, in order to counteract this movement, the electric field's direction must be "down" as well, given that the electron will be attracted to the positive terminal.

Thanks!

[jakesilove]

Thank you, less confused now:)

Alas, the confusion continues...how do I do question 18 of this paper https://www.boardofstudies.nsw.edu.au/hsc_exams/2014/pdf_doc/2014-hsc-physics.pdf (sorry i couldn't upload a photo here's the link to the 2014 HSC)

So, using the right hand rule, we can find the force on the electron. Make sure to point your thumb to the left, because it should be opposite to electron flow. That should get you a force DOWNWARDS due to the magnetic field.

Now, we what to apply an electric field to counteract the change, so we will put the positive electrode on top (to ATTRACT the electron). Electric field lines are drawn positive to negative, so the direction will be downwards.