HSC Physics Question Thread

[beau77bro]

It might just be a mistake. This is from the sample answers for the 2015 HSC paper, question 26c).

are u sure it wasnt r + a?

[blasonduo]

Hey guys can I please have some help with these questions?
BTW i was also hoping someone could explain the difference between magnetic flux and magnetic flux density?
Thanks!

ooh! This place is backlogged with questions!

I shall help with the electromagnets

The first step is to figure out the direction of the magnetic field (either left to right of right to left)

From the positive terminal you can see that the magnets are going to be ELECTROMAGNETS, you can see that it wraps around the LEFT block in clockwise way, You know it has a SOUTH facing pole (I use Nanti-Socks to remember )

To double check, we'll do the other side, from the NEGATIVE terminal, this time, it is travelling in an ANTI-clockwise way (remember to look at it standing in the middle) This means that the RIGHT block is the NORTH pole. So the magnetic field lines follow from RIGHT to LEFT.

The next part is the simple right hand palm rule, as you know the + and - terminals, follow the current lines until the right option works. In this case, B is the only one that rotates clockwise. (Left side coil current goes up (so thumb up!) and magnetic field right to left (so fingers to right) shows the palm upwards and thus it rotating clockwise.

Basically, this question is asking about your ability to figure out electromagnets, As B and D are almost the same, where D's RIGHT block is coiled the wrong way!

[blasonduo]

It might just be a mistake. This is from the sample answers for the 2015 HSC paper, question 26c).

Hmm, I just went over the question, and the answers do account for the radius of the earth, if you are unsure, would you like me to show the working?

[hobocop]

Hmm, I just went over the question, and the answers do account for the radius of the earth, if you are unsure, would you like me to show the working?

Hmm, I just went over the question, and the answers do account for the radius of the earth, if you are unsure, would you like me to show the working?

Is this not just accounting for altitude? (Not sure if my solutions are same as yours)

[CyberScopes]

ooh! This place is backlogged with questions!

I shall help with the electromagnets

The first step is to figure out the direction of the magnetic field (either left to right of right to left)

From the positive terminal you can see that the magnets are going to be ELECTROMAGNETS, you can see that it wraps around the LEFT block in clockwise way, You know it has a SOUTH facing pole (I use Nanti-Socks to remember )

To double check, we'll do the other side, from the NEGATIVE terminal, this time, it is travelling in an ANTI-clockwise way (remember to look at it standing in the middle) This means that the RIGHT block is the NORTH pole. So the magnetic field lines follow from RIGHT to LEFT.

The next part is the simple right hand palm rule, as you know the + and - terminals, follow the current lines until the right option works. In this case, B is the only one that rotates clockwise. (Left side coil current goes up (so thumb up!) and magnetic field right to left (so fingers to right) shows the palm upwards and thus it rotating clockwise.

Basically, this question is asking about your ability to figure out electromagnets, As B and D are almost the same, where D's RIGHT block is coiled the wrong way!

Can you please explain how you determined the poles of the electromagnets? Is there a rule to determine this?

[jamonwindeyer]

Thank you! What about the case of slits in a metal sheet, the eddy currents are somewhat significant there right?
So now it is my understanding that rings require the entirety of the ring for a significant current to flow whereas sheets and cubes (e.g. transformers) can have slits. Would this be correct?

Somewhat, but lessened significantly. Whether you still want to say they exist is really up to how pedantic you want to be

Hey there!

Got a question why does a three-phase generator use electricity to generate electricity? Makes no sense if someone could help.

Thank you very much!

I assume you mean the current we use setting up the magnetic field? Basically, we get out more electricity than we put in setting up that field

Hey could someone explain how transistors work? don't get the whole current flowing through emitter and base and collector thing..

You actually don't need to explain how a transistor works in the HSC course! You just need to know what they are used for - An electrical switch, or an amplifier

[jamonwindeyer]

Hey there,

could anyone please explain what eddy currents are and how we can determine its direction?

Hi Julia! Eddy currents are loops of induced current formed in conductors in response to a changing magnetic field. Remember that induced eddy currents flow to create a new magnetic field, to oppose the original change in magnetic field. We determine their direction using the right hand grip rule.

If we hold our right hand in a thumbs up/grip, then the thumb represents where we need the north pole to be. Our fingers represent the direction of the eddy current loop

For the PE effect of this question, I would discuss how Einsteins ability to explain the PE effect with the quantisation of energy which classical physics could not - evidence for quantum theory and dual wave particle model for light. However I'm not sure what I would discuss for the special relativity theory - what did it change about the model of light, besides the fact that it is constant in all frames of references?

You've nailed it, we originally thought that light travelled in a medium and that its speed would be affected by relative motion in that medium - Einstein's Special Theory establishes the fact that \(c\) is a constant. That's a change in the model

can someone explain photovoltaic cells please? i dont get the directions and attractions -the photons strike the n-p junction and the electrons travel where and why? how does it create a current from this?

i get the electrons are emitted by the photoelectric effect and travel towards the n-type since it is positively charged by the diode's depletion zone. but why does that create a current throuhgout? its only migrating a short distance? or does it go around the whole circuit? confused

It goes around the whole circuit, you need to think of the depletion zone like a voltage source. It is setting up an electric field which 'pushes' electrons around the circuit, just like a voltage source would. The electric field is directed from N to P, so electrons will flow in the opposite direction around the whole circuit

[pikachu975]

This question so hard. From qanta to qwarks can someone help me with an example of how they would answer it? TIA

(Image removed from quote.)

Both fission and fusion result in less mass of products hence the extra mass is converted into energy (E=mc^2) and released

[arunasva]

Hey yo, any help for this question is appreciate. Guess this is  the last time I'm askin for help here

[b ]the picture might be too small check out question 12 here http://www.acehsc.net/wp-content/uploads/TrialPapers/2011/Physics/2011_Physics_-_James_Ruse_Trial_with_Solutions.pdf  [/b] 

[jamonwindeyer]

Hey yo, any help for this question is appreciate. Guess this is  the last time I'm askin for help here

[b ]the picture might be too small check out question 12 here http://www.acehsc.net/wp-content/uploads/TrialPapers/2011/Physics/2011_Physics_-_James_Ruse_Trial_with_Solutions.pdf  [/b] 

Hey! Induced emf (E) is proportional to rate of change (or gradient) of magnetic flux. Magnetic flux has two regions where it is changing, both linearly. Since both changes are linear, their rate of change is constant, so we'll get a constant induced emf. This narrows down to A or B.

The rate of change of flux at the tail end of the graph is greater - Steeper gradient. So, this will cause a larger induced emf, in the opposite direction to what we had initially. The answer is A

[arunasva]

Hey! Induced emf (E) is proportional to rate of change (or gradient) of magnetic flux. Magnetic flux has two regions where it is changing, both linearly. Since both changes are linear, their rate of change is constant, so we'll get a constant induced emf. This narrows down to A or B.

The rate of change of flux at the tail end of the graph is greater - Steeper gradient. So, this will cause a larger induced emf, in the opposite direction to what we had initially. The answer is A

Thanks man !

[hobocop]

Hi, could I please get an explanation on why this is C?

Thanks.

[beau77bro]

Keep in mind that the data can be localised because the magnetic field is slightly different in every voxel, as set up by the gradient coils - So we can use this to figure out where our readings are coming from

wait so does the mri measure the change in Magnetic vector or the re-emitted radio waves? how do they make the image? so the radio wave returns in the specific larmor frequency set up by the gradient coils and what information do they carry? oh wait is the magnetic vector just a way of explaining the time taken for the relaxation to happen?

[austv99]

Hey! Induced emf (E) is proportional to rate of change (or gradient) of magnetic flux. Magnetic flux has two regions where it is changing, both linearly. Since both changes are linear, their rate of change is constant, so we'll get a constant induced emf. This narrows down to A or B.

The rate of change of flux at the tail end of the graph is greater - Steeper gradient. So, this will cause a larger induced emf, in the opposite direction to what we had initially. The answer is A

Isnt the induced emf equal to negative change in flux over change in time, meaning the graph should be flipped?

[austv99]

Would appreciate if someone could help me clarify these questions.
Braggs question: I'm not too sure how the interference pattern produced in the experiment exactly led them to conclude the crystal lattic structure. Is it because it showed a geometric/periodic structure? Is there any significance with the contructive and deconstructive interferences produced?

Quanta to Quarks: Would I includes Chadwicks discovery of the neutron and the corresponding explanation by conservation laws, then fermis demonstration of nuclear fission, increasing the understanding of the ability to break up the atomic structure for energy production. Also, would i include that he used paulis proposal of the neutrino to comprehensively explain beta decay?

TIA

Bump