HSC Physics: Suggested Answers and Discussion

[and1_98]

Had to really think on my feet for the last one!
 
Got the calculation; my explanation for the 'open switch' part wasn't too sound though.

Had to go back and essentially guess a majority of the multiple choice due to time constraints towards the end... they were insanely difficult under pressure.

Thought a lot of the questions (bar the Transformer/society and 6 mark recount of 3 technologies) actually required a sense of understanding of concepts regarding the course; which is what I like to see.

A decent exam - hard mc - and a lot to do in 3 hours

[jamonwindeyer]

Correct me if wrong, but I though 16 was A because mechanical movement creating electrical energy (generator) and the right side of coil would be north to attract to south and reduce motion (Lenz's Law), and by right hand grip rule would be A?

I agree, there was no power supply, and the cone is pushed providing a current, therefore it is a generator. I disagree with the current direction though- we are looking at conventional current, so the thumb should point to the left, and the fields go from the north to south, therefore towards the center, therefore current should be going down on the front side, and travel from X to Y (conventional). So I would say A is the correct answer. (Hope it is anyway). I think the initial answer C is correct for the elevator- we experience weightlessness as the elevator slows, which implies a lower downwards acceleration, therefore it should not fall as fast.

You guys (and others) are totally right, thanks for that spot! I totally missed it! I've made that change, legends!

Elevator is also incorrect

Care to justify? Jake and I are pretty confident on that one

If the magnets were in the same direction, it would be stronger, but the magnets have their poles near each other- so the fields would be very small and only go from N to S in small contained areas- hence very minimal interaction with coil.

Great answer Platypus, thanks heaps!

[ParallelPlatypus]

I agree, it was an exam which rewarded holistic conceptual preparation, not memorization (except for the three discoveries and applications, but there was plenty to draw from in the course- I did semiconductors/transistors, SQUID superconductor device, and generators, lots to talk about). I really liked the exam, came out feeling very accomplished.

[bohimei]

How is 10/(5.2x10^4) = 0.1 mT ?

[ParallelPlatypus]

Yeah it should be around 1.92*10^-4 T, they probably put 5*2 in calculator thinking the dot was a multiplication.

[Swagadaktal]

Fuck I'm from VCE - took a quick look at your exam and holy shit its wayyyy harder than VCE physics holy fuck

Just a q: q23b the field applied has to go upwards coz the electron is moving to the right - so current is going to the left (conventional current) , electric field going into page so the force is going downwards (using RH Slap)? So to make sure it doesnt go downwards the field has to go upwards?     

(like im just learning here not questioning solutions or anything )

[jamonwindeyer]

How is 10/(5.2x10^4) = 0.1 mT ?

Yeah it should be around 1.92*10^-4 T, they probably put 5*2 in calculator thinking the dot was a multiplication.

Cheers guys, rounding error, changed to 0.2mT which is the same as the answer above if you round to that level of significance

[bluealder]

I agree, there was no power supply, and the cone is pushed providing a current, therefore it is a generator. I disagree with the current direction though- we are looking at conventional current, so the thumb should point to the left, and the fields go from the north to south, therefore towards the center, therefore current should be going down on the front side, and travel from X to Y (conventional). So I would say A is the correct answer. (Hope it is anyway). I think the initial answer C is correct for the elevator- we experience weightlessness as the elevator slows, which implies a lower downwards acceleration, therefore it should not fall as fast.

Yeah my bad, messed up the grip rule

[jamonwindeyer]

Fuck I'm from VCE - took a quick look at your exam and holy shit its wayyyy harder than VCE physics holy fuck

Just a q: q23b the field applied has to go upwards coz the electron is moving to the right - so current is going to the left (conventional current) , electric field going into page so the force is going downwards (using RH Slap)? So to make sure it doesnt go downwards the field has to go upwards?     

(like im just learning here not questioning solutions or anything )

Hey Swag it was a brutal one this year!

Force on electron due to electric field goes out of the page, since the field goes into the page. Using Right Hand Slap, the magnetic field needs to be upwards to generate an oppositional force into the page

[Evan123]

For Q17 I don't understand the reasoning behind it being A. The elevator is travelling up and slowing down meaning the acceleration of the elevator and therefore the ball is down. This downwards acceleration on top of the downwards acceleration of gravity causes an increase in acceleration downwards compared to normal gravity and therefore is a steeper parabola.

Hope that made sense

Edit: I don't understand why it is C (I think it is A)

[gabe.ralph]

Reasonable test, can I ask why Q15 could not be C. Whoops thought Q21 was 8000,000 from the surface and calculated earths radius then added that in as well.
 
 

[Lockyb98]

Would Q17 be A? As the elevator is slowing down as it reaches the TOP floor therefore time in flight would be shorter?

[bohimei]

I'll be honest I was genuinely feeling nervous today morning, but when I saw the paper it was such a relief. How would you guys compare this paper's difficulty with the other recent ones? Just wanna check band thresholds

[ParallelPlatypus]

Imagine you are in an elevator, when you first take off from the ground, you experience a stronger downwards 'gravity'- just like a rocket ship taking off, this would cause the path shown in A. Just as you decelerate when you reach the top, imagine you are in the elevator, you get the fluttering feeling in the stomach, this is due to the feeling of lowered 'gravity', or a lower perceived weight. This lower weight means the flight will be longer. Pretty confident on this one. As for 15, I found this one hard... I think it should be A, I agree with the solutions, after lots of last minute research last night- light strikes the n or p type semiconductor around the boundary, creates the electron, hole pair (by exciting the electron out of the valence band), then the pn junction local electrical field pulls the electron towards the n type and the hole towards the p type. The light isn't changing the voltage, it is simply facilitating the electric field of the depletion zone achieving this.

[bohimei]

Would Q17 be A? As the elevator is slowing down as it reaches the TOP floor therefore time in flight would be shorter?

Time of flight is irrelevant here. The elevator is still moving up BUT slowing down, the ball is NOT slowing down along with the elevator. Because the ball isn't slowing down it'd actually move up relative to the elevator which is slowing down. Hence the dotted line above the solid line.