HSC Physics: Suggested Answers and Discussion

[Happy Physics Land]

I feel like question 20 is D, if it were DC it would be continuously clockwise, but with  AC - the brushes dont change the sides every half turn . unless they do .

But because the wire forms solenoids around the two blocks near the rotor coil, AC is also fed into that and the solenoids become electromagnets. So whilst AC is converted into DC by commutator, the polarity of the electromagnets would switch every now and then and hence it would work like a motor rotating in a clockwise direction as predicted by right hand palm rule.

[gstaah]

uh so for the 6 marker on discoveries that lead to new technologies. i put Bragg's experiment creating a better understanding of lattice structures within crystals which enabled the development of superconductivity to occur -> bcs theory  -> MRI but idk if this is correct or i would get any marks?

[bethjomay]

I found the main section ok, but multiple choice and Quanta to Quarks were hard! I got really lost in the calculation for the option. What are people thinking for a band 6 cut off?

[bethjomay]

uh so for the 6 marker on discoveries that lead to new technologies. i put Bragg's experiment creating a better understanding of lattice structures within crystals which enabled the development of superconductivity to occur -> bcs theory  -> MRI but idk if this is correct or i would get any marks?

That sounds good as part of an answer! Did you talk about generators and one other technology as well?

[Aliceyyy98]

Can someone plz explain mc q20 i dun understand? Also for q30b wat would the three marks be allocated to??

[ParallelPlatypus]

For Q20, this is actually the exact design of a "universal motor"- which was part of the Motors and Generators topic, so you can research that online if you wish. Basically, every time the AC switches, the magnetic solenoids also swap, which means the force is in the same direction. So essentially, the AC acts as a DC current.

[ParallelPlatypus]

For 30b, I would guess maybe one for identifying Back EMF/Lenz's law as a source of resistance in the coil, one for explaining what happens when the switch opens, and another for explaining how this effects the falling object. It is a pretty generous 3 marks (in my opinion).

[bethjomay]

For 30b, I would guess maybe one for identifying Back EMF/Lenz's law as a source of resistance in the coil, one for explaining what happens when the switch opens, and another for explaining how this effects the falling object. It is a pretty generous 3 marks (in my opinion).

I had a brain snap and said that the back emf would be greater when the switch was open as there would be no load on the circuit.

[gstaah]

That sounds good as part of an answer! Did you talk about generators and one other technology as well?

yeah i talked about faraday-> generators and eintsteins photoelectric effect -> solarcells

[stateofmind]

I am pretty sure the answer to MC qu 17 is A, not C as suggested. An upwards deceleration is equal to downwards acceleration. As a= (v-u)/t, the increased value of acceleration means flight time is LESS, not greater. As horizontal range=velocity x time, the reduced time means horizontal range is also reduced, as illustrated in diagram A. 

[jamonwindeyer]

uh so for the 6 marker on discoveries that lead to new technologies. i put Bragg's experiment creating a better understanding of lattice structures within crystals which enabled the development of superconductivity to occur -> bcs theory  -> MRI but idk if this is correct or i would get any marks?

Sounds awesome to me!

I found the main section ok, but multiple choice and Quanta to Quarks were hard! I got really lost in the calculation for the option. What are people thinking for a band 6 cut off?

I'm feeling low 80's for a Band 6 cut-off, because the MC was brutal and the paper on the whole was on the difficult end of the scale

For Q20, this is actually the exact design of a "universal motor"- which was part of the Motors and Generators topic, so you can research that online if you wish. Basically, every time the AC switches, the magnetic solenoids also swap, which means the force is in the same direction. So essentially, the AC acts as a DC current.

Universal Motors aren't specifically part of the syllabus, but those who touched on them probably would have found the question a little easier than deriving from first principles

[jamonwindeyer]

I am pretty sure the answer to MC qu 17 is C, not A as suggested. An upwards deceleration is equal to downwards acceleration. As a= (v-u)/t, the increased value of acceleration means flight time is LESS, not greater. As horizontal range=velocity x time, the reduced time means horizontal range is also reduced, as illustrated in diagram C. 

You are correct about the upwards deacceleration being a downwards acceleration, but it's not the ball that is accelerating. It is the lift, our frame of reference! If our frame of reference is accelerating downwards, that's like the ball accelerating upwards! It's a test of relativity. Ever been in a lift that went down really quick and you feel a bit lighter? It's just like that!

[Sanaz]

LMAO I MENTIONED BACK EMF hahaha for 30b.... thought i was wrong...

[jamonwindeyer]

LMAO I MENTIONED BACK EMF hahaha for 30b.... thought i was wrong...

Nah you got it!

[Neutron]

How well do you think we needed to explain the 3 technologies question? Cause my mind blanked and the only one i could think of was Maglev trains (discovery being superconductors) but I just said that they provided high speed transportation for society through superconductive magnets without explaining the specifics :O (i don't actually know the specifics, damn my brain couldn't function during that exam)