NEW Physics Exam Added (1 Viewer)

[Duxify]

Thought I would start a new thread since the other one was getting quite long....

I have just added a 20 question physics exam (there are now 3). I think this is the hardest yet... I am trying to make them harder as we get closer to exam time.

So far I have only had 7 people take Exam 2 (understandable since you were all studying hard for English). No perfect scores yet, but the top score so far on is 18/20. I will put up feedback on this exam when I get a few more people take it.

For those of you who don't know about it yet, I have launched a free online HSC resource at www.duxify.com.

It allows you to take online HSC style physics (and chem) exams and be ranked vs other students who have taken the same exam (rather than just your school).

Also, if you missed it on the other thread, below is updated feedback from exam 1.

Feedback from Exam 1

63 Participants

Taking out obvious outliers, the average was 13.4.

There were only 3 perfect scores (20/20).

The hardest questions were:

Q20- Meisner effect (39% correct)
Q14- AC generator vs motor (56% correct)
Q6- centripetal force and gravitational force for satellites (58% correct)
Q11- back EMF (60% correct)

Goodluck... Not long to go now!

 

[hjed]

Thanks, this one was much better than the second one, which I felt was very hard.

I did find one bug though, in the question 4 I selected the correct answer according to the explanation, but it has marked me wrong and highlighted another answer as correct?

Also I'm pretty sure all the answers in question 20 are incorrect for the purpose of the syllabus, in the syllabus the meissner effect excludes magnetic field from the super conductor. No current is induced in the superconductor or magnet (according to the model used in the syllabus). I got marked down for arguing that in my trial exam.

 

[Duxify]

Thanks, this one was much better than the second one, which I felt was very hard.

I did find one bug though, in the question 4 I selected the correct answer according to the explanation, but it has marked me wrong and highlighted another answer as correct?

Also I'm pretty sure all the answers in question 20 are incorrect for the purpose of the syllabus, in the syllabus the meissner effect excludes magnetic field from the super conductor. No current is induced in the superconductor or magnet (according to the model used in the syllabus). I got marked down for arguing that in my trial exam.

Hi hjed,

Sorry only just saw this... thanks for letting me know on Q4, will take a look at the back end and see if I can find the issue.

For question 20.

These are my full notes on magnetic levitation:

1. Magnetic levitation occurs if a magnet is brought near a superconductor below its critical temperature.
2. The magnet causes a change in flux which induces eddy currents in the superconductor.
3. The induced currents create a magnetic field that opposes the flux which caused them. This causes repulsion between the magnet and the superconductor.
4. Because none of the flux penetrates the superconducting material (as per the Meisner effect) the magnetic field created by the induced current is exactly equal too and opposite (i.e mirrors) the magnetic field that created it.This is also known as strong diamagnetism.
5. If the magnet is relatively small this repulsive force is sufficient to balance the weight of the magnet and it will levitate above the superconductor.

I found this part of the syllabus was poorly explained by all of the text books and relied mostly on external sources. Hence why the question is prob a bit tricky.

From my understanding there is a current induced in the SC (but not the magnet).

Still, I think its prob worth understanding in case of a short answer question- for example Q22 HSC 2006... The answer in success one explicitly talks about larger currents induced in cooled metals due to less resistance.

 

[Duxify]

Feedback for Physics Exam 2

There were 12 participants.

The raw average was 14.4/20. Higher than Exam 1 (where the average was 13.4/50).

The highest mark was 19. No perfect Scores!

The toughest questions were:

Q2: Projectile motion (28% got this)
Q3: Tricky escape velocity question (37% got this)
Q14: Striation patterns and discharge tubes (44% got this)

Thanks very much to all who attempted an exam.

PM me if you have any questions/feedback or want more info on your results.

I will give feedback on Exam 3 in the next few days… so give it a go if you haven't already.

Goodluck!

 

[Duxify]

Hi all,

I have put up another 20 question physics exam (there are now 4 in total).

I will put up feedback for Exam 3 tomorrow.

The highest score is 15 so far.... Give it a go if you haven't already.

Not long now... Goodluck!

 

[hjed]

Lots of mistakes in the fourth one:

There is a mistake in question 8, you state that the magnetic field is perpendicular to the motion (Correct), however you then substitute theta=45!!!
This results in the wrong answer as theta=90.
Could you please correct my results?

Also in question 12 insulation in either direction will still decrease the transfer of heat and does prevent some movement of current.

And a note on question 18, but the syllabus doesn't require you to know the critical temperature of superconductors, just that it occurs at a low temperature. It would be a fair assumption that the metal was superconducting.

In question 20, AC current is not used in superconductors as only DC currents experience DC resistance. Your reasoning for B being the answer however is that superconductors have 0 resistance. The Jacaranda text book states on multiple occasions that DC current would be used in a superconductor based power transmission line (see pg242 and 237). The most efficient voltage for power generation (as supposed to transmission) is not covered in the syllabus.

 

[Duxify]

Ok cheers for letting me know... will take a look now.

 

[Duxify]

Lots of mistakes in the fourth one:

There is a mistake in question 8, you state that the magnetic field is perpendicular to the motion (Correct), however you then substitute theta=45!!!
This results in the wrong answer as theta=90.
Could you please correct my results?

Also in question 12 insulation in either direction will still decrease the transfer of heat and does prevent some movement of current.

And a note on question 18, but the syllabus doesn't require you to know the critical temperature of superconductors, just that it occurs at a low temperature. It would be a fair assumption that the metal was superconducting.

In question 20, AC current is not used in superconductors as only DC currents experience DC resistance. Your reasoning for B being the answer however is that superconductors have 0 resistance. The Jacaranda text book states on multiple occasions that DC current would be used in a superconductor based power transmission line (see pg242 and 237). The most efficient voltage for power generation (as supposed to transmission) is not covered in the syllabus.

Q8:

I think my answer is correct.

You need to use the angle to find out the length of the wire which is in the magnetic field.

This is similar to Q7 in 2006 HSC exam which caught a lot of people out.


Q12:

I have changed the wording of the question to “least likely” to eliminate this ambiguity.

Q18:

This question is similar to Q22 HSC 2006.

Although you don’t need to know specific critical temperatures, you should know that currently the highest critical temps in type II superconductors is around -150˚C.

For most type I superconductors (which this is) critical temps are around -250˚C.

Therefore I don’t think you can say it is a fair assumption that the metal was superconducting at -100˚C. I think you would loose marks in the exam if you did.

Q20:

I don’t think the Jacaranda explanation on this topic is great. It does say if we had superconducting power lines the advantage “may” swing back to DC… but in reality AC will still be used because we would still need transformers which would therefore mean DC could not be used.

This is what this question is trying to test.

This question is similar to a question in a school trial paper.

You are obviously not likely to get something like this in a MC question, but more likely a short answer discussion question.

 

[hjed]

Q8:
I think my answer is correct.

You need to use the angle to find out the length of the wire which is in the magnetic field.

This is similar to Q7 in 2006 HSC exam which caught a lot of people out.

My apologies, I did miss that. You might want to update your explanation to make it clearer that that is what you are doing, I read it and though you had said you weren't using the angle and then used it anyway.

Q20:

I don’t think the Jacaranda explanation on this topic is great. It does say if we had superconducting power lines the advantage “may” swing back to DC… but in reality AC will still be used because we would still need transformers which would therefore mean DC could not be used.

This is what this question is trying to test.

This question is similar to a question in a school trial paper.

You are obviously not likely to get something like this in a MC question, but more likely a short answer discussion question.

On the second page I listed it specifically says that DC would be used ("the current in the transmission lines would of course be DC rather than conventional AC."). It even explains why, "the constant direction-switching in AC causes energy losses and heating. That would ... counter the low superconduction temperatures." Superconductors simply don't work that well on AC, I seem to remember there was a past HSC question on using AC in superconductors and the answer was that you shouldn't.
It would actually be far more efficient to convert current to AC, transform it, and then convert it back to DC. When AC was chosen for transmission lines this process was very inefficient, but with technological advances it is actually not that complicated and is used in computers.
Either way you need to update your explanation because you claim there is 0 loss due to 0 resistance when in fact there is not 0 resistance if you are using AC. If you are using the logic that transformations would require AC to support eliminating the other two answers then you can't then use a DC current for your reasoning to support your third answer. Ie. if they are using AC current they are all incorrect.

 

[Duxify]

My apologies, I did miss that. You might want to update your explanation to make it clearer that that is what you are doing, I read it and though you had said you weren't using the angle and then used it anyway.



On the second page I listed it specifically says that DC would be used ("the current in the transmission lines would of course be DC rather than conventional AC."). It even explains why, "the constant direction-switching in AC causes energy losses and heating. That would ... counter the low superconduction temperatures." Superconductors simply don't work that well on AC, I seem to remember there was a past HSC question on using AC in superconductors and the answer was that you shouldn't.
It would actually be far more efficient to convert current to AC, transform it, and then convert it back to DC. When AC was chosen for transmission lines this process was very inefficient, but with technological advances it is actually not that complicated and is used in computers.
Either way you need to update your explanation because you claim there is 0 loss due to 0 resistance when in fact there is not 0 resistance if you are using AC. If you are using the logic that transformations would require AC to support eliminating the other two answers then you can't then use a DC current for your reasoning to support your third answer. Ie. if they are using AC current they are all incorrect.

Ok thanks... point taken. I agree the superconducting question is probably a bit ambiguous. I will update Q20.

I have also updated Q8 to make the answer more clear.

Thanks for your feedback. Much appreciated.

 

[Duxify]

Feedback for Physics Exam 3

There were 6 participants.

The raw average was 9.3/20. Excluding marks below 5 the average was 12.4.

The highest mark was 15.

The toughest questions were:

Q8: Tricky weight force/magnetic force/parallel conducting wire/voltage problem (0% got this)
Q10: Tricky back emf/current drill question (0% got this)
Q15: Photoelectric effect/work function wavelength question (17% got this)
Q17: Tricky photoelectric effect photocurrent question (17% got this)

Thanks very much to all who attempted an exam.

PM me if you have any questions/feedback or want more info on your results.

I will put up feedback on Exam 4 tomorrow… give it a go if you haven't already.

Goodluck!

 

[Duxify]

Feedback for Physics Exam 4

There were only 4 participants.

The average was 10.5/20.

The highest mark was 16.

The toughest questions were:

Q4: Astronaut g-force (0% got this)
Q6: Orbital period ratio given radius increase (0% got this)
Q14: Calculating electric field strength from force (25% got this)

Thanks very much to all who attempted an exam.

PM me if you have any questions/feedback or want more info on your results.

Goodluck!