Multiple Choice (3 Viewers)

[Xenocide]

1A
2D
3B
4D
5C
6B
7A
8C
9B
10B
11B
12A
13A
14B
15D

Just checked these, 99% sure they are all correct.

 

[holla back girl]

i am going to go and make my complaint now...
i thought the questions were beyond my reach even though i am a competent physics student.

http://www.boardofstudies.nsw.edu.au/aa_main/contact.html

 

[Dumsum]

i am going to go and make my complaint now...
i thought the questions were beyond my reach even though i am a competent physics student.

http://www.boardofstudies.nsw.edu.au/aa_main/contact.html

You won't acheive anything, yes it was hard, but everyone had to sit the same test. It's fair.

 

[seadonkey]

1A
2D
3B
4D
5C
6B
7A
8C
9B
10B
11B
12A
13A
14B
15D

Just checked these, 99% sure they are all correct.

haha u musta copied me... got all the same as u

 

[Leedowl]

Q7 - 2cents

If you think of it via Lenz law - current flows to oppose change that caused it - as the loop rotates the number of flux lines passing down through the loop will be decreasing (eventually getting to zero as it reaches 90 degrees). Thus the induced current flows to "restore" these missing flux lines. Thus by the right-hand-grip rule the current flows clockwise around the loop to replace the downward flux lines- thus the answer is A

 

[jake2.0]

15D use right hand palm rule... electrons up holes down

but can holes physically travel??

 

[stew_g17]

anyone got reasoning why q10 is not d, wont larger eddy currents be in B and C??? probly wrong like most of my multiple choice but oh well im finished!!!

 

[AreYouAlright?]

I have a couple questions... why is Q13. a) and not C) Phosphorus... Phosphorus has more electrons in valence band so wouldn't the dopant level be higher as shown?.... Why isn't 12 B i know Planck made the hypothesis and mathematical expression but the question says 'explained'. Are you sure about Q6... isn't it because the Earth's magnetic field it doesn't matter what direction? and I'm sorry but question 7 is definately D rotate the page so the field goes left to right and mentally turn the coil ninety degrees right hand rule shows it can't be A.

 

[rowan brand]

i agree

einstein explained - but crap question writing really

i disagree about question 6. Using teh formula F = BIL sin theta

re-arranged this becomes I = F/ (BL sin theta)

Hence since i is proportional to L (length of wire), B, sin theta (which is the orientation to the external field which is the eearths field, hence orientation DOES matter), and is also proportional to force which is the speed which teh thing is rotated.

Hence the only answer can be B. as all the others are implicated in teh induction of current as seen in the above formula.

 

[AreYouAlright?]

i agree

einstein explained - but crap question writing really

i disagree about question 6. Using teh formula F = BIL sin theta

re-arranged this becomes I = F/ (BL sin theta)

Hence since i is proportional to L (length of wire), B, sin theta (which is the orientation to the external field which is the eearths field, hence orientation DOES matter), and is also proportional to force which is the speed which teh thing is rotated.

Hence the only answer can be B. as all the others are implicated in teh induction of current as seen in the above formula.

Yeah i agree but i thought thickness of wire would affect the amount of current... simply because there is less resistance and i dunno the Earth's mag field is everywhere would it really matter the most? I'm just being hopeful lol

 

[who_loves_maths]

Originally Posted by AreYouAlright?
I have a couple questions... why is Q13. a) and not C) Phosphorus... Phosphorus has more electrons in valence band so wouldn't the dopant level be higher as shown?....

i think i gave an explanation to that in my last post AreYouAlright?

 

[seadonkey]

but can holes physically travel??

yes holes r just an empty spaces within the lattice due 2 group 3 dopant.
if i got a hole wid an electron in it 2 the left and another hole on the right.. ie only a hole on the right, and the electron moves to the right. it is sed the hole moved 2 the left

 

[who_loves_maths]

Originally Posted by AreYouAlright?
Yeah i agree but i thought thickness of wire would affect the amount of current... simply because there is less resistance and i dunno the Earth's mag field is everywhere would it really matter the most? I'm just being hopeful lol

AreYouAlright?, the thickness would indeed affect the current reading as you say. but it is the LEAST likely answer because:

when/if you increase the thickness of the wire, you increase the current. but, you also increase the weight of the wire. as a result of weighing more, the wire will be rotated slower than before by the students. hence, the rate of cutting flux will be reduced - thereby diminishing the induced E.M.F. as a result, the current will also be belittled.

so the net overall effect of increasing wire thickness equates to almost nothing.

 

[Celestgirl]

i found it hard too

yeahh i found the question in multiple choice was quite hard and tricky
however this is my answer

Q1 (a) since based on the equation sqrt of ux^2 + vy^2
Q2 (d) the orbital decay is always caused by the atmosphere and this causes a drad that leads to the decrease of the speed
Q3 (B) since 1/2mv^2=GMm/r and the small m is getting eliminated the mass of space probe not affect the magnitude of escape velocity
Q4 (c) i doubt it some people in my school think that it is either c or d, but my argumentation is that the deceleration is the one that is used in the hohfmann's transfer to move to the planet that is on the inner orbit of the earth, since the Rocket P in the picture decelerated the rocket must have changed its orbit to become an elliptic orbit that is tangent to both the origin orbit and the new orbit, but it don't know
Q5 must be C!... why 90-50degress is just the same as 40
Q6 (d) i guess, because the lease likely to affect the copper wire, however i think we are all get it wrong north to south to south and east to weast means that the angle is 0degrees, not 90, and why it is maximum? because the equation of induced emf is blvcostheta, not F=bilsintheta
Q7 it must be D!
Q8 don't wannt to tell you it is something that makes me feel confused :<, i know the battery is DC but it also gets a switch?? the question is ambiqious
Q9(B) since the induced eddy current happens in both tube Q and R, however the tube R is not a full circle and because of that the area is not maximum that reduces the induced eddy current therefore the magnet that is droppd to the tube R appears to be having faster than Q
Q10 (D) i know most people think that it is B but if you use the right hand rule you will know that both B and C still having a high induced eddy current that will cause heat, why d? because in the picture D it gets large number of strips
Q11 B
Q12 A
Q13 A or C ? hehehehe it made me confused because as we know the band theory of solids, the gap between the conduction band and the valence band we call it by forbidden gap, how about if you think like this, if we add more number of electron the condution band should be doped with more layer to fill the conduction band ??? it is obviously confused us, but since we know that the number of electron is available in both bands isn't it?
Q14 (B)
Q15 (B) some students in my class think it is also D but some of them deriving an expression qvb=qe(the force between the magnetic field when it is applied to the electrons) they eliminate q then become v=e/b, which means the speed of electron (the charge e) is slows down due to magnetic field of magnitude (b) but i don't that's right!!!

i guess don't put too much worry on this, everyone knows it is hard so i hope our mark will get scaled up

 

[Celestgirl]

i found it hard too

yeahh i found the question in multiple choice was quite hard and tricky
however this is my answer

Q1 (a) since based on the equation sqrt of ux^2 + vy^2
Q2 (d) the orbital decay is always caused by the atmosphere and this causes a drad that leads to the decrease of the speed
Q3 (B) since 1/2mv^2=GMm/r and the small m is getting eliminated the mass of space probe not affect the magnitude of escape velocity
Q4 (c) i doubt it some people in my school think that it is either c or d, but my argumentation is that the deceleration is the one that is used in the hohfmann's transfer to move to the planet that is on the inner orbit of the earth, since the Rocket P in the picture decelerated the rocket must have changed its orbit to become an elliptic orbit that is tangent to both the origin orbit and the new orbit, but it don't know
Q5 must be C!... why 90-50degress is just the same as 40
Q6 (d) i guess, because the lease likely to affect the copper wire, however i think we are all get it wrong north to south to south and east to weast means that the angle is 0degrees, not 90, and why it is maximum? because the equation of induced emf is blvcostheta, not F=bilsintheta
Q7 it must be D!
Q8 don't wannt to tell you it is something that makes me feel confused :<, i know the battery is DC but it also gets a switch?? the question is ambiqious
Q9(B) since the induced eddy current happens in both tube Q and R, however the tube R is not a full circle and because of that the area is not maximum that reduces the induced eddy current therefore the magnet that is droppd to the tube R appears to be having faster than Q
Q10 (D) i know most people think that it is B but if you use the right hand rule you will know that both B and C still having a high induced eddy current that will cause heat, why d? because in the picture D it gets large number of strips
Q11 B
Q12 A
Q13 A or C ? hehehehe it made me confused because as we know the band theory of solids, the gap between the conduction band and the valence band we call it by forbidden gap, how about if you think like this, if we add more number of electron the condution band should be doped with more layer to fill the conduction band ??? it is obviously confused us, but since we know that the number of electron is available in both bands isn't it?
Q14 (B)
Q15 (B) some students in my class think it is also D but some of them deriving an expression qvb=qe(the force between the magnetic field when it is applied to the electrons) they eliminate q then become v=e/b, which means the speed of electron (the charge e) is slows down due to magnetic field of magnitude (b) but i don't think thats right!!!

i guess don't put too much worry on this, everyone knows it is hard so i hope our mark will get scaled up

 

[richz]

AreYouAlright?, the thickness would indeed affect the current reading as you say. but it is the LEAST likely answer because:

when/if you increase the thickness of the wire, you increase the current. but, you also increase the weight of the wire. as a result of weighing more, the wire will be rotated slower than before by the students. hence, the rate of cutting flux will be reduced - thereby diminishing the induced E.M.F. as a result, the current will also be belittled.

so the net overall effect of increasing wire thickness equates to almost nothing.

ok, i agree with ur explanation but how does the orientation of the wire affect the reisitance??

 

[spank_meh]

all i can say is... fuck the exam it was haaaaard!!!

i wish i did last years paper dammm

 

[Leedowl]

Q15

Now starting to think that this was a trick Q about holes and electrons moving opp directions in semiconductors

If 1/2 current from holes and 1/2 from electrons then holes will be moving into the page (with arrow) and electrons out of the page (opp direction to arrow)

Using palm rule or LHR the holes will be pushed down. However the electrons coming out of ther page you have to reverse their direction (because they are negative) thus using the LHR or palm rule they have the same downward force.

ie answer is C

 

[chensta]

i spent a while on the cooper wire questions, then i thought it this way

magnetic field is vertically up out of earth right? and the wire is horiztonal, so it is already at perpendicular....what tricked me is north/east etc, i was looking at it on a vertical plane, but really its all on horizontal...so no matter which direction, it will still be cutting the flux....

 

[who_loves_maths]

Originally Posted by chensta
i spent a while on the cooper wire questions, then i thought it this way
magnetic field is vertically up out of earth right? and the wire is horiztonal, so it is already at perpendicular....what tricked me is north/east etc, i was looking at it on a vertical plane, but really its all on horizontal...so no matter which direction, it will still be cutting the flux....

Originally Posted by xrtzx
ok, i agree with ur explanation but how does the orientation of the wire affect the reisitance??

hi xrtzx,

to answer this you will need to know the setup of the Earth's magnetic field around the Earth. essentially it is from the magnetic north pole to the magnetic south. but what this means is that (because the Earth is assumed to be a sphere) every localised point on the surface of the Earth between the poles will experience a near-uniform magnetic field going from North to South.

now, the student rotating the wire in a small elongated loop in the question is that localised point on the Earth. now, as you are aware, a current-carrying wire in a magnetic field experience no force in a magnetic field if it is parallel to its direction, while it would experience maximal force if at right angles to the field.
so note that the orientations of East-West and North-South, as specified in the question, are perdendicular to each other; and recall, from above, that the direction of the magnetic field at that point is from North to South.

can you see the rest of the explanation now? {there's still a bit left to explain, but knowing your prowess at Phys, i'm sure you can figure it out quite easily }