Multiple Choice (2 Viewers)

[Just James]

m/c answers

Ok, now that everyone has finished discussing the multiple choice, could someone please list the answers to all 15. Don't put down what you did in the exam, put down what you now believe to be the correct answers.

 

[frenzal_dude]

I also think Q8 is C because even though batteries are DC when the switch turns on there will be a quick change in the magnetic field which will affect the galvanometer and then the current will subside after when the current is flowing and thhere is no more magnetic field

Also another reason it could be C is bc even if u disregard the whether the battery is AC/DC then the battery will still go flat which will make the current less

yeh i thought the same as u, for the initial split second wouldnt there b a bit of current that would then drop to 0 because there was a change in magnetic field?

 

[HotShot]

MC
1. A or C - most probable A, cos the velocity does keep changing? doesnt it? the velocity of the ball at the top its motion zero - is also correct. but that means the velocity is changing anyway = A
2.D - atmosperhic drag, gravity is too strong orbit is stupid how was it initially in orbit? sun's solar wind do not affect low earth orbits.
3.A, mass of space probe is importnat, radius is important, and universal gravitational is a constant and does is needed. tricky, relates to the escape velocity equation,
4.Either C or D, i thinks its D, anyhow if you reduce its orbital speed, the probe will not maintains its orbital velocity and fill into a lower orbit or collide with the planet. Damn i put D its C i think.
5.B, 45 degress is optimal angle for longest range.
6.D, orientation of the wire has absolutely no effect at all on the current produced. the earths magnetics field is small and doesnt effect this, it is also relatively constant.
7.TRicky, um either A or D
8.D- transformers only work with AC current, switch is closed means current is flowing, so not A, B and C are not sine graphs.
9. B- easy, cos eddy current only affect metals, plastic no eddy current falls first, copper with slit, has smaller edy currents than the other copper solid.
10.B- easy, the first one no insulations, energy is lost as heat, C, the wire is wrapped arounf an insulator so nothing will happen and it will heat up. D, is ridiculour same as C. B the insulationing materials creates smaller eddy currents that reduce heat, therefore less energy is llost as heat.
11.B - should be obvious
12.not sure either A or B, methinks A, cos Einstein didnt explain black-body radiation, Planck did with his packets. einstein explained photoelectric effect and relativity.
13.A or C, methinks A -jsut cos a nerd told me
14.work it out = B
15. D as electrns and holes have opposite charges and the magnetic fields sends them across in different directions (though they could attract each other)? whoa nice question.

 

[DigitalFortress]

erm no....mass of space probe doesn't effect esc velocity.

 

[fizzwizz]

q4 is A - think about it. C and D are the same thing really.

 

[fizzwizz]

and here are the correct answers
1. A
2.D
3. B
4. A
5. C
6. D
7. A
8. C
9. C
10. B weird q
11. B
12. A
13. A
14. B
15. D

 

[haboozin]

and here are the correct answers
1. A
2.D
3. B
4. A
5. C
6. D
7. A
8. C
9. C
10. B weird q
11. B
12. A
13. A
14. B
15. D

how do u know these are the correct answers

 

[helper]

q4 is A - think about it. C and D are the same thing really.

Think about your answer to 2 and then how you correspond your answer to 2 with your answer to 4.

A satellite comes to Earth because it slows down in 2 but it starts spinning out in 4 because it slows down.

 

[haboozin]

Think about your answer to 2 and then how you correspond your answer to 2 with your answer to 4.

A satellite comes to Earth because it slows down in 2 but it starts spinning out in 4 because it slows down.

I think he is basing it on this:

A satellite in low Earth orbit will need an orbital velocity of approximately 28 000 km h-1 to maintain the orbit
....
[geostationary] orbital velocity is approximately 11 000 km h-1

i got tricked by that aswell, but now when i think about it, this only happens because gravitational potential energy is changed to kinetic, however in this event for it to go up it has to gain gravitational potential energy which means that it has to gain "energy" from somewere, but this space probe has already lost energy tothe rocket (due to law of conservation of momentum).

So C or D could be the only correct ones, if you do some calculations you should get D?

 

[frenzal_dude]

can someone confirm what q8 was? who put the answer of 0 current? and who put the one where it showed a bit of current which soon dropped to 0? because i think it was C, the one where it showed the graph with a bit of current droppin to 0 soon after.

 

[HotShot]

can someone confirm what q8 was? who put the answer of 0 current? and who put the one where it showed a bit of current which soon dropped to 0? because i think it was C, the one where it showed the graph with a bit of current droppin to 0 soon after.

its ac current, plus others r not sine waves, also when the switch is closed current flows, so that means there is some current. and why would it drop after sometime?

 

[who_loves_maths]

Originally Posted by fizzwizz
and here are the correct answers
1. A
2.D
3. B
4. A
5. C
6. D
7. A
8. C
9. C
10. B weird q
11. B
12. A
13. A
14. B
15. D

err... as haboozin said, how do you those are the correct answers? over-confidence?

for one thing, your question 15 is already wrong. the answer is C - both holes and electrons are forced to move to the same side by the magnetic field.

 

[HotShot]

err... as haboozin said, how do you those are the correct answers? over-confidence?

for one thing, your question 15 is already wrong. the answer is C - both holes and electrons are forced to move to the same side by the magnetic field.

how do you know that? cos holes and electrons have opposite charges which means they move it opposite direction, but they also attract each other, but then it would have neutral charge which means it could go straight forward? any explanation?

 

[who_loves_maths]

anyways, i think all the "conversial" multiple choice questions have all been discussed in this thread, so for those who are still interested, this is what i believe (ie. IMO) to be the correct answers:

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

once again, as i said, this is IMO.

 

[who_loves_maths]

Originally Posted by HotShot
how do you know that? cos holes and electrons have opposite charges which means they move it opposite direction, but they also attract each other, but then it would have neutral charge which means it could go straight forward? any explanation?

Precisely! it is for the reason that they travel in opposite direction AND they are oppositely charged that when you apply the Right Hand Rule to the question you will find they are forced to the same sides.

in this case, two opposites cancel out to give the same results!

also, in relation to your "neutral current", you are incorrect. the magnetic field is applied so that the magnetic force on the holes and the charges is perpendicular to the direction of current. Hence, under an emf, the direction or flow of the current is unaffected by the transverse movement of the charges to either side of the rod.

 

[Iceman145]

God, guys. show some sense, could we please?

1. A - i think we're agreed here.
2. D - same.
3. B - by formula, v squared = GM/r. don't see mass of the space probe there anywhere.
4. C - i'm pretty sure, it could be D but. Look, you people that put A, does that mean that if i zoom around earth really fast, and then slow down, i can fly into space?!!! (or, a satellite slowed by atmospheric drag zooms away further and further into oblivion?!) duh.. F = mv^2/r, and so when v drops, force required for circular motion drops. but Gravitational force is just as strong, resulting in too much force from gravity. and what happens when gravity is too strong? geez.
5. C
6. B - not D. it's been explained already.
7. A
8. C - hello, it's a battery. DC.
9. B - also been explained. i'm pretty sure it's b, anyway.
10. B
11. B
12. A
13. I still have no idea what the correct answer is. anyone know for sure? i guessed C. seemed reasonable.
14. B
15. C

The only ones i'm remotely unsure about are 4 (C or D), 9 (B or C) and 13 (anything's possible). Far as i know though, i'm headin for a 15/15.

 

[who_loves_maths]

13. I still have no idea what the correct answer is. anyone know for sure? i guessed C. seemed reasonable.

it is A. that has also already been explained previously in this thread - plz check again.

4. C - i'm pretty sure, it could be D but...

Q4 is D, not C. i could go into a lengthy explanation, but i won't. you just have to believe me

10. B

this is where i do not agree with many BOSers here. i believe it to be D, not B.

 

[helper]

Precisely! it is for the reason that they travel in opposite direction AND they are oppositely charged that when you apply the Right Hand Rule to the question you will find they are forced to the same sides.

Can you go through the steps in both cases?

Thumb - Current
Fingers - Magnetic field
Palm - Force

I can't seem to come up with your result.

Now is top and bottom sides?

 

[who_loves_maths]

^ okay helper:

consider the holes first - they are travelling in the direction of conventional current into the page and are taken to be positively charged.

applying the Right Hand Palm Rule, fingers in the direction of B field to the right, and thumb into the page - ie. palm is facing down.

hence, the holes are pushed to bottom of rod.

now, consider the electrons - they are travelling in the opposite direction of conventional current out of page and are negatively charged.

applying the RHP Rule, fingers in direction to B field to the right, and thumb out of the page - this would suggest palm/force is facing up.
but, the thumb represents conventional current, so the palm must be flipped once again - ie. force is downward on electrons.

hence, the electrons are pushed to bottom of rod.

Therefore, holes and electrons are pushed to the same bottom side of the rod. answer is thus C.


^ so the case with negatively charged electrons and moving opposite to positive holes is what i mean by "two opposites cancel" to give the same result in this case.
the two opposites are: 1) in charge, and 2) in direction of current flow. and since both are included factors in the RHP Rule, then two flips of the palm results in the initial orientation of the palm. so the force is same on both holes and electrons.

hope you understand now

 

[fizzwizz]

Just a thought about q15. What if minority charges carriers are considered? Will this invalidate question?