Section I - Multiple Choice (1 Viewer)

[someth1ng]

We believe it is this as you only want to detect the wavelength that corresponds to its peak wavelength, if you had a low band gap (A) then it may detect other random radiation that won't distinguish it from humans etc

Booo, but you can still get interference with the smaller range. You have less interference but you have less coming from the human...

 

[habitres]

Booo, but you can still get interference with the smaller range. You have less interference but you have less coming from the human...

That's a good point, the question isn't very specific on what it means "best". however, for a one mark question, that extra analysis etc shouldn't really be valid, further making me think it was B just because you 1) chose and approx wavelength. 2) use the equations to sub it in. it got you virtually 0.17 etc. Sort of reverse intuitive if its like "OH wait, it has to be BELOW this, ~everything above useless"

Not 100% but can't do anything so w/e.

 

[IceDingo]

Someth1ng, I like your answers, exactly the same as mine

19 is definitely D, If you had low resistance all you would get in your pot is a bunch of eddy currents that produce little heat because they don't lose as much energy from resistance in the metal. This is why you use mostly iron based cookware and not copper.

My reasoning for 20 being A is that when I checked 10 um it turned out to only be ~0.12 eV. So I checked 8um and it was still only ~0.15 eV so not enough energy still to release electrons from B. That was close enough to the peak for me so I just went meh, put A because peak energy definitely has enough to release electrons from A.

EDIT: Now looking at it though A does seem a bit too far off the scale Ahwell, onto the next exams!

 

[nifkeh]

Read the first paragraph on wikipedia, https://en.wikipedia.org/wiki/Induction_cooking

I took at look that this question today, if you think about it, high resistance means more energy released as heat which is needed to cook food more quickly and is an ideal situation, compared to a low resistance, which means even less heat energy is released and your food will still be cold lol

and if you use V=I/R , low current with high resistance = low voltage. I might be wrong with this explanation but this is from my prelim knowledge of circuits

 

[mickstarify]

I took at look that this question today, if you think about it, high resistance means more energy released as heat which is needed to cook food more quickly and is an ideal situation, compared to a low resistance, which means even less heat energy is released and your food will still be cold lol

and if you use V=I/R , low current with high resistance = low voltage. I might be wrong with this explanation but this is from my prelim knowledge of circuits

Its V=IR, lower current and higher resistance does not neccesarily = low voltage, it could equal the same voltage

e.g changing from 0.4A to 0.2A, and resistance from 5 Ohms to 10 Ohms

 

[MonsterMe]

get over it man, people I know including me are looking at 15/20 best case scenario so

+111111111111111111111111111111111111111111111111111111111

 

[qwe_r1]

Re: General Thoughts: Physics

5. is c as it is a group 3 element. which makes a P type semiconductor

 

[Nader95]

These are what I put and the reasoning behind it...:
1) D - DC generator due to split ring comm.
2) A - The speed of light is the length travelled by light in a vaccum in 1/299 792 45 of a second
3) C - The cathode beam converts kinetic energy on collision exciting the nucleus causing it to glow green
4) B - To go form -3 to -5.8 you -2.8
5) C - Extrinsic semiconductors doped with P-type are doped with group 3 elements increasing hole flow in valence band
6) C - Self explanatory
7) B - Radial magnetic field means torque is always a maximum relative to the MF leaving current as the other vector
8) A - 0.03 = 0.05 * I * (0.2/sin(30))
9) B - Self explanatory
10) D - Ratio is 1:10 and is 3:30 is the only one in that ratio
11) C - Momentum is conserved in a closed system, a downward impulse exerted down = impulse up
12) B - Centripetal force = (6*10^24) * (2(pi)(3.85*10^8/2.36*10^6)^2) / (3.83*10^8)
13) B - r^3/T^2 = r^3/T^2
14) C - According to Lenz EMF = - (delta flux)/(delta time)
15) D - Two unique properties of superconductors are the i) Meisner effect ii) Perfect diamagnetism, (i) applies here
16) C - If no external force is acting then the rotation should continue at 200 RPM without additional work needed
17) B - f/l = KI^2/d
18) C - (1/3.1^2)/(1/4.6^2) ---- Still unsure on this one, just reasoned that F= 1/r^2 if its the only variable changing
19) D - High frequency = High flux = High current = I^2 * R, The higher the resistant, the more electron collisions
20) B - Graph peaks at 9micrometers, convert to frequency (c=fl), use E=hf

Those were my answers, found the exam pretty easy overall

Edit:
For 14 I actually initially wanted to put D but considering the fact that the graph starts at 0 magnetic flux at t=0, then the induced EMF should be at 0 since there is no change yet...

 

[v1]

19) D - High frequency = High flux = High current = I^2 * R, The higher the resistant, the more electron collisions

V=IR... the lower the resistance the higher the current. and what if i told u current is made up of moving electrons.

 

[mickstarify]

These are what I put and the reasoning behind it...:
14) C - According to Lenz EMF = - (delta flux)/(delta time)

For 14 I actually initially wanted to put D but considering the fact that the graph starts at 0 magnetic flux at t=0, then the induced EMF should be at 0 since there is no change yet...

Did you miss calculus 101? In question 14, if we were to derive this graph the rate of change at the peaks are 0, therefore the EMF must be zero at these points

 

[Nader95]

Did you miss calculus 101? In question 14, if we were to derive this graph the rate of change at the peaks are 0, therefore the EMF must be zero at these points

Yea this is one of those moments where I give myself an eternal facepalm >_<, didnt do math this year, did it last year, prolly woulda saw this differently then =.=

V=IR... the lower the resistance the higher the current. and what if i told u current is made up of moving electrons.

To answer the second part:
Then I'd say your correct ^.^, current = rate of charge moving

You are wrong about the first part though, not gonna say wikipedia is reliable however this pretty much represents what I was gonna say anyway==>
http://en.wikipedia.org/wiki/Induction_cooking

Aluminum or copper alone does not work on an induction stove because of the materials’ magnetic and electrical properties. Aluminum or copper cookware is more conductive than steel, and the skin depth in these materials is larger since they are non-magnetic. The current flows in a thicker layer in the metal, encounters less resistance and so produces less heat. The induction cooker will not work efficiently with such pots.

The heat that can be produced in a pot is a function of the surface resistance. A higher surface resistance produces more heat for similar currents. This is a “figure of merit” that can be used to rank the suitability of a material for induction heating. The surface resistance in a thick metal conductor is proportional to the resistivity divided by the skin depth. Where the thickness is less than the skin depth, the actual thickness can be used to calculate surface resistance

 

[Keelan134]

Re: General Thoughts: Physics

Any mistakes?

1. d
2. a
3. c
4. b
5. d
6. c
7. a
8. c
9. b
10. b
11. d
12. a
13. b
14. c
15. d
16. c
17. c
18. a
19. d
20. a

I am not sure about 17, 18 :L

Wouldnt 5 be C, Gallium, it wanted a p-type, therefore a group 3 atom, therefore Gallium

 

[Keelan134]

I got this
1)D (straightforward)
2)A
3)C
4)B
5)C (Group 3 atoms created p types)
6)A (no idea)
7)B (key word was proportional)
8) C (angle is unnecessary)
9) D (could be B)
10) D
11) C (A is not about Launch, B is incorrect thrust direction, and I didn't think the launch pad exerted the resultant force)
12) Cant Remember what I put but I used Grav Force = Centripetal Force, and calculated the Grav Force)
13) Again can't remember, just a Kepler's Law question
14)C
15)was shit, wasn't A or C, I think I went D, because B says the superconductor creates currents in the magnet, it doesnt so Process of elimination gave me D
16)Think I went C
17)B
18)(LOL it was to scale for once, had to ask supervisor for a ruler) A
19)D
20)B (A and C aren't semiconductors, B has a lower band gap then D, therefore B.

Thought?

8

 

[mickstarify]

I got this
1)D (straightforward)
2)A
3)C
4)B
5)C (Group 3 atoms created p types)
6)A (no idea)
7)B (key word was proportional)
8) C (angle is unnecessary)
9) D (could be B)
10) D
11) C (A is not about Launch, B is incorrect thrust direction, and I didn't think the launch pad exerted the resultant force)
12) Cant Remember what I put but I used Grav Force = Centripetal Force, and calculated the Grav Force)
13) Again can't remember, just a Kepler's Law question
14)C
15)was shit, wasn't A or C, I think I went D, because B says the superconductor creates currents in the magnet, it doesnt so Process of elimination gave me D
16)Think I went C
17)B
18)(LOL it was to scale for once, had to ask supervisor for a ruler) A
19)D
20)B (A and C aren't semiconductors, B has a lower band gap then D, therefore B.

Thought?

8

6 is definitely C,
8 I worked out to be A, i am quite certain it is A & will post a solution if you like
9 I believe is B
14 is not C
18 i do believe is C
20 is debatable, HgCdTe is a semi-conductor btw

 

[Nader95]

8) C (angle is unnecessary)
Thought?

8

Angle is very necessary, your not given the LENGTH of the wire, your given the height of the magnetic field, trigonometry --> sin(30)= 0.2/length ---> length = 0.2/sin(30)

 

[Keelan134]

Angle is very necessary, your not given the LENGTH of the wire, your given the height of the magnetic field, trigonometry --> sin(30)= 0.2/length ---> length = 0.2/sin(30)

I meant when you use BilSin(theta) as the wire is perpendicular to the mag field. you use it to calculate the length of course I get ya mate

BTW Im not an outstanding student this is just what I put down.

 

[Keelan134]

6 is definitely C,
8 I worked out to be A, i am quite certain it is A & will post a solution if you like
9 I believe is B
14 is not C
18 i do believe is C
20 is debatable, HgCdTe is a semi-conductor btw

My Mistake, I did get A, I was just going from memory.
18 I may have put C actually, because A is 1.5, which is which I probably confused with question 8 when I was writing my answers just now, so I got that too
Yeah 20 I just said B

 

[choiyokbao]

MC was pretty hard compared to previous years imo, some i had no idea but overall decentish hopefully XD

 

[mickstarify]

MC was pretty hard compared to previous years imo, some i had no idea but overall decentish hopefully XD

Yeah I found some of the multiple choice quite challenging, and the very fact that we still haven't completely established the answer for 20 says something.
However Section 1 wasn't too hard, & my module was fairly decent, so all round I found this to be a very reasonable test.

 

[someth1ng]

A and C aren't semiconductors, B has a lower band gap then D, therefore B.

This is possibly the worst reasoning ever. Both A and C are semiconductors.