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# AQA Physics Unit 4 Thread - JAN 28th 2010 (Post Exam) watch

1. (Original post by WilliamWJ)
yes
WHOOP
2. (Original post by SuperMario88)
SAID 3 PI OVER TWO
I said pi/4 =/ I was debating between pi/2 and pi/4, I think I put pi/4. Might not have.
I said pi/4 =/ I was debating between pi/2 and pi/4, I think I put pi/4. Might not have.
I put pi/4.
4. (Original post by Danny A G)
I thought the exam went alright. The multiple choice was harder then I expected but I think i got most of them right. Did anyone get £3.78 for the transformers one?
yupp £3.78
5. (Original post by fabregassed)
i think i got 220uf
6. (Original post by WilliamWJ)
I put pi/4.
I thought it was pi/2?
7. (Original post by 2710)
I think i put 1.9, cant remember

I did P= IV = 300mA x 9V = 2.7W

?
i got that too

2.7W
8. (Original post by mazchaz)
Yeah tricky paper.

Some of the questions I remember:
Length of Pendulum: 1.95 or 1.98 or something

Used Mgh to work out her max kinetic energy

For the acceleration of the planet B i just used g = F/m as a=g

For the 6 marker I really screwed it up. I talked about back Emf, laminating the core, reducing the current to reduce heat loss.

for the prove it was 4£ I got something like 378pennies
Advantage of turning pc off, cheaper
Disadvantage: takes time to boot up

KE graph looked like a pair of boobs for swinging girl

Capacitors time stayed the same
21.7 as my time constant, for some reason i put units as omega farads

potential questions
Double one of them and the other stayed to same

and multiple choice was rushed and I personally found it tricky

General consensus from my school was fairly tricky but not too bad.
for units of time constant i wrote seconds lol
9. what was thee answer to question 2 in the multiple choice
10. (Original post by physics_fanatic)
for units of time constant i wrote seconds lol
correct
lol
11. (Original post by fabregassed)
for the 90% bit, i kept getting 10%
That's the point. The energy left is 10% - therefore 90% is wasted.

For the kWh I also got around 377.78 pence - i.e. £3.78 which is approximately £4.00.
12. (Original post by MB001)
That's the point. The energy left is 10% - therefore 90% is wasted.

For the kWh I also got around 377.78 pence - i.e. £3.78 which is approximately £4.00.
I got like £3.14 or something silly like that. =/ well it was close to £4 in my opinion, but lost a mark there. I think I got this because I thought you had to do 80% of the time for a year, cause it was 80% wasted and I thought at the time that, that's what they wanted cause i'm sure it said "the price for the wasted energy". =/

I dunno.
14. (Original post by WilliamWJ)
Cos is pi/4 rads out of phase with sin. The sin wave was displacement and cos wave was velocity. I think...
Aren't they the opposite way around to that

Displacement, cosine. Velocity a sine curve flipped over..
15. (Original post by WilliamWJ)
Cos is pi/4 rads out of phase with sin. The sin wave was displacement and cos wave was velocity. I think...
Um no its not. cos graph is 90 degrees out of phase with sin
16. (Original post by WilliamWJ)
Cos is pi/4 rads out of phase with sin. The sin wave was displacement and cos wave was velocity. I think...
That's what I thought, cause I drew sketches of the three diagrams and compared them and I looked at them before I went into the exam. =/

Displacement: Cosine wave
Velocity: upside down sine wave
Acceleration: upside down cosine wave.

So i figured pi/4 out of phase.
17. (Original post by Game_boy)
Some answers I remember, anyone got similar?

Multiple choice:

- The time period had r^3/GM in it
- An oscillating mass would return to its natural frequency after driving force removed
- 1000uF vs. 100uF capacitor was 10xenergy, 10xcharge stored
- displacement is pi/2 out of phase with velocity?

Paper:

- velocity was 2.21 in Q1
- Transformer increasing voltage to 12V - no effect on time to 90% discharge, quadruple energy stored
- 23.2s time constant
- 6x10^-5 current
- gpe was gp times m, for that 'state the relationship'
- Kinetic energy graph was 0 at 0, max-positive at T/4, 0 at T/2, max-positive at 3T/4 and 0 at T. I ended up with two parabolas but not sure about the shape. Is it like a sin^2(x) curve and if so how were we meant to know that shape?
- I got £4.20 for the cost, was that "about £4" or did anyone get closer? How did those people who got 3.78 get it? I got 3W for the previous part.
- How much were we meant to write for the very first part; describe energy changes in pendulum? I wrote about 3 times as much as the space required, what would the two marks be?
for multiple choice
-agree with all apart frm 1000uf vs 100uf i put x10 for energy and 1 for charge!!!
written
velocity was 2.21 in Q1 yupp
for energy changes i wrote a looooooot lol
18. Everyone Keep Going On About Sin And Cos Waves. Basicly At O Time It Is At Maximum Displacement Hence Potential Energy Is Max And Kinetic Is Zero When It Goes Through Mid Point Kinetic Energy Is Max And As In One Time Period It Goes Through Twice You Should Of Had A Graph That Looked Like A Pair Of Tits End Of !
19. I thought it was ok. Agree with most of what people said. I got pi/2 for phase difference as that is 90 degrees. I got a £4.20 for the cost. (This is differs from some people prehaps they took 90% of the 2.7W rather than getting 3W of which 2.7W is 90%.). The main thing I know I messed up was the graph of K.E. only did one peak which was stupid and wrong (grrrr.), but I suspect I lost quite a few others. I did resistance in the coil wires, and eddy currents in the core for Transformers.

How was everyone for time? I was really tight.
20. did all of you do Physics A? Is my school like the only one doing spec B?

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