Physics Discussions

PrecariousNuts · 21-06-2004 2:34pm

Q1

9.8. I got 10

Q2

590nm

Q5
a, 13N
b, 980Pa
d, 3Db
e, 0.2cm-1
h, 9 years
j, udd (educated guess)

Q6

mgh = 1/2 mv^2
2gh = v^2
v^2 = 3.92
v = 1.97 ms-1

m1v1 = m2v2
10(1.97) = 8v2
v2 = 2.47 ms-1

v^2 = u^2 + 2as
0 = (2.47)^2 + 4a
6.125 = -4a
a = -1.53 ms-2

F = ma
F = 0.01225N

Q7

(i) (energy required to bring water to 0C) + (water to ice) + (ice to -20)
mcdeltat + ml + mcdeltat
(.5)(4200)(15) + (.5)(3.3E5) + (.5)(2100)(20)
31500 + 165000 + 2100
217500J

(ii) Power rating 100W
80% effecient = 80W
For one second = 80J

(iii) 217500/80 = 2718.75s
45.31 minutes

I could have got something wrong here.

Q9

Phi = hf

4.3eV = (4.3)(1.6E-19) = 6.88E-19J

6.88E-19 = 6.6E-34f
f = 1.042E15Hz

hf = Phi + 1/2 mv^2
1/2mv^2 = hf - Phi
KE = h(c/lambda) - phi
KE = 1.37E-19J

Q10

Loss of mass: 1.389E-30
Energy = 1.25E-13J

Q12a

Mass of sun = 2E30 kg

PrecariousNuts · 21-06-2004 2:35pm

It was a metaphor for the leaving cert, I'm lovin' it

Roller Toaster · 21-06-2004 2:42pm

Did the speed of a tennis ball falling through the earth's core come up again?

subway_ie · 21-06-2004 2:52pm

Originally posted by Roller Toaster
Did the speed of a tennis ball falling through the earth's core come up again?

No... The only gravity question that came up was about calculating the mass of the sun. Which I got wrong. I got 6.7164x10^25Kg.

popinfresh · 21-06-2004 2:53pm

yeah i changed the mass into kg and got a very small speed, like .25ms^-1. Which would actuall make sense cause a little tiny bob isn't gonna accelerate to 25meters per second. But then again I'm not the best at physics. I'll be lucky if I pass. Ah well LAST EXAM, OOOOOOH YEAHH!!!!!!

PrecariousNuts · 21-06-2004 2:57pm

Yeah thats a pretty pathetic mass you have there, look at my huge throbbing 2x10^30kg

Can you confirm any of my answers?

subway_ie · 21-06-2004 2:59pm

Originally posted by popinfresh
yeah i changed the mass into kg and got a very small speed, like .25ms^-1.

You probably just made a small mistake in converting to Kgs, because if you had gotten 2.5 then it would have been roughly right. Maybe lose 3 marks? If it's just a decimal place mistake that is...

subway_ie · 21-06-2004 3:07pm

Well, Q1 seems the same as mine (10.309). Q.5 I made a mess of:
a)13N
b)980N (Feckin units.)
d)20 times (pure guess, totally wrong probably)
e)20 diopters
h) 9 years
j) udd

Q.6 all sounds about right... I've only got the answer for the horizontal force - 0.1224N

Q.7 I got wrong, messed up the formula. The efficiency part is right though...

Q.9 I started making up my own formuals for... not a good idea at all. I used c=f(lambda). Only because they gave the wavelenght and speed of light in the question. Then I used E = hf + .5mv^2 for the second part. Don't know where I got that from...

Q.11 - I said a 13 amp fuse for (c) - total guess. And the voltage-time graph, I just did a sine wave...

Q.12 (a) got a puny mass. Only to the order of 10^25. Feckin tiny, and (d) - didn't know how to explain the good conductor after 0.6v bit.

Just out of curiosity, which formula did you use for the mass of the sun thing? And why would they give c and lambda in q.9 if you didn't need it??

PrecariousNuts · 21-06-2004 3:09pm

Q11, 1A P=VI 100=230I I =0.43A

PrecariousNuts · 21-06-2004 3:12pm

Sun question... they started asking about centripical force so F = mv^2/r let it equal F = GMm/r^2 voila.

Well the energy of a photon is given by E = hf, however you are not given the frequency. Use c = f lambda; f = c/lambda to get the frequency.

subway_ie · 21-06-2004 3:13pm

Originally posted by PrecariousNuts
Q11, 1A P=VI 100=230I I =0.43A

Feck it. I had written down 1A and crossed it out. And then I heard somebody say to somebody else about using a 13A fuse so I stuck it down. Crap. And I was sure 13A was for cookers and heavy stuff like that. *THAT* is why you should never, ever listen to people cheating in exams. EVER.

subway_ie · 21-06-2004 3:16pm

Originally posted by PrecariousNuts
Sun question... they started asking about centripical force so F = mv^2/r let it equal F = GMm/r^2 voila.

Well the energy of a photon is given by E = hf, however you are not given the frequency. Use c = f lambda; f = c/lambda to get the frequency.

Ok, well for the sun question I said F = GMm/d^2 and then let that equal F=mv^2/d. Cancelled it and got v^2 = GM/d. I know that 2E30 is right, but I dunno where I messed up.

I did the c=flambda part, and I even put that into E = hf, so I might get a few marks for that. Most people in my class were saying they didnt use c or lambda, which is why I thought I had gone wrong. Well, I did go wrong, just not in that part.

PrecariousNuts · 21-06-2004 3:22pm

Gotta get ready for chemistry tomorrow aswell

Mutant_Fruit · 21-06-2004 3:23pm

(i) (energy required to bring water to 0C) + (water to ice) + (ice to -20)
mcdeltat + ml + mcdeltat
(.5)(4200)(15) + (.5)(3.3E5) + (.5)(2100)(20)
31500 + 196500 + 2100
249000J

(ii) Power rating 100W
80% effecient = 80W
For one second = 80J

(iii) 249000/80 = 3112.5s

1) Wrong. Its 31500 + 16500 + 21000
=217500joules.

ii)Every second its 80joules

iii) time is 217500/80 = about 45mins 19 seconds.

Mutant_Fruit · 21-06-2004 3:24pm

Originally posted by PrecariousNuts
Q11, 1A P=VI 100=230I I =0.43A

****!! i did everything write, but worked out I to be 4.3. So i said 5 amp fuse.

One mark down the tube.

Sarunas · 21-06-2004 3:29pm

Originally posted by subway_ie

[SNIP]

Q.9 - For the calculation of the threshold frequency I used c=f¥ë, again, probably wrong. Then I used E = hf + .5mv^2 for the second part. Don't know where I got that from...
Q.11 - I said a 13 amp fuse for (c) - total guess. And the voltage-time graph, I just did a sine wave...
[SNIP]

Good paper.

Q9- i just used e=hf and hf=work function + mv^2/2.
Threshold f, was just use E=hf, bu i forgot that threshold = hf, did the calculations but didnt say that was threshold.
For 2, used hf=work function + mv^2/2, cross multiply get v. then use e=mv^2/2 to find max kenetic energy.

Maybe messed up calculations, but AFAIK the methold is right.

Q11- fsck on this, for (c) it should have been 1A. Yeah drew a sine wave, and labelled it, the bottom is -230, top 230.

Q5- (b), that was hard, some trick q. wrote down density=vol/mass. presure at point=(density) x (m) x (g). tried to do it, dunno if it was right,

for (d), i sayd sound intensity=power/area, energy per second/area. tried to give a definition, but a didnt work out.

I did:

Q1- got 11.84 for g. easy q. 21 marks for drawing graph and finding slope.
Q3-What was this? A joke? Two expirements in one. No detail required for it, great.
Q4- Another joke, 12 marks for telling how to use micrometer, easy. Messed up calculations, got, uhm cant read my handwritting, looks like 7.2 x 10^-2 ohm m-2, or is that 1.2? As long as i have the right method.
Q5- should have know my definitions better, pretty easy.
Q6-they werent clear what they wanted for "establish the relationship: force= mass x acceleration", i kinda wobbled, derived f=ma, and wrote out the expirement to show Force is proportional to Acceleration.
To find the velocity, i changed everythind to international units, m and kg, then combined potential energy = kenetic energy.

Ep= mgh = Ek= mv^2/2. cross multiplied, got 19.8 ms-1 for v.
Second part, momentum before=momentum after. m1u1 + 0 = m1v1 + m2u2. cross multiply. forgot to write down what i got.
used v^2=u^2 + 2as, found acceleration. then f=ma to find the force.

Q9-good q, hope my vague explanations why leaves dont collapse will get me full marks.
Q10-Explaining why the momentum and energy is not conserved was a bad q, since they already tell the answer. Was running out of time so had to skip energy release during the decay. Wrote down the equation as n -> He + e- + energy, then for some strange reason i attempted to write down the equation in which chadwick discovered electron...

I was really happy to see all the expirements that i revised/crammed yesterday. Overall good paper, hoped just to get a pass, but could say a C. All depends on the examiner.

Kinda quiet up here, guess most people are getting p**sed as that was their last exam.

Can taste the FREEDOM, uhm not really, one more exam to go(engineering). BRING IT ON!

PrecariousNuts · 21-06-2004 3:31pm

Originally posted by Mutant_Fruit
1) Wrong. Its 31500 + 16500 + 21000
=217500joules.

ii)Every second its 80joules

iii) time is 217500/80 = about 45mins 19 seconds.

Ah yea that time sounds very familiar. I got it right just did the calculation wrong while typing it up. Thanks for confirming my suspicion.

subway_ie · 21-06-2004 3:34pm

Forgot to label the -230 and +230 on the sine wave, althougb I did put in voltage/time. Didn't label the time either though... Overall a nice paper, but I should have done better.

Sarunas · 21-06-2004 3:36pm

oopsssssss. forgot to label...

PrecariousNuts · 21-06-2004 3:37pm

Q5

b was very easy Pressure = pgh = density x gravity x depth. Just stick in an x for depth

d In the book it says a double in sound is equal to 3db.

I = 10log(I2/I1) I2 = second intensity, I1 first intensity

subway_ie · 21-06-2004 3:38pm

Well at least you only forgot to label one thing. All the stupid mistakes I made, arghhhhhh. So feckin annoying when you can see the small things you've done wrong and lost marks for which are obvious when you look back.

subway_ie · 21-06-2004 3:40pm

Originally posted by PrecariousNuts
I = 10log(I2/I1) I2 = second intensity, I1 first intensity

I should have written that down... but instead I just said "well, it's 10log10 times 2 = 20 times". I write the stupidest answers.

BangBeater · 21-06-2004 3:44pm

I had all the formulas right and the methods - jst made a stupid mistake with figures in pt (ii)

How many marks wud I lose with just a few figures wrong?

Earls · 21-06-2004 4:06pm

For the domestic circuit Q

In d voltage/time Q is 230v not just the Vrms value?
To draw the curve shouldnt you have to get the peak voltage which is about 325v?

Mutant_Fruit · 21-06-2004 4:11pm

its obvious now what they could ask! Why couldn't you tell me before.

All in all, i think i'm B1 standard, maybe B2. that suits me perfectly FINE.

Smellyirishman · 21-06-2004 4:46pm

Well I thought he ment to draw the diagram of how the electricity is setup through out the house. There was a load of stuff they could ask, and they did! Prollt my worst section by far though I did so badly! Said the live wire was red and never did a voltage time graph or anythin of the sort

Still im SOOO happy!

subway_ie · 21-06-2004 4:47pm

Originally posted by Mutant_Fruit
its obvious now what they could ask! Why couldn't you tell me before.

I did! Have a look at the third post on this thread:

Originally posted by subway_ie
The house circuit would porobably be the benefits of RCBs, MCBs, although it get's a little tricky describing how the live/neutral wires are connected for the different rings....

See, bad spelling and all.

subway_ie · 21-06-2004 4:48pm

It depends on the marking scheme, usually its at least 3 marks of a 9 mark question, so I'd say about 3-5 marks Max. As long as you had everything else right that is...

eire_245 · 21-06-2004 5:07pm

on Q 9....i got the fundamental frequency......but forgot to get de normal one using .......c=flamda.......but subbed in de fundamental frequency for KE=hf-phi.........and then this gave me my answer .......how many marks would i have lost for this????

DS · 21-06-2004 5:12pm

Originally posted by PrecariousNuts
mgh = 1/2 mv^2
2gh = v^2
v^2 = 3.92
v = 1.97 ms-1

m1v1 = m2v2
10(1.97) = 8v2
v2 = 2.47 ms-1

v^2 = u^2 + 2as
0 = (2.47)^2 + 4a
6.125 = -4a
a = -1.53 ms-2

F = ma
F = 0.01225N

Yep, exactly what I got.

PrecariousNuts · 21-06-2004 5:19pm

Bitchin

subway_ie · 21-06-2004 5:19pm

Originally posted by eire_245
on Q 9....i got the fundamental frequency......but forgot to get de normal one using .......c=flamda.......but subbed in de fundamental frequency for KE=hf-phi.........and then this gave me my answer .......how many marks would i have lost for this????

It's hard to tell in physics really. As a rule of thumb they give marks in lots of 3; so, if the question is 9 marks, you need 3 statements of fact, or if its a mathematical question, there's usually a few different stages and you get marks for getting to each stage. Definition type questions are alot easier to predict the marking scheme for than mathematical questions - one statement of fact for 3 marks. For maths questions, it's just too unpredictable.

eire_245 · 21-06-2004 5:23pm

yeah hearin wat ur sayin but will they give me the marks for the KE part cause i did it right but wit a wrong value cause my method was right nd only knock off marks for the specific wrong bit like in maths

DS · 21-06-2004 5:24pm

Precarious, got all the same answers as you except Q10, didn't do it.

Anyone do Q8? All I can remember was 3.24V.

DS · 21-06-2004 5:27pm

Great paper. My 3 glorious days of wasting around before yesterday paid off, I think I might have robbed an A1. What a relief, couldn't have gone better. Did an extra Q in A and in B. Experiments were really easy. Really straightforward calculation Q's like 6,7,9, really nice.

PrecariousNuts · 21-06-2004 5:30pm

Q8 I did it at the end just cause, you know..

(i) 3.76V, 2.24V
(ii) 1.12E-4C
(iii) 5.6197E-4J

Probably wrong because I don't particularly enjoy electricity

DS · 21-06-2004 5:36pm

Yeah 3.76 not 3.24. Can't remember the other 2. Electricity is crap, hope I'm not doing much of it next year. It's not real physics.

senordingdong · 21-06-2004 6:16pm

Well im enough of an adult to admit when im wrong....and i was wrong in my predictions.

It seems most of you (and most people in my class) were satisfied wit htat paper.

I wasnt, i think i failed.
Anyone thats in the same boat as me should post here.

oeNeo · 21-06-2004 6:19pm

Yeah that paper was sort of bent, you're not the only one. I haven't talked to anyone from my class as I left 20 minutes early.

Section A was fine I think, probably got loads wrong though. Section B was an utter-bollox. Q9 and Q10 were the only ones I actually might have done decent in, Q5. 6, 11, 12 were all horrendous.

Despite that I'm now finished, FOREVEAAR. GOOD BYE LEAVING CERT FORUM.

Mutant_Fruit · 21-06-2004 6:27pm

Ah right, i seemed to miss your third post there

Said the live wire was red

It can be red OR brown i thought, depending. But i tihnk brown is more correct

nesthead · 21-06-2004 6:29pm

great paper. ****ed up the particle physics question tho. i also stupidly forgot to get a value for "g" from my graph, although i got the graph perfectly right, fully labeled and all that.

couldnt belive there was only one graph question on the whole paper!!

subway_ie · 21-06-2004 6:31pm

oeNeo/senordingdong - I don't think anybody can really say that the physics paper was unfair. It was a very fair paper, with a good balance of mathematical and "definition" type questionsm probably more so than most years. It stuck to the same template as previous years, and even repeated a few parts of questions. I definetly dont think you can blame the paper for making you fail.

senordingdong · 21-06-2004 6:37pm

Originally posted by subway_ie
I definetly dont think you can blame the paper for making you fail.

Im not blaming the paper. Im just not that good at physics, full stop. I was hoping that i could write enough thrash to scrape a pass but after seeing soloutions and what not, i doubht it.
But just to clarify, i dont blame the paper, my teacher or even the constolations.

oeNeo · 21-06-2004 6:42pm

Who's blaming the paper? I thought the paper was bent, me personally. Sure a lot of people probably thought it was fine but just like senordingdong said I'm not the best at physics. I could more than likely get an A1 if I tried hard enough but I didn't put in the effort. Because of that for ME, the paper was bent.

subway_ie · 21-06-2004 6:44pm

Originally posted by oeNeo
Who's blaming the paper?

Ah, ok, I read "bent" as meaning "bad/not right"

SeanPuddin · 21-06-2004 6:46pm

My teacher is to blame for my errors. I study physics at home, and when it suits him i get a class, like once a month.

He did the spectrometer experiment with me like 2 weeks ago:

i got the wavlength part wrong, and after he goes "Yea you got that part wrong"
****in queer.

oeNeo · 21-06-2004 6:47pm

Originally posted by SeanPuddin
i got the wavlength part wrong, and after he goes "Yea you got that part wrong" ****in queer.

Hahaha, what a legend teacher.

senordingdong · 21-06-2004 7:03pm

Originally posted by oeNeo
Hahaha, what a legend teacher.

I think you mis-read that, i think sean was calling his teacher a ****ing queer, not vice versa.

Smellyirishman · 21-06-2004 7:07pm

Ye I had a crap teacher! Had to teach myself the whole course since easter.

He was out today in all his colours though seeing how everyone did and I thought I would just want to get away from him but he is human and it aint his fault he cant teach (he knows physics inside out hes a genious at it but cant teach)

So when I knew (more or less) that I passed I just let it slide cause all the work was done and I got what I wanted so I talked to him for a while very graciously (is that the word?! )

Ye my dad says that he has heard a LOT of complaints on the radio about the paper! I was like WTF but apperantly there was TOO much maths on it but I dunno I stopped listening to the radio the day before the english exam!

Cathy · 21-06-2004 7:14pm

Originally posted by PrecariousNuts
Yeah thats a pretty pathetic mass you have there, look at my huge throbbing 2x10^30kg

Can you confirm any of my answers?

Off the top of my head, I know that you have the right answer. I knew it in the exam, but I couldn't work it out - I got 4 by 10^30 or something.

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