Physics Discussions

PrecariousNuts

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

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

Roller Toaster

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

subway_ie

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

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

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

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

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

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

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.

subway_ie

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

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

Gotta get ready for chemistry tomorrow aswell

Mutant_Fruit

(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

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

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

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

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

oopsssssss. forgot to label...

PrecariousNuts

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

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

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

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

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

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

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

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

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

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

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.