# OCR Physics, Forces, Fields and EnergyWatch

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11 years ago
#81
You get 1.55 if you forget to squareroot the ratio of Kelvin temps.
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11 years ago
#82
(Original post by teachercol)
You get 1.55 if you forget to squareroot the ratio of Kelvin temps.
How many marks for 1.55?
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11 years ago
#83
(Original post by mabe_uk)
How many marks for 1.55?
3 although I guess you'd get some marks for saying KE is proportional to T, and stuff.
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11 years ago
#84
i think i lost 25 marks on that paper
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11 years ago
#85
wrong smile
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11 years ago
#86
(Original post by teachercol)
Nope havent scanned it yet - just sat down and did the paper
Usual disclaimer - I've just rocketed through the paper and havent checked these.

Q1 Looks like a nice and easy opener if you can read from the graph successfully.

a) Mgh = 0.22 -> h=1.60m (2)
b) Ke increases at a decreasing rate (2)
c) i) KE = 0.11J -> v = 3.96ms-1 (2)
ii) At terminal vel, drag force up = weight down = mg = 0.137N (1)
iii) Energy loss per second = mgh / t = mgv = 0.543W (2)
d) KE before bounce = 0.11J; v before = 3.96 ms-1 down
KE after bouce = 0.8 x 0.11J; vafter = 3.55ms-1 up
Change in v = 7.5 ms-1 (usual ctach about changing direction)
Change in momentum = 0.105 Ns or kg. m s-1 (4) Total 13

Q2 Always a tricky topic for many people
a) Internal energy = sum of random KE and PE for all paerticles in the system
SHC = energy needed to change temp of 1kg of material by 1K (3)
b) H=mcdT = 1.45E5J (3)
c)i) PV=nRt - sub first set of values to get n=2.65mol
sub second set of values to get n=4.11 mol
dn = 1.46 mol -> change in mass = 0.044kg (4)
ii) Energy is prop to T in Kelvin so ratio of speeds = ration of sqr(temps in kelvin)
= sqr(453/293) = 1.24 (3) Total 13

Q3 Straightforward motion in a circle.
a) i) Arrows marking Tension and weight (2)
ii) Resultant force must be horizontal and towards centre of circle (2)
iii) F = m v^2 / r
b)i) Vert T cos theta = mg
Horiz T sin theta = m v^2 /r
so tan theta = v^2 /rg -> tan theta = 0.36 and theta = 19.8 degrees (4)
ii) Theta will stay the same because doesnt depend on m (1) Total 10

Q4 Not too bad - seen most of this before
a) Charge is +ve so lines leave spehere and go towards plates, top and bottom
Lines must meet spehere and plates at 90 degrees. (3)
b) i) E = V / d = 1.25E6 NC-1 (2)
ii) F = EQ = 6.25 E-3N (2)
c) i) F = q1q2/4Pieor^2 -> 0.14 mN (3)
ii) 0.14 mN = 1.37g
Charges repel so must add to scale reading -> 9.58 g (2) Total 12

Q5 Not too bad - but a few might struggle.
a)i) Flemings LH Rule -> Force is vert upwards (1)
ii) F = BIL -> 4.8E-3 N (2)
b) i) T = 20E-3s f = 1/T = 50Hz (2)
ii) Standing wave has an antinode in middle
Acc is prop to amplitude
Amplitude varies so acc is not same (3)
c) i) Resonance (1)
ii) Peak at 50 Hz so mark in units of 25 Hz on scale (1)
ii) Usual sketch of damped resonance curve - lower / broader/ same resonant f. (3)
Total (13)

Q6 Routine at start, gets harder
a) Beta- decay so a neutron becomes a proton.
Z = proton number increases by one 38-39
N = neutron number decreases by one (137-55) 82 ->81 (2)
b) i) Activity = number of nuclei decaying per unit time (1)
ii) lambda = ln2/t1/2 (Dont forget to convert years to seconds) = 7.33E-10 s-1 (2)
c) i) After 15y N = 3.525E15.
A = lambda N = 2.59E6 Bq (2)
ii) N = No exp (-lambda t) Read a pair of values of N and t from graph, sub and calc No
3.525E15 = No exp (-7.33E-10 x 15 x 3.15E7)
No = 4.98E15 (3)
iii) n = No / Na = 8.3E-0 mol
m = nx mm = 8.3E-9 kg = 8.3 ug (2) Total 14)

Q7 Not a lot needed here - but you have to know this reaonably well

a) Need wavelength of order of spacing.
X ray interference (bragg law)
Diagram and Details of path difference and how to calc spacing
Electron (diffraction) interference. eg graphite
How to calc spacing from ring pattern
Lambda = h/ mv (7)
b) Atom dia 1E-10 - 1E-9
Nucleus dia 1E-15 - 1E-14

Wavelengths need to be less than size of obect to produce diffraction pattern.
(4) + (4) QWC

Without sounding like I'm nit picking, me, my brother and a few lads from school think you may be wrong on the following questions (don't hold me on it though).

4)ii 0.14mN = mg
5)ii will actually have a lower resonant frequency (very slightly) must touch at f=0 but nowhere else.
6)iii it was about 1.13E-6g (though my brother originally got your answer but realised his mistake and changed it)
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11 years ago
#87
That was quite alright in the end. I got so lucky. I did an X-ray diffraction question last night. Got 0 marks on it. Revised it. I still don't think I did too well on that question but still.

Plus that ratio of the speeds bit i might of mucked up by forgetting to convert to kelvin. Should get a few still though. I'm thinking I lost between 10 and 20.

Question 1 a)i) i did perfectly first time, then decided it was wrong, then decided it was wrong again....I got it right in the end. Damn graphs and scales etc.
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11 years ago
#88
lets just hope the ums is 60 because i might of just scraped a A
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11 years ago
#89
always marks for working out so even if you did get some questions wrong it should still build up
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11 years ago
#90
(Original post by peach plum pear)
3 although I guess you'd get some marks for saying KE is proportional to T, and stuff.
Yeah I said KE is proportional to T. Ok, paper except the last question, didnt revise xrays/electron stuff throughly. I hope mark scheme is generous.
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11 years ago
#91
That was a tough exam, but I thought It was easier than other past papers ive done like June '07. Btw does anyone know the exact date of when our January results are published?
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11 years ago
#92
sorry - just a quick query, apologies if ive been stupid. didnt 6iii) ask you to show it was approx 1*10^-6g (1 ug)?

i reached that
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11 years ago
#93
(Original post by wrockite)
Without sounding like I'm nit picking, me, my brother and a few lads from school think you may be wrong on the following questions (don't hold me on it though).

4)ii 0.14mN = mg
5)ii will actually have a lower resonant frequency (very slightly) must touch at f=0 but nowhere else.
6)iii it was about 1.13E-6g (though my brother originally got your answer but realised his mistake and changed it)
4) Yes - You need to convert F to kg, (/ 9.81 ) then convert to g. Suspect i may have x 9,81 rather than /9.81 :blush
5 c) ii is the resonance curve - yes the resonant freq will be slightly less but generally ignored at A level.
6 iii) yes there are a couple of typos above . You need to mutiply the number of moles by 0.137 to get the mass in kg. Then x 1E9 to get it into milligrams. You get the answer given on the paper.
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11 years ago
#94
Hi
my question is probably out of context but since it happend during this exam .......

Well, we were all told to only use black ink by our exam officer and we were all provided with black ink pens but for some reason i decided to do it in blue.
Do you think it will influence my mark?? (Sounds ridiculously but just to make sure that i dont get disqualified from the exam for using blue ink )
Thank you.
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11 years ago
#95
Not at all, I think they're trying to make it compulsary black but it says on the front blue or black ink. I should imagine it's to do with the scanning. In my maths exam they said to us we should use black but blue is okay but please use black in the future.

I think this went bette than I originally thought looking at those answers. I got 1.24 for that ratio woo!!
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11 years ago
#96
i had 1.24 im sure that was correct v is propotioanl to root T then u say V/root T = V/Root T for the second one
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11 years ago
#97
Thanks for the marks list

Based on that i lost 3 for the standing wave question (well, i did kinda think 3 marks was a lot if acceleration was the same, but i couldn't think of a reason it shouldn't be, so i just waffled) and a couple on diffraction.

I turned to the back first to look at the last question to give myself time to think if i needed to...i hate diffraction, i wasn't there when we did it -_- but i think i did okay.

Probably a couple of sig fig errors and something for drawing too... but i think i should have over 80 which is my target
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11 years ago
#98
(Original post by Candescence)
Based on that i lost 3 for the standing wave question (well, i did kinda think 3 marks was a lot if acceleration was the same, but i couldn't think of a reason it shouldn't be, so i just waffled) and a couple on diffraction.
Ha! I had exactly the same thought process. I can't actually remember what a standing wave is, mind.
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11 years ago
#99
So was the acceleration the same at all points on the length of wire in the field?
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11 years ago
#100
Nope as not all the wire was in the field so the displacement in the centre of the field is more than the outside and a is proportional to displacement from equilibrium.

I got that wrong in the test though
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