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# OCR A2 Physics A: G484 & G485 - 15 & 21 Jun 2017 [Exam Discussion] watch

1. (Original post by headyb)
Acoustic impedance of bone was=7.55x10^6 so density=4720kgm-3.
I'll think of some more
how did you work it out though?
2. (Original post by adsy9)
how did you work it out though?
Ir/Io=0.4 (as 40% reflected) Using Ir/Io=(Z2-Z1)^2/(Z2+Z1)2, I square rooted* both sides and rearrange to make Z2 the subject and enter value for Z1 (1.7x10^6).

Also, I think the energy used to kill cancer cells = 1.3J
3. (Original post by headyb)
Ir/Io=0.4 (as 40% reflected) Using Ir/Io=(Z2-Z1)^2/(Z2+Z1)2, I squared both sides and rearrange to make Z2 the subject and enter value for Z1 (1.7x10^6).

Also, I think the energy used to kill cancer cells = 1.3J
same, was annoying to write out, got a diff value than you for density though, did you just do Z(muscle)/1600?
4. (Original post by sadasdasdasd)
same, was annoying to write out, got a diff value than you for density though, did you just do Z(muscle)/1600?
Wasn't it asking calculate the density of the bone? I did Z(bone)/1600
5. (Original post by headyb)
Wasn't it asking calculate the density of the bone? I did Z(bone)/1600
it was on the previous page to the bone question, had to turn over to get to the bone one from the muscle one
6. (Original post by sadasdasdasd)
it was on the previous page to the bone question, had to turn over to get to the bone one from the muscle one
Oh slight memory lapse! Yeah I did 1.7x10^6/1600=1062.5 and rounded to 1063
7. there was the power transferred by x-ray question and then mass change of the star (1.8*mass of sun).
power transfer I used the exponential formula to find I, did I*A (had to multiply mm^2 by 10^-6 to get m^2) then multiplied this by 2*60.
mass of the star was just an energy calculation followed by using E=mc^2
ratio was something like 0.33, dunno
8. advantages for MRI over CAT I wrote that MRI is non-ionising as no x-rays and that CAT scans provide better contrast for soft tissues (with low Z).
9. (Original post by sadasdasdasd)
there was the power transferred by x-ray question and then mass change of the star (1.8*mass of sun).
power transfer I used the exponential formula to find I, did I*A (had to multiply mm^2 by 10^-6 to get m^2) then multiplied this by 2*60.
mass of the star was just an energy calculation followed by using E=mc^2
ratio was something like 0.33, dunno
I think the energy used to kill the cancer cells was 1.3J (A=6mm2 ,x=1.4?cm, I=2x10^6, u=5cm, t=120s). I worked out the intensity then multiplied by area for power, then time for energy.

Can you remember if it stated 2pm in the X-ray production question? I might have been 20pm actually..
10. (Original post by headyb)
I think the energy used to kill the cancer cells was 1.3J (A=6mm2 ,x=1.4?cm, I=2x10^6, u=5cm, t=120s)

Can you remember if it stated 2pm in the X-ray production question?
was 12pm, got 23j for the energy question though.
it gave a huge value for voltage for me anyway, 6.xx*10^something
11. (Original post by headyb)
I think the energy used to kill the cancer cells was 1.3J (A=6mm2 ,x=1.4?cm, I=2x10^6, u=5cm, t=120s). I worked out the intensity then multiplied by area for power, then time for energy.

Can you remember if it stated 2pm in the X-ray production question? I might have been 20pm actually..
it was 2pm
12. (Original post by sadasdasdasd)
was 12pm, got 23j for the energy question though.
it gave a huge value for voltage for me anyway, 6.xx*10^something
Yeah I ended up with a value just under 7x10^5 I think, rounded up due to the question asking for producing a minimum wavelength X-ray of 2pm.
13. (Original post by emmabrindley1)

This is my attempt at uploading it ??? Not sure if it worked
Thank you for your effort, didn't see it in time though - paper was easy enough. Magnetic resonance+sun evolution free marks..
14. (Original post by adsy9)
it was 2pm
I thought it was about that. I ended up using 10-13 by some weird mistake for the pico meter though.
15. Magnetic field:
Momentum of particle=3.456x10-23
Energy=4102eV
Time for 60deg=(1/6)(2pi)(0.018)/(v)=4.96x10-10s

Ra-222 -> Po-218 + alpha particle.
Number of radioactive isotopes in the basement=1.80x10^7
Ratio of isotopes to 40mol of air=7.46x10^-19.
Emits harmful ionising radiation and has a long half life.
16. (Original post by headyb)
Magnetic field:
Momentum of particle=3.456x10-23
Energy=4102eV
Time for 60deg=(1/6)(2pi)(0.018)/(v)=4.96x10-10s

Ra-222 -> Po-218 + alpha particle.
Number of radioactive isotopes in the basement=1.80x10^7
Ratio of isotopes to 40mol of air=7.46x10^-19.
Emits harmful ionising radiation and has a long half life.
what was your particle velocity?
lambda for the decay was something like 0.693/3days, 40/this was 2.xxx
17. (Original post by sadasdasdasd)
what was your particle velocity?
lambda for the decay was something like 0.693/3days, 40/this was 2.xxx
I worked out p first from rearranging F=mv2/r=BQv->mv=BQr, divided by m for the velocity=3.80x10^7ms-1.

Activity was 40? Well in that case t1/2 was 3.2 days, so lambda=0.693/(3.2*3600*24)=2.51x10-6 s-1.
N=40/2.51x10-6=1.6x10^7
18. (Original post by sadasdasdasd)
What did you guys get for the arc seconds a of the star?
0.2ish it was less than 1 from what I remember

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