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# OCR Physics B G495 Field and Particle Pictures June 21st 2011 Exam Thread watch

1. (Original post by TheTomD)
I also found that one tricky, although thought I knew what I had to do!

All-in-all not a bad paper, could have been much worse. Didn't get majorly stuck on anything and atleast I finished this time (damn you G491 resit!)!
did you manage to do it in the end?
I tried finding the mass with E=mc^2, then using KE = 1/2 mv^2?! but V was something like 3.8x10^8
2. (Original post by Alex.Stevens)
did you manage to do it in the end?
I tried finding the mass with E=mc^2, then using KE = 1/2 mv^2?! but V was something like 3.8x10^8
I think you were supposed to do it using the definition of the gamma factor in terms of velocities(1/the root of (1-v^2/c^2))
3. (Original post by Alex.Stevens)
did you manage to do it in the end?
I tried finding the mass with E=mc^2, then using KE = 1/2 mv^2?! but V was something like 3.8x10^8
I think you couldn't use KE= 1/2 mv^2 because the particle was moving at relativistic speeds to time dilation screws that approximation up.

I did Erest +Ek = Etotal then Etotal/Erest= Gamma
Gamma =1/SQRT(1-V^2/C^2)
So used the value of gamma rearranged the equation and got a velocity that was of the magnitude 10^7 or 10^8 ms^-1 can't remember which lol Probably wrong but the velocity came out to allow my gamma factor so I might get some marks haha
4. Thought that was about as good as we could have hoped for. I only needed 64 marks... hello Uni! Takes off heaps of pressure from my retake on Monday anyway.... so happy.
5. (Original post by Revolution is my Name)
I think you were supposed to do it using the definition of the gamma factor in terms of velocities(1/the root of (1-v^2/c^2))
yeah, and v is 2.5x10^8 ms^-1
6. (Original post by Larry777)
I think you couldn't use KE= 1/2 mv^2 because the particle was moving at relativistic speeds to time dilation screws that approximation up.

I did Erest +Ek = Etotal then Etotal/Erest= Gamma
Gamma =1/SQRT(1-V^2/C^2)
So used the value of gamma rearranged the equation and got a velocity that was of the magnitude 10^7ms^-1 Probably wrong but the velocity came out to allow my gamma factor so I might get some marks haha
Yeh thats what I did, I got gamma to equal 1.88 or something then v to equal 2.45 x 10^8
7. (Original post by Larry777)
I think you couldn't use KE= 1/2 mv^2 because the particle was moving at relativistic speeds to time dilation screws that approximation up.

I did Erest +Ek = Etotal then Etotal/Erest= Gamma
Gamma =1/SQRT(1-V^2/C^2)
So used the value of gamma rearranged the equation and got a velocity that was of the magnitude 10^7 or 10^8 ms^-1 can't remember which lol Probably wrong but the velocity came out to allow my gamma factor so I might get some marks haha
I did that tooo i got gamma to be about 1.8 if i remmeber correctly and my speed was like 2 x10^8 if that rings any bells? i think i did it right (hopefully)
8. easily the best paper Ive had this year
9. oh how I hate relativistic effects.
10. (Original post by Alford)
I did that tooo i got gamma to be about 1.8 if i remmeber correctly and my speed was like 2 x10^8 if that rings any bells? i think i did it right (hopefully)
I think it does sound familiar!
11. Section C was *******s, as expected.
12. Anyone else think that was ridiculously easy O.o Can't actually think of a mark I've dropped apart from the last question, which was well confusing...
13. (Original post by jimmeh)
Anyone else think that was ridiculously easy O.o Can't actually think of a mark I've dropped apart from the last question, which was well confusing...
I thought it was easyish.... but if u think u smashed it what did u put for the increasing permeance of the iron core? I put about increasing cross sectional area and laminate the core? wondering if thats right :/

END OF SECTION C = FLOP
14. Ah ****, I got the relativistic factor as 1.8 or something then just divided c by 1.8; ah well.
15. (Original post by Alford)
I thought it was easyish.... but if u think u smashed it what did u put for the increasing permeance of the iron core? I put about increasing cross sectional area and laminate the core? wondering if thats right :/

END OF SECTION C = FLOP
Increase cross-sectional area and decrease length was what I got.
16. (Original post by Alford)
I thought it was easyish.... but if u think u smashed it what did u put for the increasing permeance of the iron core? I put about increasing cross sectional area and laminate the core? wondering if thats right :/

END OF SECTION C = FLOP
I said increase the cross sectional area and reduce the length of the magnetic circuit I pretty sure they are both correct as permeance= Permeability*Area/Length

I think laminating the core would also possibly increase the permeability as it would decrease the eddy currents in the coil that reduce the overall magnetic flux passing through the iron core So don't worry!
17. (Original post by Larry777)
I said increase the cross sectional area and reduce the length of the magnetic circuit I pretty sure they are both correct as permeance= Permeability*Area/Length

I think laminating the core would also possibly increase the permeability as it would decrease the eddy currents in the coil that reduce the overall magnetic flux passing through the iron core So don't worry!
Reckon saying "make the core fatter" would get the mark for the cross sectional area point?
18. (Original post by Alford)
I thought it was easyish.... but if u think u smashed it what did u put for the increasing permeance of the iron core? I put about increasing cross sectional area and laminate the core? wondering if thats right :/

END OF SECTION C = FLOP
Increase permeance? Balls. Definitely read that wrong, could've sworn that it said increasing the flux...That's what you get for being confident

I put increasing the current and number of turns, but if it said increase the permeance, then those are definitely wrong!
19. (Original post by Larry777)
I said increase the cross sectional area and reduce the length of the magnetic circuit I pretty sure they are both correct as permeance= Permeability*Area/Length

I think laminating the core would also possibly increase the permeability as it would decrease the eddy currents in the coil that reduce the overall magnetic flux passing through the iron core So don't worry!
It doesn't increase permeability, it reduces the back emf, but otherwise yeah.
20. (Original post by Larry777)
I said increase the cross sectional area and reduce the length of the magnetic circuit I pretty sure they are both correct as permeance= Permeability*Area/Length

I think laminating the core would also possibly increase the permeability as it would decrease the eddy currents in the coil that reduce the overall magnetic flux passing through the iron core So don't worry!
yehhh i think your definatley right after i realised but yeh im hoping the mark scheme allows laminate the core because it would prevent eddy currents therefore increase total flux in the core praying im right need all the marks before section c as i can get !

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