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AQA Physics PHYA4 - Thursday 11th June 2015 [Exam Discussion Thread] watch

1. (Original post by Adangu)
Question 1c) june 2013 phya4/2 ? Help much appreciated as I never know how to do these

http://filestore.aqa.org.uk/subjects...2-QP-JUN13.PDF
Take the period of one and divide by the difference in period, usually works
2. I don't understand how the induced emf (rate of Change of flux) is max when the flux through it is zero!?
3. So do we always use the lowest significant figure from the question in the final answer?
4. (Original post by Sbarron)
I don't understand how the induced emf (rate of Change of flux) is max when the flux through it is zero!?
If you do AS or A level maths, it's because differentiating cosx gives (-)sinx. Otherwise it's because (maximum flux- current flux) is greatest when current flux= 0. Hence emf= (maximum flux- current flux)/time is greatest
5. (Original post by Sbarron)
I don't understand how the induced emf (rate of Change of flux) is max when the flux through it is zero!?
imagine the flux changing as a sin wave as a coil rotates. the derivative of a sine wave is cosine, ie pi/2 out of phase. when a sine wave is zero its gradient is max and when its at max and min its gradient is zero. Its the rate of change of flux not actually the flux itself
6. (Original post by HenryHein)
Lowest common multiple which is 38s. After each oscillation of the 2s pendulum the 1.9s pendulum become 0.10s more out of phase. So after 20 cycles, the phase difference between the 2s pendulum and the 1.9s pendulum becomes 2s. As this is equal to the 2s pendulum's time period, they are back in phase (in the same way that for any waveform 2pi out of phase is the same as being in phase)
(Original post by DanielWall96)
Take the period of one and divide by the difference in period, usually works
Thank you guys so much. So basically

Work out difference between time period = 0.1

Take the longer time period and divide by it 2/0.1= 20

Times this by the shorter time period 1.9 x 20 = 38s

Cheers!
7. (Original post by michaelotty)
So do we always use the lowest significant figure from the question in the final answer?
Only have to if they ask you to. Of course it is good general practice to do so anyway.
8. Can someone explain why Q14 is A? i thought it was B?...

http://www.egsphysics.co.uk/files/a_...W-QP-Jun02.pdf
9. (Original post by Sbarron)
I don't understand how the induced emf (rate of Change of flux) is max when the flux through it is zero!?
When the magnetic flux linkage through the rectangular coil is 0 ( parallel to the magnetic field) then the induced emf would be maximum because the rectangular coil would cut most of the field lines perpendicularly at this position.
10. Does anyone know how to go about answering this question? I got A but the answer is B.
11. (Original post by Boop.)
Can someone explain why Q14 is A? i thought it was B?...

http://www.egsphysics.co.uk/files/a_...W-QP-Jun02.pdf
Because the magnetic field lines are not passing through the charged particles at RIGHT ANGLES. Therefore, the magnetic force would not act where the particles would move unaffected.
12. (Original post by Boop.)
Can someone explain why Q14 is A? i thought it was B?...

http://www.egsphysics.co.uk/files/a_...W-QP-Jun02.pdf
The electron is travelling parallel to the magnetic field, so the force on the electron is 0.
13. can someone explain how they go about working out q13 on june 2010 paper? http://filestore.aqa.org.uk/subjects...W-QP-JUN10.PDF
14. (Original post by _Caz_)
If you don't mind me asking, what year paper is this question from?
no idea sorry, it's from this document https://08c6cd28b8bef288858e878e5745...e%20choice.pdf
15. Are eddy currents similar to back emf?
16. (Original post by aprocrastinator)
no idea sorry, it's from this document https://08c6cd28b8bef288858e878e5745...e%20choice.pdf
Do you have the answers for those?

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17. (Original post by CD223)
Do you have the answers for those?

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https://08c6cd28b8bef288858e878e5745...hoice%20MS.pdf
18. (Original post by Sbarron)
I don't understand how the induced emf (rate of Change of flux) is max when the flux through it is zero!?
To be honest i don't know that either.

(Original post by HenryHein)
If you do AS or A level maths, it's because differentiating cosx gives (-)sinx. Otherwise it's because (maximum flux- current flux) is greatest when current flux= 0. Hence emf= (maximum flux- current flux)/time is greatest
Yeah mathematically that proves it but if you think about it the the change in flux in the coil is no different when it passes horizontally through the magnetic field lines than it passes vertically. In fact if you think about it, the emf induced would have to be greater when the coil passes vertically through the magnetic field lines than when it passes horizontally through the magnetic field lines. But the truth is otherwise, that is the emf induced is greatest when the coil passes horizontally to the magnetic field lines and minimum when it passes vertically through the magnetic field lines. I have read explanations that the change in flux is greatest when the coil passes horizontally through the field lines and minimum when it passes vertically through the field lines. But this cannot be true as the change in flux in the coil is as equal when the coil passes horizontally as when it passes vertically through the field lines.

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19. does anyone have any good pdfs i could use to revise from
20. Thanks!!

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