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

1. (Original post by JJBinn)
Has anybody done the specimen paper for this unit? We were looking at it in class, didn't really understand parts of the SHM question. If nobody has done it, how would you work out the phase difference of two oscillating systems with their frequencies?

One was 0.2Hz and the other was 1.5Hz, and they were in phase. And when one was 10Hz and the other was 1.5Hz they were 180 out of phase. I didn't understand why. Thanks
Is it accessible on the AQA website? I'm just trying to find it

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2. (Original post by CD223)
Is it accessible on the AQA website? I'm just trying to find it

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I can't seem to find the paper online, he gave us a paper copy. I can only find a link to the mark scheme
3. http://spcmaghera.synthasite.com/res...nit_4_B_QP.pdf

It's question 2, the QWC part.
4. (Original post by JJBinn)
http://spcmaghera.synthasite.com/res...nit_4_B_QP.pdf

It's question 2, the QWC part.
Ah thank you!

I believe it's all to do with the natural frequency that you calculated in the earlier part. The natural frequency is 1.5Hz in this question. So essentially it is asking you to describe the oscillations of a mass spring system when the driving frequency is below, equal to, and above the natural frequency.

Does that make sense? I can try and brainstorm six points if that helps?

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5. (Original post by l1lvink)
I was doing the PHYA4 June 2014 paper today and got stuck on a handful of the Section A questions and was hoping that some of you might be able to help me out with the ones I haven't been able to figure out?
https://www.dropbox.com/s/308v2mbp1d...21718.jpg?dl=0
https://www.dropbox.com/s/tcu9b76utz...21749.jpg?dl=0
https://www.dropbox.com/s/ltnp00c0y6...21814.jpg?dl=0
I'm stuck on Questions 6, 7, 15, 24; the answers are B, C, C and A respectively.
Apologies in advance for the upside down photo, but this is from the 'top secret' June 2014 paper.
Thanks
(Original post by Mai.H)
For 6 i did T+mg=mv^2\r and then rearranged it for T.

For 7 i used x=cos2pift from the formula sheet and replaced it with p and q

For 24 i used the Nfi=BAN then i replaced BAN with the second emf induced in a rotating coil equation from the formula sheet by reaarranging it and i replaced the w with 2pi/T to get A since t cancels and sin(2pi)=0

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Thanks a lot. One query though; when I followed your answer to Q24, I end up with Nfi=emf/((2pi/t) x sin2pi), so I get the entire denominator to equal 0--what did you do differently?

Also, does anyone know how to solve Q15?
6. (Original post by CD223)
Ah thank you!

I believe it's all to do with the natural frequency that you calculated in the earlier part. The natural frequency is 1.5Hz in this question. So essentially it is asking you to describe the oscillations of a mass spring system when the driving frequency is below, equal to, and above the natural frequency.

Does that make sense? I can try and brainstorm six points if that helps?

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Yeah I understood the majority of the points in the mark scheme, I just really can't get my head around the phase differences and wondered if somebody could explain.

For example, when the driving Frequency is at 0.2Hz, the systems are in phase. I don't really understand the link between frequency and phase difference I don't think. Thanks if you can explain this
7. (Original post by JJBinn)
Yeah I understood the majority of the points in the mark scheme, I just really can't get my head around the phase differences and wondered if somebody could explain.

For example, when the driving Frequency is at 0.2Hz, the systems are in phase. I don't really understand the link between frequency and phase difference I don't think. Thanks if you can explain this
When the driving frequency is below the natural frequency for a lightly damped system, the oscillator and driver are in phase - the displacement of the oscillator is in phase with the driver, causing small, but not large oscillations.

When the driving frequency equals the natural frequency for a lightly damped system, the oscillator and driver are pi/2 radians out of phase - the displacement of the oscillator is pi/2 radians out of phase with the driver, meaning as the velocity is pi/2 radians ahead of the displacement, the force and velocity are in phase and the system moves with maximum amplitude.

When the driving frequency is much greater than the natural frequency, the system cannot keep up with the driver as it is oscillating too fast for the oscillator to produce oscillations with sufficient amplitude - the driving frequency is in anti phase with the displacement, meaning the amplitude severely reduces.

If you follow this through, this is how you get different "bell shaped" resonance curves as the driving frequency approaches the natural frequency.

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8. (Original post by CD223)
When the driving frequency is below the natural frequency for a lightly damped system, the oscillator and driver are in phase - the displacement of the oscillator is in phase with the driver, causing small, but not large oscillations.

When the driving frequency equals the natural frequency for a lightly damped system, the oscillator and driver are pi/2 radians out of phase - the displacement of the oscillator is pi/2 radians out of phase with the driver, meaning as the velocity is pi/2 radians ahead of the displacement, the force and velocity are in phase and the system moves with maximum amplitude.

When the driving frequency is much greater than the natural frequency, the system cannot keep up with the driver as it is oscillating too fast for the oscillator to produce oscillations with sufficient amplitude - the driving frequency is in anti phase with the displacement, meaning the amplitude severely reduces.

If you follow this through, this is how you get different "bell shaped" resonance curves as the driving frequency approaches the natural frequency.

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Okay so any value below 1.5 for the driving would've meant the systems were in phase? and any above makes them out of phase? Or is it more along the lines of, any significantly below/significantly above?

Thanks a lot
9. (Original post by JJBinn)
Okay so any value below 1.5 for the driving would've meant the systems were in phase? and any above makes them out of phase? Or is it more along the lines of, any significantly below/significantly above?

Thanks a lot
It's more along the lines of "At a frequency much below the natural frequency, they're in phase. At a frequency equal to the natural frequency they're pi/2 radiance out of phase. At a frequency much above the natural frequency, they're pi radians out of phase."

No worries!

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10. (Original post by CD223)
I hate them. It feels awful that my first "exam" is virtually a week away and I've done no prep haha!

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Yep my practical is on Monday & Wednesday, two weeks yesterday until the written part... 2 1/2 weeks until my first real exam which is a lot closer than I realised!!!
11. (Original post by Lau14)
Yep my practical is on Monday & Wednesday, two weeks yesterday until the written part... 2 1/2 weeks until my first real exam which is a lot closer than I realised!!!
Woah! 10 days until my section B, then 40 days until my first exam!

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12. (Original post by CD223)
Woah! 10 days until my section B, then 40 days until my first exam!

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40 days sounds like ages... weird having only four weeks left in college though, doesn't feel like it's ending at all?
13. (Original post by Lau14)
40 days sounds like ages... weird having only four weeks left in college though, doesn't feel like it's ending at all?
Yeah 5th June - it'll soon creep up though!

I know what you mean. They've not even released details about our leavers do yet haha.
14. (Original post by CD223)
Yeah 5th June - it'll soon creep up though!

I know what you mean. They've not even released details about our leavers do yet haha.
I think 5th June is my 4th and 5th exams :P

I think we got some details last week about the last day but I skipped tutorial for my friends birthday so I don't really know!
15. (Original post by Lau14)
I think 5th June is my 4th and 5th exams :P

I think we got some details last week about the last day but I skipped tutorial for my friends birthday so I don't really know!
Oh my - that makes me feel scared. We got given an EMPA past paper pack yesterday which made me wince haha.

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16. (Original post by CD223)
Oh my - that makes me feel scared. We got given an EMPA past paper pack yesterday which made me wince haha.

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Yeah I really need to go over the EMPA papers, probably leave it until after my practical now though!
17. (Original post by Lau14)
Yeah I really need to go over the EMPA papers, probably leave it until after my practical now though!
I have no excuses. I'll definitely start looking over them tomorrow before my shift at work :P

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18. Does anyone else advocate doing the written section prior to the multiple choice?
19. (Original post by Lau14)
Yeah I really need to go over the EMPA papers, probably leave it until after my practical now though!
You mentioned in your A2 EMPA prep, to show the examiner any anomalies on the graph--how do you show this?
20. (Original post by l1lvink)
You mentioned in your A2 EMPA prep, to show the examiner any anomalies on the graph--how do you show this?
Circle the point and label it anomalous, be sure to leave it out of your line of best fit if this is the case.

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