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

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Think about moving a spring very slowly. You'd barely be affecting the oscillations which is what its getting at with 0.2 Hz.

1.5hz is resonance as its the same as the natural frequency.

As for 10Hz, you must remember those resonance curves, right? Think of what happens to the amplitude as the driver frequency exceeds the natural frequency.

Edit: I didn't pay attention to your question. When in resonance , the driver and the displacement of the oscillation system are 90 degrees out of phase. It's just a fact.

Thanks, that helps. To confirm for the 0.2 Hz do you assume the effect is so little it's in phase because the mark scheme says in phase with driver

Reply 3741

MSB47

Original post by CD223

Which board? AQA C4 screwed me over 😭

Posted from TSR Mobile

Yes same AQA, I honestly think for the questions they gave us it was really difficult to get them all done in 90 minutes. I panicked and made so many silly mistakes, was aiming for an A* this year :frown:

Reply 3742

Mattyhouse

Original post by Lau14

Yes

Cheers

Reply 3743

davidharris96

Original post by Humza Ali

How to work out when a set of swings of varying time period will next be "in conjunction"?

Do you mean when they'll next be in phase? Do the time period difference multiplied by the smallest time period, this gives the number of the smallest time periods that take place, then you multiply by the time period to get the overall time

(edited 8 years ago)

Reply 3744

QueNNch

Guys are we allowed to write on the Section A multiple choice question paper so that we can do the calculations?

Also is it okay to underline words in the questions?

I never did these things just in case until now that I feel the need to, for this exam.

Reply 3745

samlyon

Original post by HenryHein

The natural frequency is 1.5Hz.

When the frequency of the driver is equal to the natural frequency 2)(b)(ii), the phase difference is pi/2, i.e. in phase with velocity. At 0.2Hz (i), the phase difference is nearly zero. At 10Hz(iii) the phase difference is nearly pi

In general:
when driven at freq much lower than natural: little or no phase difference
when driven at resonance: phase difference pi/2
driven at frequency much higher than resonance: phase difference approaching pi.

I'm afraid I can't explain why this is the case, so unless someone else can it's just something to blindly accept.

Yeh that makes sense I was just trying to work out the exact phase differences for the 0.2Hz and 10 Hz ones but I didn't need to

Reply 3746

Lau14

Original post by Brianna96

Hi, quick question, if anyone could answer it for me, it would be a great help!!
Why in a MS does it say E proportional to V squared in the equation of energy stored in a capacitor? Why can't you use V is proportional to E because of the equation: E=0.5QV? How do you now what equation to use when working out what is proportional to what in the energy equations?

Posted from TSR Mobile

It's because Q is dependent on V or something I think?

Original post by Mattyhouse

My normal calculator keeps going on and off, its ****ed :s not sure whether just to use my graphic instead ;/

Take both if you want them!

Reply 3747

mrno1324

What's the difference when dropping a magnet through a copper tube and a coil, if there is any?

Reply 3748

davidharris96

Original post by QueNNch

Yeah as long as your answer is clear in the box then you're all good, i scrawl all over mine with workings out haha

Reply 3749

saad97

Original post by JaySP

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Sorry some are clearer than others

Thank you, I'm just confused about how you went from V^2/(V-2)^2 to V = 2V -4 on q19?

Reply 3750

username1571649

Original post by davidharris96

Thanks for this answer, its come up twice and this helps loads

Reply 3751

moneus

Original post by Brianna96

that eqn is for energy stored in a capacitor.

The question is asking about the CELL so use w= itv

Reply 3752

davidharris96

What are people feeling that the 6 mark written question will be on?

Reply 3753

AR_95

Right, I'm off for tonight. I'll see you guys after the exam tomorrow.

Best of luck!

Reply 3754

Plasmapause

Could someone explain why the answer to this question (question 4bi) is 7.4x10^11? When I did it I got 7.4x10^8

Reply 3755

JaySP

Original post by saad97

Thank you, I'm just confused about how you went from V^2/(V-2)^2 to V = 2V -4 on q19?

I tend to skip steps sorry.

Square root both sides, times both sides by (V-2)

Reply 3756

moneus

Original post by mrno1324

What's the difference when dropping a magnet through a copper tube and a coil, if there is any?

none that i can think off.

Both would experince a mag field.

Reply 3757

Lau14

Original post by mrno1324

What's the difference when dropping a magnet through a copper tube and a coil, if there is any?

My best guess would be that as a coil has more turns (N is higher) than for a tube (which has only 1), the magnitude of induced emf would be higher?

Reply 3758

saad97

Original post by JaySP

I tend to skip steps sorry.

Square root both sides, times both sides by (V-2)

Ah that makes sense now, thanks. Are you prepared for tomorrow?

Reply 3759

HenryHein

Original post by Humza Ali

How to work out when a set of swings of varying time period will next be "in conjunction"?

The short answer is to find the lowest common multiple.

If you can't do that (or have to explain how you got to your answer), first calculate the phase difference (in seconds).

Then either work out what you must multiply the phase difference by to get to the larger of the two time periods. Then multiply the SMALLER time period by that number.

OR work out what you must multiply the phase difference by to get to the smaller of the two time periods. Then multiply the LARGER time period by that number.

For example, I think it was in june 2014 where one system had a time period of 2s and the other 1.90s. So the phase difference is 0.1s. To get to 1.90s, multiply 0.1 by 19. Then 2 * 19 = 38s.