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

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Original post by QueNNch
Past grade boundaries:1434020555233.jpg

I reckon
51 for A
57 for A*


64/5 for 120 UMS?


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Original post by NEWT0N
The graph was for discharing, so my analysis applies there


But the point is, it was charged to that max current without that resistance and then discharged so the only thing R affects is the time taken to discharge
Original post by Humbers
1. Is newtons as rate of change of momentum is force (newtons 2nd law)
21. I put A as it had the longest side perpendicular to the magnetic field. Its a couple and not looking for flux (so largest area is irrelevant) force on couple is F=BIl so biggest couple was A as it had largest length perpendicular to field
22. I think is 1/root 2 as kinetic energy is proportional to the square of velocity

yes i agree
For Coulombs law, would I get any marks for quoting the formula and saying what each symbol meant?
Original post by Fred Cantoni
64/5 for 120 UMS?


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I hope you're right, last year it was 69/75 for 120 UMS
Original post by chizz1889
Average I got 0.0475. Worked out what I was at 90• (0.0950) then divided by 2 ☺️ (Think I may have needed to divide by root2 though as I think RMS applies


Nah I don't think you do. It's average because the rate of change of flux may not be constant, it's just the change in Nphi/ t


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Original post by Will177
But the point is, it was charged to that max current without that resistance and then discharged so the only thing R affects is the time taken to discharge


V=IR the first basic law, V remains the same , R increases therefore I decreases
Original post by Humbers
1. Is newtons as rate of change of momentum is force (newtons 2nd law)
21. I put A as it had the longest side perpendicular to the magnetic field. Its a couple and not looking for flux (so largest area is irrelevant) force on couple is F=BIl so biggest couple was A as it had largest length perpendicular to field
22. I think is 1/root 2 as kinetic energy is proportional to the square of velocity


Yeah, newton is equal to acceleration.

For 21 i meant the same thing. I have written the one with the greatest width
22 asked for the ratio of the forces snd not velocity


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well I just lost my uni place for september!
Original post by TheRAG
For Coulombs law, would I get any marks for quoting the formula and saying what each symbol meant?


From past paper mark schemes, that's fine and you should get full marks :biggrin:
The Electric field strength should have been negative as the charge was negative. -1560 Vm^-1.

I got this wrong.

Also any clues on the damping question? I wrote there would be more damping without the ring as the total energy given to the system with the ring will be greater as GPE=mgh. The work done against air resistance is the same as time period is not affected - because the pendulum system has more energy with the ring it will take longer for the initial energy to be lost through doing work against air resistance. Thus the amplitude will decrease at a slower rate.
Original post by TheRAG
For Coulombs law, would I get any marks for quoting the formula and saying what each symbol meant?


Its possible but they said to write in words so its a 50/50 chance coz sometimes they allow formulas as long candidates write what each symbol means
Reply 4152
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smelon
Original post by Fred Cantoni
No. It's force.


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F=m(v-u)/t so I don't see how it can be force? Surely the unit of force is kgms^2, whereas momentum has a unit of kgms^1
Original post by Amanzz
Nah I don't think you do. It's average because the rate of change of flux may not be constant, it's just the change in Nphi/ t


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That's not the law for a rotating coil
Reply 4154
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k9000
2
For the capcitance and 2mj question did you guys get B (the energy taken from the battery was 2mj) that was incorrect and some energy would have been wasted so more energy was actually taken?
Original post by Mehrdad jafari
Yeah, newton is equal to acceleration.

For 21 i meant the same thing. I have written the one with the greatest width
22 asked for the ratio of the forces snd not velocity


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A newton is not the same unit as acceleration

Force = newton = rate of change of momentum mv-mv/t = kgms-2
Acceleration = rate of change of velocity = ms-2


for 22. i know its the ratio but if it has half the kinetic energy it has root 2 of the velocity F=BQV so force is proportional to velocity
Surely A because the longest length was perpendicular to the field?
Original post by k9000
For the capcitance and 2mj question did you guys get B (the energy taken from the battery was 2mj) that was incorrect and some energy would have been wasted so more energy was actually taken?


Yep
Original post by M1llward
The Electric field strength should have been negative as the charge was negative. -1560 Vm^-1.

I got this wrong.

Also any clues on the damping question? I wrote there would be more damping without the ring as the total energy given to the system with the ring will be greater as GPE=mgh. The work done against air resistance is the same as time period is not affected - because the pendulum system has more energy with the ring it will take longer for the initial energy to be lost through doing work against air resistance. Thus the amplitude will decrease at a slower rate.


Less damping, mass is increased and so therefore is momentum. Increased momentum results in resistive forces having a lesser effect on the motion of the pendulum, thus less damping.
Original post by DanTheMan358
Ahh i think you may be right.


Do you remember how the questions was worded?

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