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

1. this one was definitely harder than last year - we're so pressed for time in this exam its so annoying. gonna have to boss phya5
2. (Original post by QueNNch)

I reckon
51 for A
57 for A*
64/5 for 120 UMS?

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3. (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
4. (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
5. For Coulombs law, would I get any marks for quoting the formula and saying what each symbol meant?
6. (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
7. (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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8. (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
The voltage provided by the capacitor as it discharges is unchanged
The resistance of the load is greater than before as it discharges
Then just use V=IR like I showed to find the current in the discharging circuit
9. (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
10. (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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11. well I just lost my uni place for september!
12. (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
13. 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.
14. (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
15. (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
16. (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
17. 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?
18. (Original post by ssargithan)
21 A I put the same as you fro the same reason but people are saying B due to bigger area?
Yes B had the biggest area. The force is only on the vertical pieces of the wire because the horizontal ones are parallel to the field and therefore experience no force. F=BIL with I=5 for all the currents. Then the couple is Fd-BILd=BI*Area of the rectangle. BI is constant so the couple is proportional to area of loop.
19. (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
20. Surely A because the longest length was perpendicular to the field?

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