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# AQA Physics PHYA4 - 20th June 2016 [Exam Discussion Thread] watch

1. (Original post by Music With Rocks)
How do you do this question?

Water of density 1000 kg m–3 flows out of a garden hose of cross-sectional area7.2 × 10–4m2 at a rate of 2.0 × 10–4m3 per second. How much momentum is carriedby the water leaving the hose per second?

A 5.6 × 10–5N s
B 5.6 × 10–2N s
C 0.20 N s
D 0.72 N s
Find the velocity by dividing (2.0x10^-4 by (7.2x10^-4 m^2) , then find the mass of water leaving the hose per second, so density x volume (which you are given) , then p=mv (per second)
2. (Original post by Terminatoring)
Only if you take the equilibrium as 0 potential energy , but the the bottom has NEGATIVE GRSVITSTIONAL POTENTIAL - problem!!
I had the same questions...I came to the conclusion it is being dumbed down because it's a levels....because realistically kinetic energy would only be gained from highest point down to equilibrium....everything else is conversion of elastic to grav potential

I think you just assume on the way up its elastic-kinetic, past equilibrium it's Ke to ge...it's dum I know but this way of thinking is what's in the Merck schemes
3. (Original post by kingaaran)
No, I am talking about when you are doing energy calculations. The negative sign simply indicates that you have lost energy and it has been expelled into another form.

Take a particle performing vertical circular motion and your zero level to be at the highest point the particle reaches into its motion. Then when you go beneath that zero level, you have lost potential energy, which will give you a negative sign. Doesn't mean energy is negative - just that it has been lost to another form.
C
I'm not going to sit here and claim you can have negative energy, because that of course makes no sense. I would have thought it'd have been obvious that I am talking about energy losses.
Okay so you've lost energy. SO HOW IS THE TOTAL POTENTJAL ENEGY MAXIMUM.?
4. Time to rest. Best of luck tomorrow folks.
5. (Original post by Terminatoring)
Okay so you've lost energy. SO HOW IS THE TOTAL POTENTJAL ENEGY MAXIMUM.?
What kind of oscillation is the question talking about? Mass/Spring?
6. (Original post by Terminatoring)
Why does the graph of potential energy in 2014 show a vertically oscillating Spring mass system as having 0 potential energy at some points? Surely at the equilibrium it has gravitational potential energy and at the bottom elastic?
The graph shows total potential energy. The potential energies cancel when the mass spring is at equilibrium position. The Gpe is the same as the Elastic potential at this point.
7. Im dreading it hopefully its fine tomorrow
8. Please ignore me lads and get some rest because im gonna confuse a few innocent souls
9. (Original post by ombtom)
Momentum = mv
m = density * volume
v = volume per second divided by area

Does this help?
A little, still not fully there haha

so v=(2.0*10^-4)/(7.2*10^-4) = 5/18

then m = 1000* ?

Not quite sure what to use as volume to calculate mass
10. Alright guys I've got it sorted out. Give me the chance to explain myself once and for all.
When coil's plane face is PARALLEL to the field lines(looking from the side, imagine two wires forming an area with its plane face facing up and down), the FLUX LINKAGE is minimum because that's when no flux line cuts through it, and from this position if the face was to turn slightly, it goes from no flux line cutting through it at all to suddenly having flux line cutting through it, so the total flux lines cut through the face suddenly increases by a lot! so the the RATE of change of flux linkage is max(thus Faraday's Law), which means induced emf is maximum.
When coil's place face PERPENDICULAR to the field lines, the FLUX LINKAGE is maximum because that's when the most flux lines are cutting through and from this position if the face was to turn slightly, the total flux lines cut through the face decreases by a tiny bit, so the RATE of change of flux linkage is zero(which is emf because emf proportional to rate of change of flux linkage), therefore induced emf is minimum.
I realised it my before comments I've been half wrong and half right, so I hereby present my apology by means of getting this right.
11. (Original post by Music With Rocks)
A little, still not fully there haha

so v=(2.0*10^-4)/(7.2*10^-4) = 5/18

then m = 1000* ?

Not quite sure what to use as volume to calculate mass
Volume in 1 second. So V/s becomes V.
Alright guys I've got it sorted out. Give me the chance to explain myself once and for all.
When coil's plane face is PARALLEL to the field lines(looking from the side, imagine two wires forming an area with its plane face facing up and down), the FLUX LINKAGE is minimum because that's when no flux line cuts through it, and from this position if the face was to turn slightly, it goes from no flux line cutting through it at all to suddenly having flux line cutting through it, so the total flux lines cut through the face suddenly increases by a lot! so the the RATE of change of flux linkage is max(thus Faraday's Law), which means induced emf is maximum.
When coil's place face PERPENDICULAR to the field lines, the FLUX LINKAGE is maximum because that's when the most flux lines are cutting through and from this position if the face was to turn slightly, the total flux lines cut through the face decreases by a tiny bit, so the RATE of change of flux linkage is zero(which is emf because emf proportional to rate of change of flux linkage), therefore induced emf is minimum.
I realised it my before comments I've been half wrong and half right, so I hereby present my apology by means of getting this right.
Mate I still think you're not quite right. Flux linkage is not the same as flux lines being cut, in fact they are the opposite. See this pic from a good revision website

look at the diagram then read the italic text
13. (Original post by ombtom)
Volume in 1 second. So V/s becomes V.
I am probably (definitely) being really dumb here but how would I get volume?

Because I have area but not length?
14. (Original post by Music With Rocks)
I am probably (definitely) being really dumb here but how would I get volume?

Because I have area but not length?
I've lost your original question now but I'm sure it gave the volume per second.
15. (Original post by Music With Rocks)
I am probably (definitely) being really dumb here but how would I get volume?

Because I have area but not length?
the volume is given in the question. it gives in m^3 per second and you model it as one second, so it is essentially the volume of the cylinder!
16. predictions for 6 markers?
17. (Original post by Music With Rocks)
A little, still not fully there haha

so v=(2.0*10^-4)/(7.2*10^-4) = 5/18

then m = 1000* ?

Not quite sure what to use as volume to calculate mass
the volume leaving the hose per second times by density = mass leaving hose per second

the momentum per second is just mass per second x velocity of the water
18. (Original post by d14m)
the induced EMF is directly proportional to the rate of change of flux linkage, the rate of change of flux cut is greatest when the coil is parallel to the field

When the coil is perpendicular to the magnetic field the rate of change of flux cut is zero, but the flux linkage is at a maximum. You can see this graphically by looking at the gradient of a flux linkage graph

I remember it by thinking of it in terms of a differential, it's pretty confusing initially
(Original post by kingaaran)
.
Again this is confusing me... The induced EMF is greatest when it's 90degrees to give sin90=1, making the emf induced a maximum and so the rate of change of flux linkage a maximum....

This is starting to **** me off now
19. (Original post by tanyapotter)
predictions for 6 markers?
Was just about to ask this. I'm going to guess something about magnetic fields. Hopefully not damping/natural frequencies or an experiment (do they still do that in unit 4?)
20. (Original post by abro1089)
Mate I still think you're not quite right. Flux linkage is not the same as flux lines being cut, in fact they are the opposite. See this pic from a good revision website

look at the diagram then read the italic text
Okay, the italics texts say this.

"The induced emf has its greatest value when the MOVEMENT of the coil is perpendicular to the field"
Imagine when the PLANE of the coil is parallel to the field line, the MOVEMENT of the coil will be perpendicular to the field. Say the field lines going from right to left. and say the coils(looking from sideways) is is turning anticlockwise, with its plane face facing up and down, now that's when the MOVEMENT of the coil is perpendicular because left wire is moving down as the right wire is moving up(turning anticlockwise you see)
If you still can't understand this then I can't even even anymore

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