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

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

Only if you take the equilibrium as 0 potential energy , but the the bottom has NEGATIVE GRSVITSTIONAL POTENTIAL - problem!!

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.?

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?

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

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.

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?

I am probably (definitely) being really dumb here but how would I get volume?

Because I have area but not length?

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 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

.

predictions for 6 markers?

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