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

1. (Original post by DanTheMan358)
But still? B statement was correct?
No it wasn't as the battery used more than 2mJ as 2mJ was supplied to the capacitor and a little extra to the wires
2. (Original post by M1llward)
Im sure a couple is the force acting perpendicular to the axis of rotation multiplied by the distance between the two equal and opposite forces. So the couple is equal to BI(vertical length)*(horizontal length) ie proportional to the area. So the coil with the largest area had the greatest couple and that was B.
I put A
3. For the iron bar and coil question i thought current was not induced because there was no relative motion between the magnetic field from the coil and iron. But in the second part after the variable was turned out the magnitude of the magnetic field has changed and then induces current.

After reading what you guys have put I'm gonna assume I'm wrong
4. does anyone remember a brief summary of each question? e.g. like question 1 was resonance then damping.
5. (Original post by _pirate_pete)
The damping question with ring and cone. The damping force is air resistance, which depends on shape of object, how streamlined it is. It does not depend on mass. hence ring has no effect on the damping forces. But the effect of the damping forces will be less since the ringed cone has more inertia. The question is what the mean by damping? The dissipative forces, or the final effect?
Exactlllyylylyy
6. (Original post by _pirate_pete)
The damping question with ring and cone. The damping force is air resistance, which depends on shape of object, how streamlined it is. It does not depend on mass. hence ring has no effect on the damping forces. But the effect of the damping forces will be less since the ringed cone has more inertia. The question is what the mean by damping? The dissipative forces, or the final effect?
Damping is the final effect, not the amount of force, the impact of the force is reducing and so therefore so is the damping (I got it wrong but that is correct)
7. (Original post by chizz1889)
No it wasn't as the battery used more than 2mJ as 2mJ was supplied to the capacitor and a little extra to the wires
8. (Original post by DanTheMan358)
Also for the ring and cone question, i put it has no affect on damping? Because damping is caused by resistive forces and weight doesn't affect acceleration downwards?
Gravity?
9. (Original post by TheRAG)
what did y'all put for the six marker?
This is a brief summary but I included extra details in my answer:

Newton's first law applies because if the centripetal force is removed the ball will move in a straight line tangential to its circular path at that point. (The horizontal component of velocity remains constant and gravitational acceleration increases the vertical component downwards, so it follows a parabolic curve.)

Newton's second law F=ma applies because the centripetal acceleration is v^2/r and the centripetal force is mv^2/r, which is mass times centripetal acceleration i.e. F=ma

Newton's third law applies because the string exerts a centripetal force on the ball and the ball exerts and equal and opposite tension force on the string to the right

The string isn't perfectly horizontal because there is a vertically downwards component of force on the string due to the ball's weight. But this is much smaller than the tension on the string to the right see previous paragraph for why there is a tension) So the vector triangle of forces on the string is: large tension to the right, weight of ball vertically downards. The resultant force on the string is therefore at a small angle below the horizontal causing it to be not horizontal
10. (Original post by DanTheMan358)
The question asked what was incorrect.. B was incorrect so that was the correct answer (confusing ik)
11. Hello guys, what was the Answer to the rod question , current increases? then for the second part it decreases as more power is wasted in primary due to higher resistance, also does it sort of act like a transformer?
12. For the height, I put some energy lost as it's transferred to heat energy (internal heat in motor)

and I also put that there would be lost volts due to internal resistance of the battery?
13. (Original post by king cobra)
Not in the spec so doubt it

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It's just lenz's law. A flux moving in the opposite direction causes the coil to set up a field in the opposite direction to the flux. Initially it sets up a field opposite to a flux moving in direction X (say), then it sets up a field in the opposite direction to -X. X is opposite to -X so the induced current the second time round is opposite.
14. (Original post by Fvthoms)
What do those symbols represent?

Momentum, p=mv [kgms^-1]
Rate of change= (delta)mv/t [kgms^-2]
Dimensions, there are 3 basic dimensions to every unit in physics (ie you can make a unit for anything using these three including tesla and things like that's) M=mass L=length T=time. Doing m3 in maths that is how I approach those questions as once you know, it's the easiest way to do it. Trust me it is definitely weight.
15. Do you think Ill get the mark for flux linkage unit if I just put 'Wturns'?

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16. (Original post by k9000)
The question asked what was incorrect.. B was incorrect so that was the correct answer (confusing ik)
ffs, that's annoying
17. (Original post by rohan9777)
Maybe for the two areas to be roughly equal.
That would make sense but then again someone sitting an exam has no way of accurately estimating this so I don't think it would be useful. Also what happens to people who got confused and just assumed they wanted us to indicate that the current is lower the second time (their reasoning would be something like "there is no way on earth that they want us to draw y axis intercept to scale" )
18. (Original post by imyimy)
does anyone remember a brief summary of each question? e.g. like question 1 was resonance then damping.
Question one asked you to describe what "forced oscillations" and "resonance" mean with regard to amplitude, frequency and phase difference.

There was a definition of Coulomb's Law
19. I wrote Wb Turns too!

(Original post by louisg565)
Do you think Ill get the mark for flux linkage unit if I just put 'Wturns'?

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20. Aw I thought it was Wb

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