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

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I'm struggling with this one too :frown:


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Original post by gcsestuff
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I keep getting B, any help ? :smile:


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It should be the maximum flux linkage, shouldn't it? To be honest if it is i don't even know why it is


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Original post by Mehrdad jafari
For a rotating coil in a magnetic field:

When the coil is vertical the flux linkage is greatest and when the coil is horizontal the induced emf is greatest

I thought induced emf is the change of flux linkage so why when the coil is at right angle to the magnetic field lines the induced emf us zero? This sounds so unreasonable


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The angle in question i believe is that between the field lines and the normal to the coil. When coil is vertical, angle = 0 therefore flux = max, when coil is horiz, angle = 90 flux =0
Flux = BANcostheta
Magnitude of emf = d/dt(N.flux)
Reply 2803
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Original post by gcsestuff
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I'm struggling with this one too :frown:


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I am guessing C.


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Original post by Ilovemaths96
The angle in question i believe is that between the field lines and the normal to the coil. When coil is vertical, angle = 0 therefore flux = max, when coil is horiz, angle = 90 flux =0
Flux = BANcostheta
Magnitude of emf = d/dt(N.flux)


Thanks for your response :smile:

Yeah, mathematically that explains it but I'm having trouble with the idea that when the flux linkage is maximum the induced emf is zero


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Original post by a.a.k
I am guessing C.


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How do you assume that the collision is inelastic?


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Reply 2806
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Original post by Mehrdad jafari
How do you assume that the collision is inelastic?


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Is the answer right.
Btw i am assuming since they both moving together it cant inelastic otherwise they would be moving in opposite direction

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Reply 2807
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Original post by Mehrdad jafari
Thanks for your response :smile:

Yeah, mathematically that explains it but I'm having trouble with the idea that when the flux linkage is maximum the induced emf is zero


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I always relate it to how many lines cut through. Though i would suggest this as an exam answer

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Original post by a.a.k
Is the answer right.
Btw i am assuming since they both moving together it cant inelastic otherwise they would be moving in opposite direction

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Your answer can't be right as the momentum is always conserved and can not change.
And also not necessarily. Looking at the trucks one has a less mass than the other so during the interaction it might only exert such a force as to not move in the opposite direction to its original velocity


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Reply 2809
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Original post by Mehrdad jafari
Your answer can't be right as the momentum is always conserved and can not change.
And also not necessarily. Looking at the trucks one has a less mass than the other so during the interaction it might only exert such a force as to not move in the opposite direction to its original velocity


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And this is why is said i am guessing.

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Original post by a.a.k
And this is why is said i am guessing.

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It's cool man :smile:
But guessing is not a good thing to do. We need to have confidence in our thinking. During my mocks i would never guess an answer to a question even though choosing an option would be of no harm as i wouldn't loose marks. So I would leave it blank. For me if i don't know, then i don't know :smile:


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Reply 2811
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Original post by Mehrdad jafari
It's cool man :smile:
But guessing is not a good thing to do. We need to have confidence in our thinking. During my mocks i would never guess an answer to a question even though choosing an option would be of no harm as i wouldn't loose marks. So I would leave it blank. For me if i don't know, then i don't know :smile:


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I do the same. But if i cant do them until the exam finishes then i just guess in the last minutes of the exam.

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Reply 2812
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Original post by Mehrdad jafari
It's cool man :smile:
But guessing is not a good thing to do. We need to have confidence in our thinking. During my mocks i would never guess an answer to a question even though choosing an option would be of no harm as i wouldn't loose marks. So I would leave it blank. For me if i don't know, then i don't know :smile:


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Whats the answer btw

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Answer is b to the momentum one. :smile:


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Original post by a.a.k
I do the same. But if i cant do them until the exam finishes then i just guess in the last minutes of the exam.

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That's true. If you guess you still have 25% chance of getting it right which is a good thing


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Original post by a.a.k
I am guessing C.


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I would say B: Clearly momentum doesn't change. For the kinetic energy, we know that the collision will slow down their overall speed (clearly they can't travel together at more than or equal to the speed of the first car!). But their mass is increased. kinetic energy is proportional to mv^2, where m gets bigger and v gets smaller, as we just showed. But v is squared, so v getting smaller causes more affect overall, so mv^2 get smaller overall. Hence the kinetic energy decreases. So: momentum is unchanged, kinetic energy decreases --> option B.
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Original post by gcsestuff
Answer is b to the momentum one. :smile:


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Aww thnx i was nearly got it right

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Original post by a.a.k
Aww thnx i was nearly got it right

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Yeah c was just below b :wink:


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Reply 2818
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Original post by Mehrdad jafari
Yeah c was just below b :wink:


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And A was just above b :awesome:

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Original post by gcsestuff
Answer is b to the momentum one. :smile:


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I think i understand now why that it. Looking at the trucks, the moving one has a less mass than than the stationary one. This means that if the collision is perfectly elastic it would transfer some of its momentum( as it has a less mass)to the other truck, leaving itself with a lower velocity (in the opposite direction pehaps)so they couldn't have the same velocity after the collision.


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