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

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Original post by Fvthoms


The mark scheme for 2 a) (ii). for this question, don't need numerical advice, just a description of the thought process and the method taken to calculate the answer.
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In the earlier part of the question you know that the polar orbiting satellite is in an orbit of radius of 7370km and so the satellite requires a certain amount of force to stay in its orbit as its weight is pulling it towards the earth.

As stated in the mark scheme there are more than one approaches to finding the centripetal force. You are already given the time period of the orbit of the satellite and that is extremely useful because it indicates the time taken by the satellite to orbit around the earth without falling towards the earth. If this time was greater it would definitely fall.

The quickest method is the one involving less steps of calculation as it would be the first one in the mark scheme. Because you have the time period you don't need to take account of the Inverse square law of gravity as as this would be needed when you are given the radius to work out that time period or vice versa. You could still use it but then you would have to look at the formula sheet to get the mass of the earth. So because you have the time period you can simply consider the satellite as an object tied to a string and swung in a horizontal circle with the same time period. Then using F=mw2r would give you the centripetal force which will be equal to the weight of the satellite at that height above the earth.

You might be thinking working with F=mv2/r but then you would have to know the linear velocity of the satellite, that is the velocity at which the satellite travels the circumstance of the orbit. But because the angular velocity is constant you just need to use the first equation as i said above or the first one in the mark scheme.
Le me know if you still have any questions




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Original post by NEWT0N
Sir, I frame no hypothesis.


Kudos to science super star

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Original post by NEWT0N
I be not understanding yεy^{\varepsilon} word Kudos that you use, but mere science super star I am not, for I am a science nebula cloud.


For the sake of your magnificiant hair we must stop this conversation as it serves no purpose for the physicians on this forum.
If u dont believe me then good luck proving scientifically:ahee:

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Original post by NEWT0N
Sir, I frame no hypothesis.


Yes! But you have to admit that you have not been able to discover the cause of those properties of gravity from phenomena.


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Original post by NEWT0N
I have a rigorous proof by Alchemy.

But again you are mistaken, for contrary to popular belief, that is not my hair but bits of my large surface area brain flowing out of my head.

Your most humble servant,

Is. Newton


Oneday a tree grew freshly new apples weighing a ton.

It was a newton of apples

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Original post by NEWT0N
Ha ha ha, I have not become this merry since the time when one of my students had the audacity to ask "what use is this book Euclid's Elements?" Funny lad.


I appreciate your admiration of my jolly jok.

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Original post by NEWT0N
God is the first and last cause, a most subtle gravitational spirit that pervades all and lies hid in all gross bodies.


So then why just as the world was created from dark chaos through the bringing forth of the light and through the separation of the airy firmament and of the waters from the earth, our work brings forth the beginning out of black chaos and its first matter through the separation of the elements and the illumination of matter?


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Original post by NEWT0N
I am much obliged for the amusement you provided me.


Most welcome. Perhaps someday you might return my favour in some other form.

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Hi guys I need help on doing this question as I find it really difficult, could someone please post an answer on this question so I could see how you worked it out in each step. I need to know how to do I would appreciate it if anyone could.
Thanks(urgently)
q1)Pyrometers are used to measure the temperature of substancesremotely where direct measurement is not practical or possible. One suchexample is measuring the temperature of blast furnaces in steel making. Theyuse Stefan’s Law to calculate the temperature based on the amount of energyreceived. If the amount of energy received by the pyrometer is 7.01x105Wm-2calculate the temperature of the furnace in °C. Stefan’s Constant, σ, = 5.7x10-8Wm-2K-4.
q2)

1.

Blast furnaces are made from steel andlined with refractory bricks to keep as much heat energy in and to prevent thefurnace from melting. If the temperature within the furnace is as above, thethermal conductivity of the refractory brick is 1.4, the thermal conductivityof the steel is 43, the thickness of the protective brick lining is 1m, thethickness of the steel casing is 500mm, and the energy transferred through bothlayers is as above, calculate the temperature at the boundary of the lining andthe casing if the outside temperature of the blast furnace is 50°C.

Hi guys I need help on doing this question as I find it really difficult, could someone please post an answer on this question so I could see how you worked it out in each step. I need to know how to do I would appreciate it if anyone could.
Thanks(urgently)
q1)Pyrometers are used to measure the temperature of substancesremotely where direct measurement is not practical or possible. One suchexample is measuring the temperature of blast furnaces in steel making. Theyuse Stefan’s Law to calculate the temperature based on the amount of energyreceived. If the amount of energy received by the pyrometer is 7.01x105Wm-2calculate the temperature of the furnace in °C. Stefan’s Constant, σ, = 5.7x10-8Wm-2K-4.
q2)

1.

Blast furnaces are made from steel andlined with refractory bricks to keep as much heat energy in and to prevent thefurnace from melting. If the temperature within the furnace is as above, thethermal conductivity of the refractory brick is 1.4, the thermal conductivityof the steel is 43, the thickness of the protective brick lining is 1m, thethickness of the steel casing is 500mm, and the energy transferred through bothlayers is as above, calculate the temperature at the boundary of the lining andthe casing if the outside temperature of the blast furnace is 50°C.

Original post by NEWT0N
Hint: Unit 2 wave theory (nodes, etc.)

Yes, it's sad how they expect you to remember AS physics because most people forget the unnecessary details (ones that aren't "built on" in A2) after a while. If they insist on including AS stuff, I would much prefer if they did AS+A2 in one paper at the end of A2. Having it so far away from the A2 paper and still expecting everyone to know it is silly.


Yes it is annoying that they want us to remember AS knowledge.

But I still do not understand the question. I wasn't particularly good at Unit 2.
3 full days of smashing unit 4. I can't wait


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Hello everyone, I've done the JAN '11 paper and I've come upon questions that I were stuck and I was wondering if you could help me.

Section A - Q14, Q21 and Q24.

Section B - Q5(c)

Thank you in advance.
Original post by Disney0702
Hello everyone, I've done the JAN '11 paper and I've come upon questions that I were stuck and I was wondering if you could help me.

Section A - Q14, Q21 and Q24.

Section B - Q5(c)

Thank you in advance.


for 14 use F=EQ and F=ma with the values of q and m for an election, for 21 use linkage=BANCostheta and subtract the values at 0 and 50 and for 24 you need to consider how a transformer works if voltage increases then current decreases and the secondary coil has more turns so there more flux linkage.

Question 5c goes right back to unit 2 where the fundamental harmonic was one loop and thus wavelength/2 shove the numbers in the formula(v=frequency*wavelength) to see it does give the fundamental vibration and thus resonates.
@NEWT0N; did you manage to work out the final question in my last post?
Original post by Dalek1099
for 14 use F=EQ and F=ma with the values of q and m for an election, for 21 use linkage=BANCostheta and subtract the values at 0 and 50 and for 24 you need to consider how a transformer works if voltage increases then current decreases and the secondary coil has more turns so there more flux linkage.

Question 5c goes right back to unit 2 where the fundamental harmonic was one loop and thus wavelength/2 shove the numbers in the formula(v=frequency*wavelength) to see it does give the fundamental vibration and thus resonates.


Thank you for your explanations, they've really helped.

I'm afraid I'm still a little confused on Q5(c), I know it is Unit 2 but I jut cannot understand this question.

Would it be too much to ask if you could do a step by step detailed explanation because I do not see how I can explain how a large amplitude vibration is produced.

Thank you in advance.
Can someone explain Q1(a)(ii) from JUN '11.

Why is R = mg - mv2/r

I assumed it'd be R = mg + mv2/r

As weight and centripetal force acts in the same direction.
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Original post by Disney0702
Can someone explain Q1(a)(ii) from JUN '11.

Why is R = mg - mv2/r

I assumed it'd be R = mg + mv2/r

As weight and centripetal force acts in the same direction.




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Original post by Disney0702
Can someone explain Q1(a)(ii) from JUN '11.

Why is R = mg - mv2/r

I assumed it'd be R = mg + mv2/r

As weight and centripetal force acts in the same direction.


No, the centripetal force IS Mg-R. Therefore, you can say Mg-R = Mv^2/r and hence R = mg-mv^2/r. I made this mistake too
Original post by a.a.k


That photo has just solved 3 days of issues thanms


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