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# OCR Physics B G495 Field and Particle Pictures June 21st 2011 Exam Thread watch

1. (Original post by JoeCarr)
I'd also like to post a final challenge to the uncertainty answer in the section C answer booklet, as it says the implied uncertainty for 9.81 is 0.01, but surely it's half of that because you would be rounding up anything 0.005 below or down from above, so isn't the uncertainty +/- 0.005??
0.01 means that the range of uncertainty is 0.01, it is a valid way of stating uncertainty, less used than +/- 0.005
2. (Original post by Alex.Stevens)
Yeah I thought that as well, don't see whats wrong with the word! I mean its nearly centripetal just a couple of letters different!
But on a serious note, isn't centrifugal the right term as its acting away from the rotation axis??
Centrifugal is the more correct term because the earth is no longer an inertial frame, because it is rotating. So in the element's frame of reference, it is feeling a 'mysterious' force. This mysterious force causes the element to increase in volume (and thus reduce its force due to pressure) in order for the gravitational force to out-balance the force due to the pressure, causing it to rotate.
3. (Original post by Summerdays)
EXACCCCCCTTTTTTTTTTTTTTTLLLLLLLL LLLLLYYYYYYYY.

But apparently, it doesn't matter.
aha, if it comes up in the exam I would write that little justification, and also it nearly did matter because it was close to the uncertainty in the time, and the next question asked which was larger so if it had been, we would have been WRONG
4. (Original post by Summerdays)
Centrifugal is the more correct term because the earth i no longer an inertial frame, because it is rotating. So in the element's frame of reference, it is feeling a 'mysteriou' force. This mysterious force causes the element to increase in volume (and thus reduce its force due to pressure) in order for the gravitational force to out-balance the force due to the pressure.
Ok, yeah your getting a A*.
5. (Original post by JoeCarr)
aha, if it comes up in the exam I would write that little justification, and also it nearly did matter because it was close to the uncertainty in the time, and the next question asked which was larger so if it had been, we would have been WRONG
Shaking my head at OCR B.
6. As a fellow Carr, JoeCarr, you have a brilliant surname.
7. maybe because it's rotating it requires a centripetal force...

considering force at poles, if we assume that the only force acting on them is gravity, which pulls the surface toward the centre

but at the equator... the same force must provide the pulling force and a force for centripetal acceleration - hence it can't 'pull as much' so the radius increases a little.

Consider a young, non-spinning molten Earth. (This isn’t strictly necessary for the argument – in what follows even a solid Earth will deform plastically over a long enough time) The gravitational force will try to pull all parts of the planet as close as possible together.
A stable spherical shape is formed when the gravitational force on a small element of volume is in
equilibrium with the outward force due to the pressure difference across the element.
When the Earth is spinning then the gravitational force on an element in the equatorial plane must be bigger than the force due to the pressure difference so that there is a resultant force on the element towards the axis of rotation to provide the centripetal force for circular motion.
Hence the element moves outwards to where the force due to the pressure difference is smaller than the gravitational force on the element. Thus the equatorial radius becomes greater and so the polar radius must become smaller so that volume is conserved.
8. (Original post by Sereni)
0.01 means that the range of uncertainty is 0.01, it is a valid way of stating uncertainty, less used than +/- 0.005
I all ways thought that uncertainty = range/2 ?!
9. (Original post by Alex.Stevens)
Ok, yeah your getting a A*.
I still don't understand why the earth bulges I can't give a DEFINITE reason.
10. (Original post by kttt101)

but at the equator... the same force must provide the pulling force and a force for centripetal acceleration - hence it can't 'pull as much' so the radius increases a little.

One question: why is that always the case?
11. (Original post by Summerdays)
One question: why is that always the case?

what do you mean?
12. (Original post by Summerdays)
One question: why is that always the case?
this is going to sound incredibly basic, the earth will have a finite amount of energy which it needs to split between gravitational and rotational?
and for some reason its greater in the centre?!
13. (Original post by Summerdays)
I still don't understand why the earth bulges I can't give a DEFINITE reason.
Some of the gravitational force at the equator is used to keep the earth spinning therefore there is less force pulling in the middle of the earth that the poles and so it bulges. Imagine spinning a blob of clay, it bulges in the middle because the resultant force holding it together in the middle is less than the resultwnt pulling the top and bottom in
14. (Original post by Swindan)
As a fellow Carr, JoeCarr, you have a brilliant surname.
agreed man, we may have to put up with ****ty brum brum jokes in primary school, but at the end of the day it's a great surname.

want to hear something that takes it to the next level though? My mum's maiden name was..............Driver. No.word.of.a.lie. Imagine how cheesy a chat up line it would sound if my dad asked for her name, and then said when she asked for his, 'I can be the car to your driver' or w.e....
15. (Original post by Sereni)
Some of the gravitational force at the equator is used to keep the earth spinning therefore there is less force pulling in the middle of the earth that the poles and so it bulges.
That makes sense. But why is it the equator that is used to keep the earth spinning?
16. Anyone care to clarify electromagnetic machines?
Transformer: AC -> Alternative Flux -> Eddy currents due to AF causing current in the core -> Current produced in secondary
Generator: Movement -> Current
Motor: Current -> Movement
Last two confuse me a bit, any help in how its formed.
17. (Original post by Alex.Stevens)
I all ways thought that uncertainty = range/2 ?!
The range that you are uncertain about covers 0.01, it is more or less the same as +/- 0.005, and may be used in preference when you dont have equal uncertainties each side eg + 0.0075/- 0.0025. Both are valid, if you feel more comfortable with +/- then yse that one, just be away that the other exists
18. (Original post by Anthz)
Anyone care to clarify electromagnetic machines?
Transformer: AC -> Alternative Flux -> Eddy currents due to AF causing current in the core -> Current produced in secondary
Generator: Movement -> Current
Motor: Current -> Movement
Last two confuse me a bit, any help in how its formed.
You seem to undersand the basics, i wwouldnt worry about the harder details too much, the answer is usually ----- induces ---- which causes -----. There are more marks involved in the calculations. Make sure you can draw flux lines, look over the hand rules, and practice some emf flux flu density calculations etc.

As for motors ang generators just remember change in flux linkage induces an emf, an emf causes flux, a changing emf (AC) produces a changing flux which causes an emf and eddy currents. Also that whatever is generated acts to oppose what generated it, so the emf caused by the changing flux caused by the changing emf opposed the changing emf.
19. (Original post by Summerdays)
That makes sense. But why is it the equator that is used to keep the earth spinning?
read this... I Think I understand it now...
http://en.wikipedia.org/wiki/Equatorial_bulge
20. (Original post by JoeCarr)
agreed man, we may have to put up with ****ty brum brum jokes in primary school, but at the end of the day it's a great surname.

want to hear something that takes it to the next level though? My mum's maiden name was..............Driver. No.word.of.a.lie. Imagine how cheesy a chat up line it would sound if my dad asked for her name, and then said when she asked for his, 'I can be the car to your driver' or w.e....
I'm betting that something along those lines must have definitely been said.

They didn't fancy hyphenating then?

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