June 2011 G485-Fields, Particles and Frontiers of Physics Watch

Oh my Ms. Coffey
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(Original post by m92singh)
Its in the revision guide for OCR physics A so, i'd go along with it. Hence the fact Workdone= QV as you said.

Increasing the tube voltage gives them more kinetic energy.
Increasing the Current( just more electrons) means there are now more electrons in the same amount of time meaning work done which equals energy transffered is more becuase its more energy (more electrons) for the same amount of time.
Likewise if they ask you about increasing the intensity of the tube I was playing around with formula and managed to get this.

I=(Qv/t)/A
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ziigmund
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anyone have jan11 paper/markscheme? ty
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m92singh
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(Original post by Oh my Ms. Coffey)
Likewise if they ask you about increasing the intensity of the tube I was playing around with formula and managed to get this.

I=(Qv/t)/A
Yes, this links with the X-ray attenuation coefficient where I=Ioe^-ux ( u = mew)

and then the half value thickness x1/2 is the thickness of the material required to reduce the intensity to half its original intensity.

this is where X1/2=-ln2/u ( u= mew) This can be derived by the I=Ioe^-ux equation

where you treat I the intensity as 1 and Io as 2 becuase you want the intenstity to half its original and then divide I/Io = e^-ux then I/Io = 1/2 take natural longs of both sides gives ln2=-ux then rearrange to give X= -ln2/u

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sulexk
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(Original post by m92singh)
Yes, this links with the X-ray attenuation coefficient where I=Ioe^-ux ( u = mew)

and then the half value thickness x1/2 is the thickness of the material required to reduce the intensity to half its original intensity.

this is where X1/2=-ln2/u ( u= mew) This can be derived by the I=Ioe^-ux equation

where you treat I the intensity as 1 and Io as 2 becuase you want the intenstity to half its original and then divide I/Io = e^-ux then I/Io = 1/2 take natural longs of both sides gives ln2=-ux then rearrange to give X= -ln2/u

Hi there, I was wondering can anyone outline the main principles of an MRI scan, in terms of referring to the equipment used such as the gradient coils etc.

Also- there is something that troubles me, why do they setup different larmor frequencies across the patient? Because surely different tissues will have different relaxation times due to their surroundings, so why the use of gradient coils?

I think it is possible for this to come up in the paper - however I most certainly think that the gamma camera and Positive emission tomography scanning, may also come up, it could be 2 out of 3. I guess it would also be very helpful if someone could let us know how radioactive tracers are used to diagnose organs and tissue as well as blood flow in the body.

I was thinking tonight and monday night we have a quiz(keywords).

Tonight we could do Medical imaging and cosmology+ nuclear physics and maybe tomorrow capacitors, electric+magnetic fields.

Hows that sound. I guess we could do it like this:

Just post a keyword question ( I could be the mod in this case for the quiz)
and you post your answers, receive 1 rep for correct answer and nothing otherwise!

Just a way of marking lol!
I dont know if this would work but it could if we have about 5-10 people!


Thank you
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jam.wa
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(Original post by CoffeeStinks)
Could someone write down bullet points on evidence supporting the big bang model please.
I think if it's evidence you want, these are the shortest and simplest:

-Lots of EM produced by early universe
-Universe's radiation is istropic (3K)
-Redshift suggests expansion
-Redshift suggests starting point
-Early universe must have been very hot
-Helium abundance
-Helium formed in BB

These will easily get you through any 4 marker on evidence for the Big Bang.
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jam.wa
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(Original post by sulexk)
Hi there, I was wondering can anyone outline the main principles of an MRI scan, in terms of referring to the equipment used such as the gradient coils etc.

Also- there is something that troubles me, why do they setup different larmor frequencies across the patient? Because surely different tissues will have different relaxation times due to their surroundings, so why the use of gradient coils?

I think it is possible for this to come up in the paper - however I most certainly think that the gamma camera and Positive emission tomography scanning, may also come up, it could be 2 out of 3. I guess it would also be very helpful if someone could let us know how radioactive tracers are used to diagnose organs and tissue as well as blood flow in the body.

I was thinking tonight and monday night we have a quiz(keywords).

Tonight we could do Medical imaging and cosmology+ nuclear physics and maybe tomorrow capacitors, electric+magnetic fields.

Hows that sound. I guess we could do it like this:

Just post a keyword question ( I could be the mod in this case for the quiz)
and you post your answers, receive 1 rep for correct answer and nothing otherwise!

Just a way of marking lol!
I dont know if this would work but it could if we have about 5-10 people!


Thank you
Your idea is Ultra Sound.
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Gawd
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(Original post by jam.wa)
Your idea is Ultra Sound.
I think I just soiled myself.
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muffingg
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There are some things that I just don't understand. If someone could kindly give brief explanations to the followings please:

* Fusion and fission
* In a transformer how does the flux linkage change? I understand that the primary coil is connected to an ac supply, but doesn't that mean that the flux linking the secondary coil stays constant? But surely it is changing !?
* Right hand rule or left hand rule? When to use which?

Nothing else is confusing me at the moment.

Thanks.
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sulexk
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(Original post by muffingg)
There are some things that I just don't understand. If someone could kindly give brief explanations to the followings please:

* Fusion and fission
* In a transformer how does the flux linkage change? I understand that the primary coil is connected to an ac supply, but doesn't that mean that the flux linking the secondary coil stays constant? But surely it is changing !?
* Right hand rule or left hand rule? When to use which?

Nothing else is confusing me at the moment.

Thanks.
Hello,

The equation for flux linkage = BAN - if any of these change then the magnetic flux linkage changes which gives rise to an induced EMF. Now if the numbers of turns is the same then this has no effect. If the area remains the same this has no effect, so the magnetic flux density must be changing, i.e. we must have a varying magnetic flux.

Now in the case of the transformer we have an iron core- which is used to ensure that all the flux created by the primary coil is linking the secondary coil. If we have a varying EMF then we must look at it in reverse. Induced EMF = rate of change of flux linkage. So we have a chaning emf in the primary coil(since we have an alternating EMF), and so from the equation we have a varying rate of change of flux linkage- which means we have a varying change in flux linkage. So the flux linking the secondary coil is constantly changing and so on the other side we have an induced EMF. IF we have a constant rate of change of flux linkage for the secondary coil then we have a constant induced EMF.

I hope this helps.
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m92singh
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[QUOTE=sulexk;32294865]Hi there, I was wondering can anyone outline the main principles of an MRI scan, in terms of referring to the equipment used such as the gradient coils etc.


Everthing Everyone needs to know for this Exam in 20 Pages!!!!!

Find attached file

My suggestion is learn everything and don't do some thing in details ( except you could do Nuclear physics in detail as its worth the most)
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Oh my Ms. Coffey
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Oh how I hate this course book.
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mauvetard
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Do we need to know anything about diffraction of x-rays to probe atoms?
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rebmu
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(Original post by muffingg)
There are some things that I just don't understand. If someone could kindly give brief explanations to the followings please:
* Fusion and fission
* In a transformer how does the flux linkage change? I understand that the primary coil is connected to an ac supply, but doesn't that mean that the flux linking the secondary coil stays constant? But surely it is changing !?
* Right hand rule or left hand rule? When to use which?
Nothing else is confusing me at the moment.
Thanks. [IMG]file:///C:/DOCUME%7E1/umbie/LOCALS%7E1/Temp/msohtml1/01/clip_image001.gif[/IMG]

Fusion and Fission are both processes that release energy.
remember binding energy is mass defect x c^2 so is the original energy released when the nucleons came together and a small amount of mass was converted to energy, so to release energy you need to increase the binding energy per nucleon
if you look at the binding energy per nucleon graph attached, you can see the the two ways to release energy are either by fusing small nuclei together to create bigger nuclei or by splitting really large elements into two medium sized elements

basically your second point is all to do with Faraday's law and also what alternating current is, basically the flux linkage is constantly changing as the current is always changing direction in A.C so it is not actually constant so you do get a change in flux linkage so you do get an induced emf in the second coil.

with regards to third point my teacher says to think Australian
so with a genarighter you use the right hand rule to work out which way the electric current will flow in an
and then the left hand rule is for working out which way a motor will turn, there's no real way to remember that but there's only two to choose from [IMG]file:///C:/DOCUME%7E1/umbie/LOCALS%7E1/Temp/msohtml1/01/clip_image002.gif[/IMG]
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sulexk
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(Original post by muffingg)
There are some things that I just don't understand. If someone could kindly give brief explanations to the followings please:

* Fusion and fission
* In a transformer how does the flux linkage change? I understand that the primary coil is connected to an ac supply, but doesn't that mean that the flux linking the secondary coil stays constant? But surely it is changing !?
* Right hand rule or left hand rule? When to use which?

Nothing else is confusing me at the moment.

Thanks.
Right hand rule or left hand rule.

Let us unravel this problem with an example. Suppose an aircraft is flying east to west, its wings are an electrical conductor cutting across the earth's magnetic flux. In the northen hemisphere, which wingtip will become positively charged?

This seems like a very challenging question at first, but really all we need to do is to find the direction of the force between the wingtips- telling us which direction the positive charge will be flowing. So we use flemings left hand rule for direction of force:

NOTE- northern hemisphere- so magnetic field into the paper, positive charge has a velocity east to west, since the positive charge is between the wings and so we have a current in the direction the plane is moving in, and now you can use flemings left hand rule to workout the direction of the force. So positive moving east to west, direction of magnetic field into the page, and it should give thumb going down the page. We can use the right hand rule to check if this is correct.
Using Right hand rule
magnetic field into the page, thumb east to west, what direction is induced current?

down the page(not into the page, down the page)

I hope this helps!
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M_I
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(Original post by m92singh)
They cant ask you how increasing the tube voltage gives the electrons more kinetic energy; in fact its just something you need to remember and you'll get marks for.

Just state:

the intensity of the X-ray beam is the power (energy per second) per unit area passing through a surface (at right angles). There are two ways to increase the intensity of the X-ray Beam:

1). Increase the tube voltage. this gives the electrons more kinetic energy. Higher energy electrons can knock out electron from shells deeper within the tungsten atoms.

2). Increase the current supplied to the filament. This liberates more electrons per second, which then produces more X-ray photons per second.

These are facts you can remember them its all good.

Good luck
Ok good. Thanks.

(Original post by jam.wa)
Did they do S to N within a magnet? I think that's what happens on the inside. No idea why.
On the outside it's always N to S
There is no external magnetic field if thats what you mean. Its just says that is how protons behave, the spin property causes them to behave like magnets but their arrow point from S to N.
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yokabasha
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(Original post by sulexk)
Right hand rule or left hand rule.

Let us unravel this problem with an example. Suppose an aircraft is flying east to west, its wings are an electrical conductor cutting across the earth's magnetic flux. In the northen hemisphere, which wingtip will become positively charged?

This seems like a very challenging question at first, but really all we need to do is to find the direction of the force between the wingtips- telling us which direction the positive charge will be flowing. So we use flemings left hand rule for direction of force:

NOTE- northern hemisphere- so magnetic field into the paper, positive charge has a velocity east to west, since the positive charge is between the wings and so we have a current in the direction the plane is moving in, and now you can use flemings left hand rule to workout the direction of the force. So positive moving east to west, direction of magnetic field into the page, and it should give thumb going down the page. We can use the right hand rule to check if this is correct.
Using Right hand rule
magnetic field into the page, thumb east to west, what direction is induced current?

down the page(not into the page, down the page)

I hope this helps!
I just used RHR you switched the fingers and that's assuming the answer doesn't want the conventional current, do these types of questions always ask for electron flow not conventional current if it doesn't state in the question?
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muffingg
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(Original post by sulexk)
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(Original post by rebmu)
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Thanks a lot guys. Really appreciate this. Sorry sulexk, I already gave u reps so can't do it again. I just read through your replies quickly and they seem very helpful for now. I'll look at them again in greater detail later today as I'm heading to a revision session now. But thanks a lot guys! Wasn't expecting that detailed answers.

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Ralphus J
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Do we need to know anything about the actual ultrasound transducer? Such as dampening and what not, its not on the specification list

Whilst im on the topic of ultrasound can someone help me please with (i) explain qualitatively how the doppler effect can be used to determine the speed of blood.

ASAP would be really great
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JONNYCMW
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In the specification where it says "describe qualitatively the evolution of the universe.... to present", does qualitatively mean we don't have to learn the temperatures and numbers associated with it?
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sulexk
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(Original post by yokabasha)
I just used RHR you switched the fingers and that's assuming the answer doesn't want the conventional current, do these types of questions always ask for electron flow not conventional current if it doesn't state in the question?
Well for that question in particular I assume the earth's magnetic field to be into the page- and so I obtained the answer that the left wingtip becomes positive, which happens to be correct!

I assume that is true, unless they mention electron flow then we can assume positive charge.

Thank you!
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