AQA Physics PHYA5 - Thursday 18th June 2015 [Exam Discussion Thread]

An Airy disk is the central bright circular region of the pattern produced by light diffracted when passing through a small circular aperture

Reply 2205

Original post by miraj3

For anyone doing Astrophysics,

what is the definition of an Airy disc?

An Airy disk is the central bright circular region of the pattern produced by light diffracted when passing through a small circular aperture

Reply 2206

Original post by JordanL_

I'm reading completely different things in different places about radioactive decay. The Nelson Thornes textbook says that alpha decay occurs in nuclides above the line of stability, but the notes and the guide I'm using both say that is occurs in proton-rich nuclides which would mean below the line, wouldn't it?

I believe alpha decay occurs above the line as in past a proton number of around 80

Reply 2207

_Caz_

10

Original post by JordanL_

I'm reading completely different things in different places about radioactive decay. The Nelson Thornes textbook says that alpha decay occurs in nuclides above the line of stability, but the notes and the guide I'm using both say that is occurs in proton-rich nuclides which would mean below the line, wouldn't it?

I think alpha emission is just heavy nuclides in general.
Beta plus/electron capture is for proton rich ones
Beta - is for neutron rich ones.

Pretty certain that's right.

Reply 2208

Original post by JordanL_

I'm reading completely different things in different places about radioactive decay. The Nelson Thornes textbook says that alpha decay occurs in nuclides above the line of stability, but the notes and the guide I'm using both say that is occurs in proton-rich nuclides which would mean below the line, wouldn't it?

Reply 2209

you-only-live-once

Original post by DCMed96

To be honest i have no idea!!! there are so many things to memorise.... and I seriously hope something nice would come out considering today's biology R paper wasn't nice at all...

is R paper meant to be harder..? Yeah i hope something nice would come up too.. I'm thinking more of X-rays or eyes though

Reply 2210

CD223

OP

10

Original post by sykik

I thought N was due to N number of molecules present because if you don't have N then the equation only applies for a single molecule??

That's also true, but in the definition of the mean square speed N is on the bottom. In order to sub in the mean square speed for u squared on the top, you must multiply by N on top so the equation for the pressure remains the same.

ImageUploadedByStudent Room1434558314.429169.jpg

ImageUploadedByStudent Room1434558314.429169.jpg

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How to do this one?

Original post by JamGrip

Ok. I posted basically the same thing back on page 50 but 2b(ii) here: http://filestore.aqa.org.uk/subjects/AQA-PHYA5-2A-QP-JUN12.PDF (June 2012 astro)
http://filestore.aqa.org.uk/subjects/AQA-PHYA52A-W-MS-JUN12.PDF (markscheme)

It doesn't multiply the answer by two afterwards even though it says it's an orbital radius. Any reason why?

Thanks

my guess is you need the diameter of the satellite as your using theta=s/r with s being arc length which is approx the diameter of the satellite and r is the radius from circle which is basically the distance from earth to the satellite. Which is why it multiplied by two in this situation and not in the past paper, I hope this helps :smile:

Reply 2213

Klaxoii

Original post by Sam.B57

Can anyone doing turning points explain to me how the electron tunnelling microscope works? It doesn't make much sense :P

There's two types - constant height and constant current, either is applicable but I find constant height easier to understand.

Basically, the microscope has a really sharp tip, which is negatively charged, the surface of the sample is positively charged. This means since electrons exhibit wave like properties they propagate between the gap (only from - to +). This transfer of electrons means there's a current between the tip and the surface. The currents magnitude depends on the gap between tip and surface - bigger the gap the less electrons that transfer and hence a smaller current. So as the tip scans across the surface the current increases and decreases if there is a dip or peak, and hence you can build up an image of the surface.

Probably not very clear sorry

Reply 2214

MSB47

Original post by 000alex

How to do this one?

Firstly because only 1 out of 400 photons are detected multiply the corrected count rate by 400 to give you the count rate of the detector if it detected all photons incident on it. Then divide that by the area of detector because that will give you the intensity of the source at the distance it is from and because it is uniform in all direction the intensity is constant at this point therefore, you can use the inverse square law equation I=I₀/4pi(r)² and re arrange to make I₀the subject and 4pi(r)² is the area of the sphere the gamma photons have spread out at that distance so I₀is activity of source because it gives the number photons emitted per second which is same as activity (number of disintegrations per second)

Hope that helps

Reply 2215

AR_95

21

How much do we need to know about quantum tunnelling?

Reply 2216

Sam.B57

Original post by Klaxoii

There's two types - constant height and constant current, either is applicable but I find constant height easier to understand.

Basically, the microscope has a really sharp tip, which is negatively charged, the surface of the sample is positively charged. This means since electrons exhibit wave like properties they propagate between the gap (only from - to +). This transfer of electrons means there's a current between the tip and the surface. The currents magnitude depends on the gap between tip and surface - bigger the gap the less electrons that transfer and hence a smaller current. So as the tip scans across the surface the current increases and decreases if there is a dip or peak, and hence you can build up an image of the surface.

Probably not very clear sorry

In mark schemes for questions I've seen something to do with limited probability, whats that? Apart from that though that does help :smile:

Reply 2217

CD223

OP

10

Original post by AR_95

How much do we need to know about quantum tunnelling?

Essentially nothing for the core unit. The alpha particle gains kinetic energy as a result of its gain of binding energy and has a greater probability of overcoming the Coulomb barrier keeping it within the nucleus.

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Reply 2218

gcsestuff