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

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That makes sense! But then again I guess we could say that discrepancy between estimates of the age of the universe is due to uncertainty in

{H_0}

?

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Yeah the hubble constant value is different according to different sources. I think AQA have it at 65 haha

Reply 801

CD223

Original post by JJBinn

Yeah the hubble constant value is different according to different sources. I think AQA have it at 65 haha

Yeah they do. Is there an accuracy that they want it to be in when you convert it to

s^{-1}

?

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Reply 802

frankiejayx

Hi, could anyone doing astrophysics help me on chapter 2.3 Summary Question 4 I cant seem to get the right answer

(edited 8 years ago)

Reply 803

JJBinn

Original post by CD223

Yeah they do. Is there an accuracy that they want it to be in when you convert it to

s^{-1}

?

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I think the answer is supposed to come out at about 15000billion years

Reply 804

CD223

Original post by frankiejayx

Hi, could anyone doing astrophysics help me on chapter 2.3 Summary Question 4c I cant seem to get the right answer

[br]P=\sigma A T^{4}[br]

[br]{T_X} = 4,000K[br]{T_Y} = 8,000K[br]{T_Z} = 20,000K[br]{M_X} = -2[br]{M_Y} = +4[br]{M_Z} = +10[br]

The surface area is proportional to the diameter squared, so:

[br]\dfrac{{A_X}}{{A_Z}} = \left(\dfrac{{D_X}}{{D_Z}}\right)^{2}[br]

As in the course notes, the equation for ratios can be re-written as:

[br]\dfrac{{P_X}}{{P_Z}} = \left(\dfrac{{D_X}}{{D_Z}}\right)^{2} \left(\dfrac{{T_X}}{{T_Z}}\right)^{4} [br]

The power output ratio is dependent on the absolute magnitude difference between the two stars.

There is a difference of

(100)^{\frac{1}{5}}

between each magnitude due to the logarithmic scale.

In which case, as there is a difference of 12 magnitudes between X and Z:

[br]\dfrac{{P_X}}{{P_Z}} = (100)^{\frac{12}{5}}[br]

So for X and Z:

[br](100)^{\frac{12}{5}} = \left(\dfrac{{D_X}}{{D_Z}}\right)^{2} \left(\dfrac{{4,000}}{{20,000}}\right)^{4} [br]

From which the ratio:

[br]\dfrac{{D_X}}{{D_Z}}[br]

can be found.

If you do the same with the figures for Y and Z, you'll end up with answers of:

[br]\dfrac{{D_X}}{{D_Z}} = 6300[br]

and

[br]\dfrac{{D_Y}}{{D_Z}} = 100[br]

Long post (sorry), but hope it helps. :biggrin:

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(edited 8 years ago)

Reply 805

CD223

Original post by JJBinn

I think the answer is supposed to come out at about 15000billion years

Yeah

although my teacher said estimates for

{H_0}

can vary from 50 to 100 depending on the source, and as the rate is accelerating the value is subject to change as time goes on, producing a graph of V against D with a curved gradient instead of a straight line haha.

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Reply 806

CD223

Is the ratio of:

[br]\dfrac{number\ of\ \gamma\ photons\ incident\ on\ a\ detector}{number\ of\ \gamma\ photons\ produced\ by\ a\ source}[br]

Always equal to:

[br]\dfrac{surface\ area\ of\ detector}{4 \pi r^{2}}[br]

?

Also, for the equation:

[br]I=\dfrac{k{I_0}}{x^2}[br]

Does

I

represent the activity and count rate? Because doesn't

[br]I = \dfrac{Power}{Area}[br]

(edited 8 years ago)

Reply 807

Disney0702

Original post by CD223

Is the ratio of:

[br]\dfrac{number\ of\ \gamma\ photons\ incident\ on\ a\ detector}{number\ of\ \gamma\ photons\ produced\ by\ a\ source}[br]

Always equal to:

[br]\dfrac{surface\ area\ of\ detector}{4 \pi r^{2}}[br]

?

Also, for the equation:

[br]I=\dfrac{k{I_0}}{x^2}[br]

Does

I

represent the activity and count rate? Because doesn't

[br]I = \dfrac{Power}{Area}[br]

Hello ☺

I believe the answer to the first ratio you were asking about can be found in Q13b)i) in the following link.

http://www.cyberphysics.co.uk/Q&A/KS5/radioactivity/Q13.html

I'm afraid I do not know the answer to the second ratio yet, I will keep trying 😊

Reply 808

CD223

Original post by Disney0702

Hello! Thank you for that - it really helped :smile:

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Reply 809

CD223

For determining the radius of a nucleus, how is the following equation found? I know it's electron diffraction and that:

[br]\sin\theta = \dfrac{0.61\lambda}{R}[br]

Can be rewritten as:

[br]\sin\theta = \dfrac{1.22\lambda}{D}[br]

For the radius R, or diameter D of a nucleus, but how is this derived from the diffraction pattern?

In the CGP guide it says: "This is where the first minimum appears":
Is it related to the single slit diffraction pattern we did last year?

ImageUploadedByStudent Room1431853044.282049.jpg

ImageUploadedByStudent Room1431853044.282049.jpg

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Reply 810

0151

Anyone gathered together the grade boundaries for the past papers to see which are the most difficult?

Reply 811

CD223

Original post by 0151

Anyone gathered together the grade boundaries for the past papers to see which are the most difficult?

The grade boundaries don't necessarily indicate a harder paper. Last year's paper was manageable compared to previous years and the boundaries were ridiculously low.

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Reply 812

Lau14

Original post by 0151

Anyone gathered together the grade boundaries for the past papers to see which are the most difficult?

Had nothing to do while watching tv so, unit five grade boundaries (for the astrophysics option - I might do them for the others if I have spare time but no promises).

phya5a boundaries.png

Reply 813

CD223

Original post by Lau14

Had nothing to do while watching tv so, unit five grade boundaries (for the astrophysics option - I might do them for the others if I have spare time but no promises).

phya5a boundaries.png

Oh cool thank you! May I nab these for the first post please? :biggrin:

I wasn't aware they varied with each option lol.

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Reply 814

Lau14

Original post by CD223

Oh cool thank you! May I nab these for the first post please? :biggrin:

I wasn't aware they varied with each option lol.

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Of course!

They must do because the optional bits can change, but I can't say I've ever checked!

Reply 815

CD223

Original post by Lau14

Of course!

They must do because the optional bits can change, but I can't say I've ever checked!

True - how's your revision going? :smile:

do you have many papers left?

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Reply 816

Absent Agent

Is this thread only for astrophysicists? :smile:

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Reply 817

CD223

Original post by Mehrdad jafari

Is this thread only for astrophysicists? :smile:

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Not at all! All options - there just seems to be a bit of a bias towards Astro as that's what the majority of people on this thread so far seem to be doing - although medical/turning points/applied options are welcome! :smile:

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Reply 818

Absent Agent

Original post by CD223

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That's cool :smile:

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Reply 819

Lau14

Original post by CD223

True - how's your revision going? :smile:

do you have many papers left?

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Still pretty much ignoring physics, I've got 3 PHYA5 and 2 PHYA4 papers left and I'd like to go through and try most of the six markers again at some point. You? :smile: