AQA Physics PHYA5 - 28th June 2016 [Exam Discussion Thread]

Original post by Ainsleyy

Bump

First you need to find the difference between the absolute magnitude of the two stars

Then because every +1 change in magnitude gives a x2.5 change in brightness, you need to do 2.5 raised to the power of the difference in absolute magnitude

So 2.5^9.8 which is around 8000

(edited 7 years ago)

Reply 362

Ainsleyy

Original post by HollyAliciaMay

First you need to find the difference between the absolute magnitude of the two stars

Then because every +1 change in magnitude gives a x2.5 change in brightness, you need to do 2.5 raised to the power of the difference in absolute magnitude

So 2.5^9.8 which is around 8000

@McDerdactyl

Ah okay thanks guys. Do you know if we have to learn
b = L/(4pi(Rsquared) or m = -2.5logb or m2 - m1 = -2.5log(b2/b1)?
They dont seem to be in the spec but they are in the book.

Reply 363

Franckenstar

Original post by rory58824

Predictions on 6 marker for section A and the Applied Physics section?

Discuss the main features of a flywheel and how these features may be used in the real world.

Reply 364

Original post by twallien

In the June 2013 exam it asks you to work out the energy released when 4 hydrogen atoms are converted in to helium and two positrons. In the mark scheme they ignore the electron in the hydrogen atom so the mass defect is:

4 x mass_proton - mass_helium - 2xmass_electron

Can anyone explain this please?

bump

Reply 365

ajaxkiller

7

wag1 mandem i have 1 day left, ive been too focused on fp2 and 3, do you think theres still time to start revision adequately on this

Reply 366

ReeceFraser

What are fission fragments and why are they unstable?

Reply 367

boyyo

6

Original post by ReeceFraser

What are fission fragments and why are they unstable?

Is this from the June 14 paper, I was confused on this q too, can anyone explain this?

Also, in a nuclear reactor, when the thermal neutron undergoes fusion with the uranium, and releases a few neutrons and energy, is beta minus decay happening?

Reply 368

Original post by ReeceFraser

What are fission fragments and why are they unstable?

The daughter nuclei of a fissioned nucleus. They're unstable because they normally contain more neutrons than thier stable isotopes.

Reply 369

Original post by boyyo

Is this from the June 14 paper, I was confused on this q too, can anyone explain this?

Also, in a nuclear reactor, when the thermal neutron undergoes fusion with the uranium, and releases a few neutrons and energy, is beta minus decay happening?

I answered the previous post but i'll put some more in detail. If you look at the N vs Z graph. The ratio of N/Z is higher for larger nuclei. Now imagine that If you halve that nucleus, its ratio will basically be the same, this means its N/Z will be higher than its stable isotope (above the stable isotope). Too many neutrons result in instability, neutron rich isotopes will decay via beta minus.

(edited 7 years ago)

Reply 370

boyyo

6

Original post by twallien

I answered the previous post but i'll put some more in detail. If you look at the N vs Z graph. The ratio of N/Z is higher for larger nuclei. Now imagine that If you halve that nucleus, its ratio will basically be the same, this means its N/Z will be higher than its stable isotope (above the stable isotope). Too many neutrons result in instability, neutron rich isotopes will decay via beta minus.

Ahh ok. You see where you said the daughter nuclei has more neutrons, but the parent Nuclei absorbs thermal neutron and undergoes beta minus decay, doesn't it lose neutrons, so the daughter has less?

Reply 371

Original post by micycle

Could someone explain how during fission, energy is released even though new nuclei have higher binding energy per nucleon.

Not sure if this has been answered properly. You have to imagine the binding energy is the energy REQUIRED to separate the nucleus in to its constituent protons and neutrons. the higher the binding energy per nucleon, the more stable it is. Nuclei above Iron have a higher potential energy which is released when they are fissioned.

Reply 372

McDerdactyl

9

Original post by Ainsleyy

@McDerdactyl

Ah okay thanks guys. Do you know if we have to learn
b = L/(4pi(Rsquared) or m = -2.5logb or m2 - m1 = -2.5log(b2/b1)?
They dont seem to be in the spec but they are in the book.

I've never heard of them and one of my teachers is an examiner so I shouldn't think so :smile:

Reply 373

HollyAliciaMay

Original post by twallien

The daughter nuclei of a fissioned nucleus. They're unstable because they normally contain more neutrons than thier stable isotopes.

They'll also have excess energy due to the massive temperatures in the reactor core, so I reckon they'd be in an excited state too
This links in to why spent fuel rods are much more hazardous than unused fuel rods, as they'll now emit beta/gamma - just in case that comes up :smile:

Reply 374

BioStudentx

16

Original post by twallien

bump

Mate, I'm not sure what nucleuses (probs not a word) you've been looking at but the ones I've seen don't have electrons. 10/10. gr8 banter.

Reply 375

Original post by boyyo

Ahh ok. You see where you said the daughter nuclei has more neutrons, but the parent Nuclei absorbs thermal neutron and undergoes beta minus decay, doesn't it lose neutrons, so the daughter has less?

Okay so the question asks "With reference to Figure 1, explain why fission fragments are unstable and explain what type of radiation they are likely to emit initially"

It fully depends on the elements involved. Yes it emits neutrons but generally speaking the ratio of N/Z will still be higher than its stable isotope.

A worked example: Uranium_235 splits in to Rubidium_92 and Caesium_140 as well as 3 neutrons. N/Z for uranium is about 1.55. N/Z for Cs is 1.545. Cs_133 is the only stable isotope meaning Cs has more neutrons than its stable isotope. I'm sure there's examples where what i have said don't apply but the question asks for general terms.

Reply 376

cowie

7

Any expectations for Tuesday's exam part A/B(Astro)??
I'm not sure how I'm feeling about it.. Getting consecutively 100% ums' but have no faith in myself whatsoever ahah.. Anyone recommend any other relevant spec papers or old spec papers which were challenging/different?

Reply 377

ReeceFraser