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

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Why do you think energy isn't released?

Posted from TSR Mobile

Ok if two products are heavier than the parent then that means that more mass is present which would mean that the energy hasn't been released but kept and just converted into mass??

Reply 1401

Absent Agent

Original post by Jimmy20002012

Could anyone explain STM microscopes doing turning points?

Posted from TSR Mobile

Have you looked at teacher's notes for turning points? It's very understandable. I think that should be swapped with students' notes booklet lol

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Reply 1402

Sbarron

Original post by Mehrdad jafari

Lol, that was only to give you a better insight.

I didn't explicitly talk about difference in masses but that change in mass is a result of the energy change before and after the fission. So if the nuclides formed have a less energy content they will have a less mass than the nuclide fissioned due to mass energy equivalence.

Posted from TSR Mobile

But less energy would mean more mass because the energy has to go somewhere and in all my physics books it says the energy is converted to mass because E = mc2

Reply 1403

Jimmy20002012

Original post by Mehrdad jafari

Have you looked at teacher's notes for turning points? It's very understandable. I think that should be swapped with students' notes booklet lol

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I will try and find them now!!

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Reply 1404

Absent Agent

Original post by Sbarron

Ok if two products are heavier than the parent then that means that more mass is present which would mean that the energy hasn't been released but kept and just converted into mass??

Well, during fission the mass of the products are less than the mass of the initial nuclide. Also mass and energy are equivalent on the scale E=mc2, meaning that a small change is mass accompanies a very large difference in energy

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Reply 1405

Sbarron

Original post by Mehrdad jafari

Also, it's not the energy holding the nucleons which is what i think you try to associate with the binding energy

Posted from TSR Mobile

Ok I didn't think that until I watched a YouTube clip which made me think the mass lost in fusion held the products together in the form of energy

Reply 1406

Klaxoii

Stupid question, but do AQA give us a table with all the nuclear masses on?

Reply 1407

tom-physics

Original post by betbi3etwerrd

To find energy given out during fission you simply find the difference in binding energies before and after.

Either way, if tried accounting for the neutrons you would be forced to account for the kinetic energy of the neutrons as well.

Also take note that if you took account for the neutrons (without KE) you would appear to have lost energy!

Oh and by the way I don't understand it myself but just seems illogical but here's a quote that might help:

"The amount of energy released by fission can bedetermined based on either the change in mass that occurs during the reactionor by the difference in binding energy per nucleon between the fissile nuclideand the fission products."

I'm assuming the fissile and fission products only include nuclei (not neutrons) if you're working it out the energy using binding energy per nucleon.

Cheers mate i understand this now :smile:

, if i think about the energy released being the difference in the binding energy's of the fissile nuclide and fission products, then it works because neutrons aren't bound to anything or any other nucleons so they have no binding energy.

Reply 1408

Absent Agent

Original post by Sbarron

But less energy would mean more mass because the energy has to go somewhere and in all my physics books it says the energy is converted to mass because E = mc2

Actually less energy means less mass.
And it's quite wrong interpreting that equation that way. You are thinking that the energy "released" during the fission is kept within the nuclides formed. If that was the case then the mass of the nuclides formed would be the same as the mass of the nuclides fissioned initially and there would be no change in mass. During fission energy is released, meaning that mass is released since mass and energy are equivalent

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Reply 1409

examiningboard

Original post by Dante991

I know an A* Is roughly about 52 out of 75, but what is 120(full) UMS? I did really well in PHYA4, so I want to do as best as possible to achieve an A*

Posted from TSR Mobile

Just use the AQA UMS converter:
http://www.aqa.org.uk/exams-administration/about-results/uniform-mark-scale/convert-marks-to-ums

Also depends on what option you're doing, i'm doing Astro and last year it was 58 for full ums :smile:

Reply 1410

Lau14

Original post by QueNNch

Ah that's the perfect explanation I was looking for! Thanks a lot!

Posted from TSR Mobile

It does decrease contrast though as less light is coming in overall

Reply 1411

Sbarron

Original post by Mehrdad jafari

Right so Let me go back.... When you separate a nucleus (fission) you end up with less mass and this is accounted for by energy loss.... And in fusion you end up with more mass because of energy gain? Is that right? Have I got that far?? Lol I'm so sorry I feel like I'm wasting everyone's time with this... I think I need to start again

Reply 1412

Sbarron

Original post by Sbarron

And is the energy released in fission the same amount of energy required to fuse the products back together?

Reply 1413

Sbarron

Original post by tom-physics

Cheers mate i understand this now :smile:

And neutrons have no mass right

Reply 1414

ASDuh97

Predictions for 6 marker on Astro?

Reply 1415

Absent Agent

Original post by Sbarron

No you're alright. I understand that textbook says if you separate a nucleus into it's constituents then the mass if the products are greater but that's only true when the fission is induced, that is artificially because energy is supplied to the nuclide to be separated into its constituents. But if the fission happens naturally, that is without any energy being supplied to the targeted nuclide, then the mass of the nuclides formed is less than the mass if the nuclide fissioned initially because no energy(mass) was given to the nuclide and that only energy was released.

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(edited 8 years ago)

Reply 1416

betbi3etwerrd

Original post by Sbarron

And neutrons have no mass right

I think you're mistaken :biggrin:

Reply 1417

tom-physics

Original post by Sbarron

And neutrons have no mass right

did you mean mass defect?

Reply 1418

Absent Agent

Original post by Sbarron

And is the energy released in fission the same amount of energy required to fuse the products back together?

Theoretically yes but once a nuclide is fissioned then the nuclides formed require a huge amount of energy to be fused together as the electrostatic repulsion between the protons of each nuclide doesn't allow the nuclides fuse as the nuclides would have to be so close together for the strong nuclear force to "grab" them together

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Reply 1419

AR_95

Original post by Mehrdad jafari

Have you looked at teacher's notes for turning points? It's very understandable. I think that should be swapped with students' notes booklet lol

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