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

1. (Original post by cajach)
Thank you, it makes a bit more sense now

1) In fission, energy must be supplied to the nucleus to separate it. The energy is absorbed by the nucleus and transformed into mass. Therefore the mass of the nucleus > mass of its fission products
2) When comparing the mass of a He-4 nucleus with the mass of its constituent protons and neutrons (no actual fission reaction taking place): Binding energy must be released from the He-4 nucleus to be separated. Mass is converted into energy internally in the He-4 nucleus and released, so the mass of the He-4 nucleus < mass of its protons and neutrons.
3) In fusion, mass of the reactants (e.g. nuclei) > mass of products (e.g. the formed nucleus), because the formed nucleus needs to absorb energy to keep itself intact. Mass is transformed into energy and the energy is stored in the nucleus as the strong nuclear force.

Hope my understanding is correct!

I should also have mentioned that in my previous example, the neutron is fired at the uranium-235 in a reactor core. This would be "induced fission" right?

So would mass of reactants >/</= mass of products if it was induced fission?

I appreciate the help!
To me point one and two contradict each other but I don't fully understand it
2. (Original post by cajach)
Thank you, it makes a bit more sense now

1) In fission, energy must be supplied to the nucleus to separate it. The energy is absorbed by the nucleus and transformed into mass. Therefore the mass of the nucleus > mass of its fission products
2) When comparing the mass of a He-4 nucleus with the mass of its constituent protons and neutrons (no actual fission reaction taking place): Binding energy must be released from the He-4 nucleus to be separated. Mass is converted into energy internally in the He-4 nucleus and released, so the mass of the He-4 nucleus < mass of its protons and neutrons.
3) In fusion, mass of the reactants (e.g. nuclei) > mass of products (e.g. the formed nucleus), because the formed nucleus needs to absorb energy to keep itself intact. Mass is transformed into energy and the energy is stored in the nucleus as the strong nuclear force.

Hope my understanding is correct!

I should also have mentioned that in my previous example, the neutron is fired at the uranium-235 in a reactor core. This would be "induced fission" right?

So would mass of reactants >/</= mass of products if it was induced fission?

I appreciate the help!
I thought that in fission the mass of the products combined was greater than the mass of the nucleus they came from because the have absorbed energy to become separated which has converted into mass??
3. For anyone doing turning points, why do the gas in a discharge tube have to be at a low pressure?
4. (Original post by Mehrdad jafari)
The beginning of your post is correct because that fission is not induced fission, that is no energy was supplied to cause the fission and so the energy released during the process resulted in a decrease in mass of the products.

In the case of the june 13 question the nuclide is separated into its constituents. Naturally this doesn't happen and so energy equal to the binding energy of the targeted nuclide must be supplied to break it into its constituents. This energy supplied is now stored in the constituents of the nuclide in the form of mass, resulting in an increase of the mass of the separated protons and neutrons of the fissioned nuclide.

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That makes so much more sence..... So if it was induced energy was supplied and the mass becomes increased.... But if it was not induced it needed to use some of it's own mass as energy so mass decreased??? Please tell me that's right??
5. (Original post by Sbarron)
I thought that in fission the mass of the products combined was greater than the mass of the nucleus they came from because the have absorbed energy to become separated which has converted into mass??
I've been trying to get my head around this for days so I don't think I would be the best person to explain it to you... What I said above is what my tutor says though. BBC bitesize also agrees and explains it quite well:

http://www.bbc.co.uk/bitesize/higher...ns/revision/2/

"The total mass of a nucleus is less than the total mass of the nucleons that make up the nucleus. This difference is known as the mass defect and is equivalent to the binding energy of the nucleus, using E = mc^2.

In fission, an unstable nucleus is converted into more stable nuclei with a smaller total mass. This mass defect is the binding energy that is released.

In fusion, the mass of the nucleus that is created is slightly less than the total mass of the original nuclei. Again the mass defect is the binding energy that is released, since the nucleus that is formed is more stable."
6. I do astro but if I had to guess, it would probably be something to do with the molecules of the gas moving slowly, so they can collide or absorb things easier at a better rate. - just a guess

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7. (Original post by cajach)
Thank you, it makes a bit more sense now

1) In fission, energy must be supplied to the nucleus to separate it. The energy is absorbed by the nucleus and transformed into mass. Therefore the mass of the nucleus > mass of its fission products
2) When comparing the mass of a He-4 nucleus with the mass of its constituent protons and neutrons (no actual fission reaction taking place): Binding energy must be released from the He-4 nucleus to be separated. Mass is converted into energy internally in the He-4 nucleus and released, so the mass of the He-4 nucleus < mass of its protons and neutrons.
3) In fusion, mass of the reactants (e.g. nuclei) > mass of products (e.g. the formed nucleus), because the formed nucleus needs to absorb energy to keep itself intact. Mass is transformed into energy and the energy is stored in the nucleus as the strong nuclear force.

Hope my understanding is correct!

I should also have mentioned that in my previous example, the neutron is fired at the uranium-235 in a reactor core. This would be "induced fission" right?

So would mass of reactants >/</= mass of products if it was induced fission?

I appreciate the help!
It's really your number 2 I think I don't get....if binding energy is RELEASED wouldn't the mass decrease??
8. (Original post by Dante991)
I do astro but if I had to guess, it would probably be something to do with the molecules of the gas moving slowly, so they can collide or absorb things easier at a better rate. - just a guess

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Cheers, I think you're along the right line, I just wanted to see what others thought too haha
9. (Original post by cajach)
I've been trying to get my head around this for days so I don't think I would be the best person to explain it to you... What I said above is what my tutor says though. BBC bitesize also agrees and explains it quite well:

http://www.bbc.co.uk/bitesize/higher...ns/revision/2/

"The total mass of a nucleus is less than the total mass of the nucleons that make up the nucleus. This difference is known as the mass defect and is equivalent to the binding energy of the nucleus, using E = mc^2.

In fission, an unstable nucleus is converted into more stable nuclei with a smaller total mass. This mass defect is the binding energy that is released.

In fusion, the mass of the nucleus that is created is slightly less than the total mass of the original nuclei. Again the mass defect is the binding energy that is released, since the nucleus that is formed is more stable."
I think I understand that.... But the last paragraph is only correct if it is spontaneous....

I think before I had trouble with confusing the separation of the products of fusion with fission
10. (Original post by Sbarron)
I think I understand that.... But the last paragraph is only correct if it is spontaneous....

I think before I had trouble with confusing the separation of the products of fusion with fission
Yeah.. I think that's what I had trouble understanding as well.

With fission/fusion, mass of the reactants is ALWAYS > mass of the products.

But the June 2013 question is just asking to compare the mass of a He-4 nucleus with the mass of the protons and neutrons if separated. There is no fission reaction going on. The answer is mass of the He-4 nucleus (the reactant) < mass of the protons + mass of the neutrons (the products), because there is a small mass devoted to the binding energy (in the He-4 nucleus).

I's confusing at first, but I think I get it now.

This video I watched might help too:

11. Are there any questions about astrophysics in set B? If so, where can I find a compilation of it?
12. (Original post by Sbarron)
I think I understand that.... But the last paragraph is only correct if it is spontaneous....

I think before I had trouble with confusing the separation of the products of fusion with fission
So if a neutron is fired at a uranium-235 atom in a reactor core, is this induced or not?

If so, energy is supplied to the uranium-235. Energy is stored in the form of mass in the uranium-235. Therefore the mass of the uranium-235 + neutron (the reactants) > mass of the products?

How is this any different to spontaneous fission though?
13. (Original post by cajach)
Yeah.. I think that's what I had trouble understanding as well.

With fission/fusion, mass of the reactants is ALWAYS > mass of the products.

But the June 2013 question is just asking to compare the mass of a He-4 nucleus with the mass of the protons and neutrons if separated. There is no fission reaction going on. The answer is mass of the He-4 nucleus (the reactant) < mass of the protons + mass of the neutrons (the products), because there is a small mass devoted to the binding energy (in the He-4 nucleus).

I's confusing at first, but I think I get it now.

This video I watched might help too:

Thankyou.. What you've written makes sense and I'll watch the link now
14. (Original post by cajach)
So if a neutron is fired at a uranium-235 atom in a reactor core, is this induced or not?

If so, energy is supplied to the uranium-235. Energy is stored in the form of mass in the uranium-235. Therefore the mass of the uranium-235 + neutron (the reactants) > mass of the products?

How is this any different to spontaneous fission though?
In spontaneous fission the products have a lower mass because energy is released.... I think
15. (Original post by cajach)
Thank you, it makes a bit more sense now

1) In fission, energy must be supplied to the nucleus to separate it. The energy is absorbed by the nucleus and transformed into mass. Therefore the mass of the nucleus > mass of its fission products
2) When comparing the mass of a He-4 nucleus with the mass of its constituent protons and neutrons (no actual fission reaction taking place): Binding energy must be released from the He-4 nucleus to be separated. Mass is converted into energy internally in the He-4 nucleus and released, so the mass of the He-4 nucleus < mass of its protons and neutrons.
3) In fusion, mass of the reactants (e.g. nuclei) > mass of products (e.g. the formed nucleus), because the formed nucleus needs to absorb energy to keep itself intact. Mass is transformed into energy and the energy is stored in the nucleus as the strong nuclear force.

Hope my understanding is correct!

I should also have mentioned that in my previous example, the neutron is fired at the uranium-235 in a reactor core. This would be "induced fission" right?

So would mass of reactants >/</= mass of products if it was induced fission?

I appreciate the help!
True, the fission of Uranium in core reactors is induced fissioned, I shouldn't have used the phrase induced fission for separating the nucleus into its constituents.

1) if the fission is a decay process happening naturally without any energy being supplied to the nucleus (bombarding the nucleus with a neutron can be neglected), in which the initial nucleus is separated into two or more nuclei, and not its constituents, then energy is released in the process because the product nuclei have a greater binding energy. The energy released results in a decrease in mass of the products compared to the initial nuclei.
• if the fission is a process in which a nucleus, He-4 for example, is separated into its constituents, that is 2 protons and 2 neutrons all separated from each other then energy equal to the binding energy of nucleus, or He-4 for our example, must be supplied to the nucleus to separate it into its constituent protons and neutrons. Because the energy was supplied (assuming that there was no energy dissipation), this energy which has an equivalent mass that is stored in the constituent protons and neutrons ( note that the initial nucleus no longer exits for its mass to be greater than its constituents because the initial nucleus has already fragmented into its constituents). Therefore the mass of the initial nucleus < the mass of its constituent protons and neutrons.

2) you mentioned a very good point there. Comparing the mass of a He-4, for example, with its constituent protons and neutrons, with the constituents not being the product of fission of the He-4 nucleus you would still find that the mass of the He-4 is less than the mass of its constituents, thats the mass of 2 protons and 2 neutrons all separated from each other. This is because neutrons and protons separated from each other are in a higher energy state than when they are joined together. When they join energy is released in the process and so protons and neutrons ( constituents of all nuclei) have a greater mass than when they are joined because they have a greater energy content when separated.

3) that's true. In fusion the mass of the nucleus formed is less than the total mass of the initial nuclei but this is because energy is released when the initial nuclei joined together. Since energy has mass this means that mass was released from the initial nuclei during fusion and so the mass of the product nucleus will be less than the total mass of the initial nuclei.

In sum:
in fission( decay process):
Mass of initial nucleus > mass of the product nuclei
( fission happening naturally never results is breaking the nucleus into its constituents but only results in the formation of two or more smaller nuclei)

In fission ( breaking a nucleus into its constituents):
Mass of the initial nuclei < mass of the constituents of the nucleus

In fusion :
Mass of the initial nuclei > mass of the product nuclei.

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16. (Original post by cajach)
Yeah.. I think that's what I had trouble understanding as well.

With fission/fusion, mass of the reactants is ALWAYS > mass of the products.

But the June 2013 question is just asking to compare the mass of a He-4 nucleus with the mass of the protons and neutrons if separated. There is no fission reaction going on. The answer is mass of the He-4 nucleus (the reactant) < mass of the protons + mass of the neutrons (the products), because there is a small mass devoted to the binding energy (in the He-4 nucleus).

I's confusing at first, but I think I get it now.

This video I watched might help too:

Just watching the link.... So far so good
17. (Original post by Mehrdad jafari)
True, the fission of Uranium in core reactors is induced fissioned, I shouldn't have used the phrase induced fission for separating the nucleus into its constituents.

1) if the fission is a decay process happening naturally without any energy being supplied to the nucleus (bombarding the nucleus with a neutron can be neglected), in which the initial nucleus is separated into two or more nuclei, and not its constituents, then energy is released in the process because the product nuclei have a greater binding energy. The energy released results in a decrease in mass of the products compared to the initial nuclei.
• if the fission is a process in which a nucleus, He-4 for example, is separated into its constituents, that is 2 protons and 2 neutrons all separated from each other then energy equal to the binding energy of nucleus, or He-4 for our example, must be supplied to the nucleus to separate it into its constituent protons and neutrons. Because the energy was supplied (assuming that there was no energy dissipation), this energy which has an equivalent mass that is stored in the constituent protons and neutrons ( note that the initial nucleus no longer exits for its mass to be greater than its constituents because the initial nucleus has already fragmented into its constituents). Therefore the mass of the initial nucleus < the mass of its constituent protons and neutrons.

2) you mentioned a very good point there. Comparing the mass of a He-4, for example, with its constituent protons and neutrons, with the constituents not being the product of fission of the He-4 nucleus you would still find that the mass of the He-4 is less than the mass of its constituents, thats the mass of 2 protons and 2 neutrons all separated from each other. This is because neutrons and protons separated from each other are in a higher energy state than when they are joined together. When they join energy is released in the process and so protons and neutrons ( constituents of all nuclei) have a greater mass than when they are joined because they have a greater energy content when separated.

3) that's true. In fusion the mass of the nucleus formed is less than the total mass of the initial nuclei but this is because energy is released when the initial nuclei joined together. Since energy has mass this means that mass was released from the initial nuclei during fusion and so the mass of the product nucleus will be less than the total mass of the initial nuclei.

In sum:
in fission( decay process):
Mass of initial nucleus > mass of the product nuclei
( fission happening naturally never results is breaking the nucleus into its constituents but only results in the formation of two or more smaller nuclei)

In fission ( breaking a nucleus into its constituents):
Mass of the initial nuclei < mass of the constituents of the nucleus

In fusion :
Mass of the initial nuclei > mass of the product nuclei.

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Yaaaayy I finally get it!!! It's all about wether the reaction needs energy supplied to make it happen or wether it happens naturally due to instability!
So in fission (breaking a nuclei into all it's constituent parts) energy would be required so a greater mass in the products.
In natural fission (including the bombardment of uranium) you are releasing energy to make more stable products so the mass of the products is less!!!

😊
18. (Original post by Mehrdad jafari)
True, the fission of Uranium in core reactors is induced fissioned, I shouldn't have used the phrase induced fission for separating the nucleus into its constituents.

1) if the fission is a decay process happening naturally without any energy being supplied to the nucleus (bombarding the nucleus with a neutron can be neglected), in which the initial nucleus is separated into two or more nuclei, and not its constituents, then energy is released in the process because the product nuclei have a greater binding energy. The energy released results in a decrease in mass of the products compared to the initial nuclei.
• if the fission is a process in which a nucleus, He-4 for example, is separated into its constituents, that is 2 protons and 2 neutrons all separated from each other then energy equal to the binding energy of nucleus, or He-4 for our example, must be supplied to the nucleus to separate it into its constituent protons and neutrons. Because the energy was supplied (assuming that there was no energy dissipation), this energy which has an equivalent mass that is stored in the constituent protons and neutrons ( note that the initial nucleus no longer exits for its mass to be greater than its constituents because the initial nucleus has already fragmented into its constituents). Therefore the mass of the initial nucleus < the mass of its constituent protons and neutrons.

2) you mentioned a very good point there. Comparing the mass of a He-4, for example, with its constituent protons and neutrons, with the constituents not being the product of fission of the He-4 nucleus you would still find that the mass of the He-4 is less than the mass of its constituents, thats the mass of 2 protons and 2 neutrons all separated from each other. This is because neutrons and protons separated from each other are in a higher energy state than when they are joined together. When they join energy is released in the process and so protons and neutrons ( constituents of all nuclei) have a greater mass than when they are joined because they have a greater energy content when separated.

3) that's true. In fusion the mass of the nucleus formed is less than the total mass of the initial nuclei but this is because energy is released when the initial nuclei joined together. Since energy has mass this means that mass was released from the initial nuclei during fusion and so the mass of the product nucleus will be less than the total mass of the initial nuclei.

In sum:
in fission( decay process):
Mass of initial nucleus > mass of the product nuclei
( fission happening naturally never results is breaking the nucleus into its constituents but only results in the formation of two or more smaller nuclei)

In fission ( breaking a nucleus into its constituents):
Mass of the initial nuclei < mass of the constituents of the nucleus

In fusion :
Mass of the initial nuclei > mass of the product nuclei.

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You are a godsend! Thanks so so much

19. Why does an increase in volume mean that the gradient of the line is reduced? I thought increasing volume would create more pressure as there are more molecules

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20. (Original post by Sbarron)
Yaaaayy I finally get it!!! It's all about wether the reaction needs energy supplied to make it happen or wether it happens naturally due to instability!
So in fission (breaking a nuclei into all it's constituent parts) energy would be required so a greater mass in the products.
In natural fission (including the bombardment of uranium) you are releasing energy to make more stable products so the mass of the products is less!!!

😊
I get it too now! Glad we could help each other out

Good luck in the exam!

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