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

how comes density here is 2.4 where as in my nelson thornes book it says 3.4

Clearly a massive typo on one or the other?

I'll try to explain it in a nutshell:1. binding energy is the energy you need to seperate a nucleus into its constituent nucleons. 2. the most stable isotopes have around 8.8MeV binding energy per nucleon e.g iron. 3. When you split a nucleus (fission) you get energy out because the products have a higher binding energy per nucleon than what you started with. The reason you actually get out this energy is because the mass of the product is less than the mass of its seperted nucleons therefore you have extra energy.4. You will get more energy from fusion because there is a larger difference in the binding energy per nucleon between the reactants and products (it helps if you look at the graph). hopefully this helps a little bit? this might help too:
https://www.youtube.com/watch?v=UkLkiXiOCWU

https://www.youtube.com/watch?v=rXer6qidxQM

Density isn't always the same but it is around the same so I wouldn't be surprised if in two different past papers they used two different values of density.

Not a number you need to know.

Scratch that, you should know the number approximately but not exactly.

I really don't understand point three onwards at all 😩
All I understand is that when two nucleons are joined, some energy is released (the binding energy) which is the same amount as the energy required (and taken in) to split them.... This energy is also the mass defect which explains why the two separated have a larger mass because the binding energy absorbed is converted to mass..... That is as far as I can understand....
Thankyou for the links.... I will have a look and see if they help...

I'll try to explain it in a nutshell:1. binding energy is the energy you need to seperate a nucleus into its constituent nucleons. 2. the most stable isotopes have around 8.8MeV binding energy per nucleon e.g iron. 3. When you split a nucleus (fission) you get energy out because the products have a higher binding energy per nucleon than what you started with. The reason you actually get out this energy is because the mass of the product is less than the mass of its seperted nucleons therefore you have extra energy.4. You will get more energy from fusion because there is a larger difference in the binding energy per nucleon between the reactants and products (it helps if you look at the graph). hopefully this helps a little bit? this might help too:
https://www.youtube.com/watch?v=UkLkiXiOCWU

https://www.youtube.com/watch?v=rXer6qidxQM

So it isn't constant for every nucleus, it does vary?

What about Ro, does that vary or is it a constant?

I think I'm right in saying that Ro is more of a real constant in comparison to the density of a nucleus.

But I don't think we need to know what Ro is. I think we'd get it given in the question.

I'm not too sure though because google says Ro is like 1.3 x 10^-15 but book says 1.4 x 10^-15 which also makes me confident that we'd get it given in the exam similar to density of a nucleus.

Energy is needed to form a nucleus from two seperate nuclei

Energy is released when a nucleus seperates into two seperate nuclei
The extra energy released is the gain of mass of the two seperate nuclei, in which the sum of their masses is larger then the initial sole nucleus

Lol this is as far as I've got to in my head which won't answer a 6 marker on it!! It's as soon as I start reading about "higher binding energy per nucleon that I get so lost!!"

It's really unnerving when I see a question on part of the spec I had no idea existed, then I realise it applied to an optional topic 😂


Posted from TSR Mobile

Only understand the first two sentences of what you wrote then it's just gobbledygook to me 😳
I feel so stupid as I'm usually good at the concepts and struggle with the maths but this I just can't see what it's on about.

Energy is needed to form a nucleus from two seperate nuclei

Energy is released when a nucleus seperates into two seperate nuclei
The extra energy released is the gain of mass of the two seperate nuclei, in which the sum of their masses is larger then the initial sole nucleus

Is it 931.3MeV, right, not 931.5?


Posted from TSR Mobile

Nelson thornes says 931.3

Ive seen in the mark scheme that they accept both before

E=mc^2 you know the equation right

When the large nucleus is separated into two smaller nucleus, energy is released, so the total energy is increased

An increase of mass is related to an increase of energy.
Because energy is released, so the total energy is increased, this must mean that the mass of the nuclei has also increased

The data sheet handed to you in the exam says 931.5

If extra energy is released when it splits then surely they will have even less mass than they did originally!???

Think of it this way

Largue nucleus X has energy

Large nucleus X can split into nucleus Y and Z

When large Nucleus X splits into Y and Z, energy is released because Y and Z exert forces of repulsion onto each other. Energy is not lost from the nucleus, it is released because the particles exert a force onto each other and you know the equation W=Fx (work = force x distance)

The two nuclei gain this energy that has been released, there fore their mass must increase