OCR Physics A G485 - Frontiers of Physics - 18th June 2015

I don't like the spec's open universe - I swear it should have an increasing gradient over time? It just looks like a flat universe which takes longer to approach its finite size(?).

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Reply 981

rachelc142

11

Original post by Elcor

I don't like the spec's open universe - I swear it should have an increasing gradient over time? It just looks like a flat universe which takes longer to approach its finite size(?).

yes !!! me and my teacher had this discussion. its an awful graph.

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Reply 982

gothmog827

2

Original post by rachelc142

yes !!! me and my teacher had this discussion. its an awful graph.

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I think maybe it's like a rootx graph, which doesn't actually tend towards a limit but has forever decreasing gradient... just a guess though

Reply 983

Jigardubal

Original post by Elcor

Can someone explain how the temperature of the Universe and the cosmic background radiation are linked?

After the Big Bang, fusion occurred which meant that helium was formed and in the process, gamma rays were released, over time due to the cooling of the universe, the wavelength of the gamma rays increased, and hence gamma rays became microwaves, which are present at this moment in time. Hope this helps

Reply 984

kate8

2

Hi, can someone please explain binding energy and how it links to E=mc2. Don't really understand it at all!

Reply 985

L'Evil Fish

OP

18

Original post by Elcor

I don't like the spec's open universe - I swear it should have an increasing gradient over time? It just looks like a flat universe which takes longer to approach its finite size(?).

Btw binding energy is the

*minimum* amount of energy to separate all the nucleons in a nucleus

I am in bed rn, going to have breakfast, make a cuppa, and do June 13 with some music, this is the type of revision I enjoy

Reply 986

Engineering Lad

5

What does it mean when mark schemes refer to a 'Z value' in medical imaging? Z is used for acoustic impedance in ultrasound, so it doesn't make any sense. Unless it's the proton number?
Can anyone clarify?

Edit: I've realised it is the proton/atomic number.

(edited 8 years ago)

Reply 987

Elcor

14

Original post by L'Evil Fish

Btw binding energy is the

*minimum* amount of energy to separate all the nucleons in a nucleus

I am in bed rn, going to have breakfast, make a cuppa, and do June 13 with some music, this is the type of revision I enjoy

Thanks

I'm going into school for some final classes

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Reply 988

Oraeng

9

Original post by kate8

Hi, can someone please explain binding energy and how it links to E=mc2. Don't really understand it at all!

Binding energy is the minimum amount of energy required to separate all the individual nucleons in a nucleus.
So for example if it were possible to separate all of the individual nucleons in a uranium-235 nucleus, there would be a difference between the mass of the nucleus itself and the sum of the masses of the individual nucleons after they were separated. This mass difference is called the 'mass defect'. The nucleus has a binding energy, so energy must be supplied in order to free the nucleons from the nucleus.
The binding energy is equal to the mass defect x c^2.

Reply 989

Oraeng

9

Original post by Engineering Lad

What does it mean when mark schemes refer to a 'Z value' in medical imaging? Z is used for acoustic impedance in ultrasound, so it doesn't make any sense. Unless it's the proton number?
Can anyone clarify?

A Z value is the specific acoustic impedance for a boundary between two specific materials. For example the acoustic impedance of the boundary between muscle and fat is different to the acoustic impedance of the boundary between muscle and bone.

Is this what you're asking? Could you provide an example of where in the mark scheme it has confused you?

Reply 990

kate8

2

Original post by Oraeng

Binding energy is the minimum amount of energy required to separate all the individual nucleons in a nucleus.
So for example if it were possible to separate all of the individual nucleons in a uranium-235 nucleus, there would be a difference between the mass of the nucleus itself and the sum of the masses of the individual nucleons after they were separated. This mass difference is called the 'mass defect'. The nucleus has a binding energy, so energy must be supplied in order to free the nucleons from the nucleus.
The binding energy is equal to the mass defect x c^2.

So the energy you put into pulling a nucleus apart is converted into mass?

Reply 991

Engineering Lad

5

Original post by Oraeng

A Z value is the specific acoustic impedance for a boundary between two specific materials. For example the acoustic impedance of the boundary between muscle and fat is different to the acoustic impedance of the boundary between muscle and bone.

Is this what you're asking? Could you provide an example of where in the mark scheme it has confused you?

It was on the topic of contrast media, which I realise now are elements with a high atomic number (Z value).

Reply 992

L'Evil Fish

OP

18

Original post by kate8

So the energy you put into pulling a nucleus apart is converted into mass?

Yeah basically once separated

Reply 993

Oraeng

9

Original post by kate8

So the energy you put into pulling a nucleus apart is converted into mass?

Rather the mass lost in separating the nucleons is converted into energy as there is a decrease in mass.

Reply 994

gothmog827

2

Original post by Engineering Lad

It was on the topic of contrast media, which I realise now are elements with a high atomic number (Z value).

never even thought about this before, always just wrote had a high z value thinking it was referring to acoustic impedance

Reply 995

kate8

2

Original post by Oraeng

Rather the mass lost in separating the nucleons is converted into energy as there is a decrease in mass.

But the mass of separate nucleons is greater than the mass of combined nucleons?

Reply 996

Engineering Lad

5

Original post by gothmog827

never even thought about this before, always just wrote had a high z value thinking it was referring to acoustic impedance

If you think about it, it wouldn't make any sense for it to be acoustic impedance, since it has the unit of pascal second per cubic metre, which wouldn't make an awful lot of sense with X-rays. Whereas if Z refers to the atomic number, it makes sense, since a high Z value would indicate the atoms of the medium have many electrons with which X-rays interact with, making a medium of high Z value more absorbing.

Reply 997

chem@uni

9

List all the forces acting on the nucleons and explain them? (6 marker)

Anyone help!!!

Reply 998

actanide

2

Original post by chem@uni

List all the forces acting on the nucleons and explain them? (6 marker)

Anyone help!!!

you have gravitational, because nucleons have mass. This force is pretty much negligble.

you have electrostatic that repulses positive protons because theyre positive.

and then you have the strong nuclear force which has a very short range. It holds the nucleons together and at the same time it prevents them from being squashed