PHYA5 ~ 20th June 2013 ~ A2 Physics

Fission reactions give off more energy because, although the change in binding energy per nucleon is smaller, there's normally about 50x more nucleons involved. Look it up, it works out to be about 10x more energy for the average fission reaction than the fusion reaction

The mass of a positron is the same as the mass of an electron I think, which is given in the table.

Ah I'm so retarded, sorry I was talking about the electron neutrinos, I forgot there were 2 positrons as well, never mind what I just said.

These are the answers I got; (the ones I think are correct anyway)

U= 12th of the mass of a carbon-12 atom
Fission is possible in stars because:
- high temperature = high Ke of particles
- this high Ke is needed for 2 particles to overcome the repulsive electrostatic force between them when colliding
- existence of plasma in stars makes this possible as molecules travel very quickly in plasma.
Energy of He: 24.7 MeV

To decrease power output of nuclear reactor:
-lower control rods (made of boron) which absorb neutrons and lower the rate of fissions
- this decreases power output as the energy of the fissions is used as the reactors source of power.
Nature of radiation of moderator's molecules;
- molecules are in an excited state due to the gain in Ke of the molecules from the collisions with the neutrons.
- as the molecules de-excite, a photon of varying energy is emitted.
- the nature is therefore gamma radiation.
Source of radiation in nuclear waste;
-spent fuel rods
- which contain U-235, U-238, plutonium-239, etc
I completely failed the thermal physics question so not posting the answers I got.
Ideal gas;
A gas that obeys boyle's law (pV= constant) at all pressures.
No. Of moles;
128.78 = 129 mol
I think I did the density question wrong, I couldn't remember if molar mass was divided by mass or the other way round, I think the mass was 5.5 kg, but I put the other answer down, will I still get some marks if I explicitly wrote m= 5.5kg but use another value?

Astrophysics section;

The object has to be beyond 2F for a diminished image.
Focal length;
- 0.46 m
Lens, eyepiece or objective?
-eyepiece, short focal length, objective takes up most of the length of a telescope, this was not the case.
I can't remember most of the astrophysics questions...
Similarity in appearance between the 2 stars; brightness
Difference; colour? Since different spectral classes?
Which is further, K.
- K's temperature range is much higher than the other one, therefore since p is proportional to T^4, K has a later luminosity and therefore further away since it appears just as bright as the less luminous star.
Black hole question, radius;
- 1.21x10^10
Rayleigh's criterion;
- states if the bright maximum of either image is within the first minima ring of the other, resolution of 2 stars is impossible. Draw a diagram showing diffraction patterns merging if angle of separation too small.
Can't remember the actual answer for the min. Angle of separation but it was really small.
Hubble's constant; 71ish, quit accurate as the actual value is 65.
Age of universe? 14 billion years but I put the answer in years? Do I still get full marks? Got like something x 10^17 years I think.

lots said colour as well however if you dont mind me asking when you look up at stars can you tell the difference in colour of the stars?

bit early for this thread haha. Liking unit 5 a lot more than unit 4 so far, applied physics feels like a breeze because we've done most of the stuff in M4 and the rest is engines

What did people get for the density question?

I think mine was really high, like 26 kgm^-3.. which seemed strange

to the winning guy, K is not further away F is further away because it is hotter.... you should get the marks for the last question though as long the value you have is correct

I think I got the letters wrong but isn't star F the one in the spectral class K?

Ah I'm so retarded, sorry I was talking about the electron neutrinos, I forgot there were 2 positrons as well, never mind what I just said.

These are the answers I got; (the ones I think are correct anyway)

U= 12th of the mass of a carbon-12 atom
Fission is possible in stars because:
- high temperature = high Ke of particles
- this high Ke is needed for 2 particles to overcome the repulsive electrostatic force between them when colliding
- existence of plasma in stars makes this possible as molecules travel very quickly in plasma.
Energy of He: 24.7 MeV

To decrease power output of nuclear reactor:
-lower control rods (made of boron) which absorb neutrons and lower the rate of fissions
- this decreases power output as the energy of the fissions is used as the reactors source of power.
Nature of radiation of moderator's molecules;
- molecules are in an excited state due to the gain in Ke of the molecules from the collisions with the neutrons.
- as the molecules de-excite, a photon of varying energy is emitted.
- the nature is therefore gamma radiation.
Source of radiation in nuclear waste;
-spent fuel rods
- which contain U-235, U-238, plutonium-239, etc
I completely failed the thermal physics question so not posting the answers I got.
Ideal gas;
A gas that obeys boyle's law (pV= constant) at all pressures.
No. Of moles;
128.78 = 129 mol
I think I did the density question wrong, I couldn't remember if molar mass was divided by mass or the other way round, I think the mass was 5.5 kg, but I put the other answer down, will I still get some marks if I explicitly wrote m= 5.5kg but use another value?

Astrophysics section;

The object has to be beyond 2F for a diminished image.
Focal length;
- 0.46 m
Lens, eyepiece or objective?
-eyepiece, short focal length, objective takes up most of the length of a telescope, this was not the case.
I can't remember most of the astrophysics questions...
Similarity in appearance between the 2 stars; brightness
Difference; colour? Since different spectral classes?
Which is further, K.
- K's temperature range is much higher than the other one, therefore since p is proportional to T^4, K has a later luminosity and therefore further away since it appears just as bright as the less luminous star.
Black hole question, radius;
- 1.21x10^10
Rayleigh's criterion;
- states if the bright maximum of either image is within the first minima ring of the other, resolution of 2 stars is impossible. Draw a diagram showing diffraction patterns merging if angle of separation too small.
Can't remember the actual answer for the min. Angle of separation but it was really small.
Hubble's constant; 71ish, quit accurate as the actual value is 65.
Age of universe? 14 billion years but I put the answer in years? Do I still get full marks? Got like something x 10^17 years I think.

haha thats exactly what I thought! couldnt think of anything better to put though, so i left it xD could you?

What mass of ice did people get?

For the last one on astro, do you mean you got 4 x10^17 seconds? cause 10^17 isnt a billion years, but i got the power 17 of seconds :P