OCR Physics A - G485: Fields, Particles & Frontiers of Physics - June 2012
Physics exam discussion - share revision tips in preparation for GCSE, A Level and other physics exams and discuss how they went afterwards.
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Re: OCR Physics A - G485: Fields, Particles & Frontiers of Physics - June 2012LOOOL the thought had crossed my mind aswell(Original post by mack94)
This exams got me thinking about taking a gap year loool -
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Re: OCR Physics A - G485: Fields, Particles & Frontiers of Physics - June 2012the death of a star depends on its mass so ill do all of them(Original post by Jetblast)
Binding energy of a nucleus is the minimum energy needed to separate all its nucleons.
Can someone go through the death of a star? does stars with the mass like our sun become a red dwarf, then turn to a blue/white dwarf then a dark body?
low mass stars - hydrogen fuel is depleted, radiation pressure ceases, the core contracts but doesnt have a high enough increase in temperature so helium fusion isnt possible, the star contracts until it is a white dwarf, the mass of the core is less than 1.4 solar masses so electron degeneracy pressure prevents it from collapsing further
medium mass stars - hydrogen fuel is depleted, the radiation pressure ceases and the core contracts, the loss in PE results in an increase of KE resulting in temperature increase and this temperature is high enough for helium fusion to begin, this produces radiation pressure once again and pushes the outer layers of the star outwards to form a red giant star. once helium fuel is depleted, the core contracts but the temperature isnt high enough for further fusion reactions to take place, the star sheds its outer layer in ion sprays and a white dwarf star remains, the mass is below 1.4 solar masses so electron degeneracy pressure prevents the star from collapsing further
high mass stars - hydrogen fuel is depleted, the radiation pressure ceases and the core contracts, the loss in PE results in an increase of KE resulting in temperature increase and this temperature is high enough for helium fusion to begin, this produces radiation pressure once again and pushes the outer layers of the star outwards to form a red giant star/super red giant star. once helium fuel is depleted, the core contracts and the temperature is high enough for further fusion reactions to take place, they can go all the way up to iron but its not possible after this as it isnt energetically favourable. the core of the star is greater than 1.4 solar masses so electron degeneracy pressure cannot prevent further collapse so the electrons combine with protons to form neutron and neutrinos where the neutrinos escape and leaving a neutron star core. the outer layers of the star rebound off the neutron star core and it explodes in a supernova
the neutron star core remains
if the gravitational pressure is so large that it overcomes neutron gas pressure, the core collapses to a point with infinite density, a black hole
hope this helped!
our sun becomes a red giant and then a white dwarf star then eventually a black dwarf star as it cools, im not sure whether theres such thing as a red dwarf or blue dwarf though!
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To charge a capacitor, all you need to do is connect it to a battery. Charge (the electrons from the battery) will travel in one direction.around the circuit. More of the electrons will gather on one of the plates, making it negatively charged. These electrons will repel the fewer electrons on the other plate, causing them to gather on the first. So we now have one positively charged plate, and one negatively charged plate. The capacitor will be fully charged when the p.d. across it = the e.m.f. of the battery. By adding a voltmeter (in parallel with the capacitor), to the circuit you can observe when it's fully charged.(Original post by moh.alt)
Does anybody know how a capacitor discharges and charges? I am soo confused.
To discharge a capacitor, all you need to do is remove the power source. This is done by simply opening the switch (The switch is usually moved to a new circuit containing a voltmeter, ammeter and a resistor. The reason for the inclusion of the resistor is that a resistor resists the flow of charge so the capacitor will discharge more slowly (through the resistor); so energy is provided for longer.
Those are the basics. Let me know if you want to know more about the shape of the Q, V & I graphs and why they look like they do
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Re: OCR Physics A - G485: Fields, Particles & Frontiers of Physics - June 2012Thank you, but how does the phosphor screen convert the x-ray to many visible light photon? Compton scattering?(Original post by Picture~Perfect)
They make an X-ray image clearer. (This is due to reduced exposure time)
Its made up of two phosphor screens and consists of an anode and cathode in a vacuum.
The X-rays are incident on the first phosphor screen. This results in the X-ray photons producing a large number of visible light photons. These strike a photocathode where electrons are released. The electrons are accelerated and focused by the positively charged anode, so that they strike a second phosphor screen. This gives out visible light.
The final image can be views on a monitor. -
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You don't need to know(Original post by hevlar.kelmet)
Thank you, but how does the phosphor screen convert the x-ray to many visible light photon? Compton scattering?
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Re: OCR Physics A - G485: Fields, Particles & Frontiers of Physics - June 2012Alright then, that's fine by me.(Original post by DavidMRoper)
You don't need to know
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Re: OCR Physics A - G485: Fields, Particles & Frontiers of Physics - June 2012I'm pretty sure we don't need to know this depth but yes radio frequencies are made by having a coil with an alternating current that causes the secondary coil to vibrate. But just saying a coil emits a PULSE of radio waves I think will be sufficient.(Original post by ReTurd)
Ok thanks, especially about 3).
But are you absolutely sure about the radio waves being used (e.g. the EM radio wave) and not an alternating magnetic feild which resonates a frequency of radio waves. I ask this because in the book it shows the radio 'waves' beeing made by a radiofrequency (RF) coil, how is a coil suposed to produce actual radio waves/photons? Surely its what i said which is an alternating current in the wire produces a magnetic feild which oscillates at radio frequency.
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70-80 raw for a*(Original post by NerdyMcNerdNerd)
Does anyone know roughly what the grade boundaries in raw marks are for an a* and 100%/full ums? Just to see haha
80-90 raw for full ums
Depending on paper
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Re: OCR Physics A - G485: Fields, Particles & Frontiers of Physics - June 2012Phosphor screen? Whats that used for?(Original post by DavidMRoper)
You don't need to know
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It's like a crystal thing - sodium iodide crystals. It's basically, some sparkly **** that gives of visible light when it absorbs X-rays :P(Original post by ReTurd)
Phosphor screen? Whats that used for?
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Re: OCR Physics A - G485: Fields, Particles & Frontiers of Physics - June 2012Hi,
I'm a bit confused with exactly why fission releases energy. I get that the total binding energy per nucleon of the two products (daughter nuclei) is greater than the original nucleus' binding energy per nucleon, so this releases energy - I just accept it!
But, the mass of the 2 daughter nuclei in total is greater than the original nucleus, and I thought that if mass of system goes up then energy must be put in, not released!? (Using E=mc^2)
Please could somebody make me less confused?
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The mass is less!(Original post by alixbaldwin)
Hi,
I'm a bit confused with exactly why fission releases energy. I get that the total binding energy per nucleon of the two products (daughter nuclei) is greater than the original nucleus' binding energy per nucleon, so this releases energy - I just accept it!
But, the mass of the 2 daughter nuclei in total is greater than the original nucleus, and I thought that if mass of system goes up then energy must be put in, not released!? (Using E=mc^2)
Please could somebody make me less confused?
Thanks
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Re: OCR Physics A - G485: Fields, Particles & Frontiers of Physics - June 2012I like that explanation(Original post by DavidMRoper)
It's like a crystal thing - sodium iodide crystals. It's basically, some sparkly **** that gives of visible light when it absorbs X-rays :P
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No, seriously though, does it give out many visible light photons or just one per x-ray?
I always get confused because the gamma camera scintillator crystal gives out many light photons per gamma ray. -
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It's actually the same thing as in the gamma camera - minus the photomutilplier tube!(Original post by wibletg)
I like that explanation
No, seriously though, does it give out many visible light photons or just one per x-ray?
I always get confused because the gamma camera scintillator crystal gives out many light photons per gamma ray.
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Re: OCR Physics A - G485: Fields, Particles & Frontiers of Physics - June 2012Thanks mate, cleared something up for me - and it's an easy way to remember it too(Original post by DavidMRoper)
It's actually the same thing as in the gamma camera - minus the photomutilplier tube!
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Just the Rutherford one I think- nothing else in the book!(Original post by JohnyTheLad)
What are the experiments that we need to know?
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our sun becomes a red giant and then a white dwarf star then eventually a black dwarf star as it cools, im not sure whether theres such thing as a red dwarf or blue dwarf though!
