June 2011 G485-Fields, Particles and Frontiers of Physics Watch

MarieLyon
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(Original post by phillichilli)
sorry in advance these notes are going to be in no particular order, i cant seem to upload documents !

the nature of the universe :

stars like the sun form from clouds of dust and gas. The main elements were hydrogen and helium, where it was denser the suns own gravity causes material to pull together and contract to form a denser mass. This has a stronger pull so more p=matter is pulled in and so on, this is known as gravitational collapse. The fusion of hydrogen and helium is known as hydrogen burning.
The parsec is defined as the distance that gives a parallax angle of 1 arc second (1/3600)
olbers paradox :
• the universe is infinite, uniform and static.
• for an infinite uniform and static universe the night sky should be bright because of light received in all directions from stars.
• the universe is not static – its expanding
• the universe is (probably) not infinite
• the finite age of the universe and the finite speed of light means that light from the most distant galaxies has yet to reach us
• as distant galaxies recede, their light is red-shifted. This means that it is less energetic and therefore dimmer.
Age of the universe = 1/hubbles constant (gradient of graph)
(gradient of graph * 103) / one mega parsec – 3.1x1022
1/ans = age of universe in seconds
age of universe in seconds / how many seconds in a year – 3.16x107s = age in years.

the evolution of the universe

a further piece of evidence for the hot big bang theory comes from the temperature of the universe today, the radiation detected in space is cosmic microwave background radiation. It corresponds to a very low temperature 2.7k.all intensities of a particular wavelength were the same in all directions backing up the cosmological principle and the isotropic nature of the universe.
Cosmological principle – the universe has the same large scale structure when observed from any point in the universe.
The universe has expanded because space itself is expanding; this itself has the effect of stretching waves as they move through space. Stretched waves have longer wavelengths and longer wavelengths correspond to a lower temperature.
So the three major pieces of evidence supporting the standard ht big bang model of the universe are:
• galaxies receding (and therefore an expanding universe)
• chemical composition of early galaxies (mostly hydrogen with 25% traces of helium, little bits of beryllium and lithium)
• cosmic microwave background radiation, which corresponds to a temperature of the universe of about 2.7k
we believe that most of the mass of the universe consists of dark matter which emits or reflects little em radiation making it hard to detect but we know it exists because we observe its gravitational pull on distant galaxies. There is also the chance that he fate of the universe will be determined by dark energy, a form of binding energy thinly distributed throughout the universe which may be causing the universe to accelerate
• density of the universe > p0: Gravitational forces eventually halt the expansion of matter and reverse the process towards a big crunch, the universe is closed. (bouncing universe)
• density of the universe < p0: Gravitational forces cannot halt the expansion of matter. The universe expands forever, the universe is open.
• density of the universe = p0: The universe will expand forever but the rate of expansion tends to 0 after an infinite time. The universe is flat.

ultrasound in medicince:

ultrasound is any wave that has a frequency above the upper hearing limit of human hearing.
This is usually 20khz.
Ultrasound is produced by a vibrating source, the frequency of the source is the same as the frequency of thewaves it produces. In ultrasound the ultrasonic waves are produced by a device in which varying voltage is used to generate ultrasound, this is known as a transducer.
At the heart of the transducer is a piezoelectric crystal. When a voltage is applied across it in one direction, it shrinks slightly, when a voltage is reversed it expands slightly. So an alternating voltage can cause it to contract and expand at the same frequency (f). The voltage induces a strain in the crystal.
The transducer also acts as the detector of ultrasound waves, it can do this because the piezoelectric crystals can work in reverse. A varying stress will produce a varying e.m.f across the crystal. The optimum size of the crystal is half the wavelength of the ultrasound frequencies.

Acoustic impedance (z) of a material depends on the density and speed of sound in the material.
Z = pc (units – kgm-2s-1

atomic structure:

baryon number, charge and strangeness are all conserved.
Hadrons - protons & neutrons, all affected by the strong nuclear force.
Leptons – electrons, unaffected by the strong nuclear force.
Proton – (uud)
neutron – (udd)
p+ meson – (ud)
phi meson – (ss)
nucleus : Electrostatic repulsion and strong nuclear force.
Coulombs law (esa) : F = qq/4??0r2
nuclear attraction : F = gmm/r2
rutherford scattering – plum pudding model.
• shooting alpha particles at thin gold foil
• most alpha particles went through but some were deflected at different ranges of angles
• this led to the decision that an atom was made up of mostly empty space apart from the central nucleus.
Isotopes are atoms of the same element with a different number of neutrons.

radioactivity :

positrons and electrons interact with each other, annihilating each other and the energy is then converted in the form of gamma rays.
You can separate alpha beta and gamma radiation by passing them through either and electric field or a magnetic field.
Alpha particles can be detected using a geiger-muller tube, when you move the radioactive source back and forth it is simple to show that the particles only penetrate a small amount of air,
nuclear decay !
Spontaneous
• the decay of a particular nucleus os not affecte by other nuclei
• the decay of nuclei cannot be affected by external factors
random
• the probability of nuclei decaying in a sample is the same per unit time
• it is impossible to detect when one nuclei will decay
carbon dating is used to find out how much time has passed since a piece of living material died.
Decay constant – the probability that an individual nuclei will decay per unit time.
Activiy – the rate of decay / disintegration
half life – the mean time for half of active nuclei in a sample to decay
the weak interaction between quarks is responsible for beta plus decay.
Most ionising, least penetrating alpha
least ionising, most penetrating – gamma

nuclear physics:

nuclear fission :
1. Splitting apart into 2 separate and more stable nuclei = nuclear fission
2. Neutron colliding with a large unstable nucleus = induced nuclear fission
nuclear fusion :
Light nuclei becoming more stable (the sun). High energy and density are required for hydrogen nuclei to overcome the electrostatic repulsion between them so that they can fuse together.
E=mc2
• the mass of a system increases when energy is put into it.
• the energy released from a system decreases when its mass decreases.
Mass defect – the mass defect of a nucleus is equal to the difference between the total mass of the individual separate nucleons and the mass of the nucleus.
Binding energy- the minimum energy needed to pull a nucleus apart into its separate nucleons.
• the neutrons in fission are very energetic, because they are moving at high speeds the probability of them reacting with uranium nuclei is unlikely so the moderator (graphite) slows them down without absorbing them in inelastic collisions.
• the control rods are made of neutron absorbing materials such as boron, when the chain reaction is needed to slow down the control rods are lowered, when the chain reaction needs to be sped up the control rods are lifted up.
• waste materials from nuclear reactors are highly flammable and pose a threat to the environment.

xrays

when fast moving electrons are rapidly decelerated, the lost energy is converted into x-rays.
The interaction force of electrons are photons, x-rays are high energy photons. X-rays are a form of e.m radiation, with a short wavelength and high frequency. They are effectively the same as gamma rays but are produced differently.
Bremsstrahlung radiation – breaking radiation
intensity is the power per unit csa. I = p/a
characteristic radiation arises in different ways in which an electron loses its energy when it crashes into the anode.
The attenuation of x-rays as they pass through a uniform material is given by :
I = i0 e-µx
where i0 is the initial intensity, i is the transmitted intensity and µ is the attenuation coefficient.
The gradual decrease in the intensity of a beam of x-rays as it passes through matter is called attenuation.
In digital systems, image intensifiers are used. The incoming x-rays strike a phosphor screen producing electrons of visible light photons. These then release electrons (by photoelectric effect) from the photocathode. The electrons are accelerated and focused by the positively charged anode so that they strike a screen which gives out visible light. The image on this screen can be viewed via television camera and can also be stored electronically.


absorption mechanisms (i cant remember i think this is in the medical section)

photoelectric effect :

In the photoelectric effect an xray photon with a low energy is absorbed by an electron in the target metal. It gains enough energy to move energy levels and escape the metal.



Compton scattering :

In compton scattering an electron with a high range of energy loses only a fraction of its energy to an atomic electron in the absorbing metal, the interaction between the photon and the electron is inelastic. The scattered x-ray photon has less energy than before so its wavelength is greater, the compton electron goes off in a different direction because momentum has to be conserved. Pair production :

In pair production a high energy x-ray photon passing through the electric field of the nucleus suddenly produces an electron – positron pair. The positron is annihilated when it collides with the electron. This process is not important in diagnostic x-rays because the x-ray energies are usually too low.

contrast media (more xrays)

it is often desirable to show up different soft tissues that absorb x-rays equally. In order to do this contrast media are used.
A contrast medium is a substance such as iodine or barium which is a good absorber of x-rays. The patient may swallow the barium containing liquid or have a similar liquid injected into the tissue of interest. This tissue is then a better absorber of x-rays and its edges show up more clearly on the final image.
Contrast media are elements with high atomic numbers; this means that their atoms have any electrons with which the x-rays interact, so they are more absorbing. The attenuation mechanism is mainly the photoelectric effect, for which the attenuation coefficient is proportional to the cube of the proton number. Soft tissues mainly consist of compounds such as hydrogen carbon and oxygen (low atomic numbers) while bone has heavier elements such as calcium and phosphorus (high atomic numbers)


more medicine :

When choosing a suitable radionuclide you need to choose one with a short half life, 2 reasons being :
1. It will give out its radiation quickly so that only a small amount is needed to form an image in the gamma camera
2. Any nuclide that remains in the patient will soon decay away, ensuring that they are not exposed to hazardous levels of radiation.
But if a hospital keeps buying a short-lived radioisotope it will already be decaying before it reaches the hospital. One solution of this is using technetium – 99m . The “m” indicates that it is metastable , that is, it remains in its energetic state for some time decaying by beta – emission.
(technetium-99m is an isotope of technetium with nucleon number – 99. Tc-99m is produced when molybdenum-99 undergoes beta decay. This happens in 2 stages :
1. 99mo42 99tcm43 + electron + anti-neutrino (half life 67 hours)
2. 99tcm43 99tc43 + gamma ray (half life 67 hours)
99tc43 decays by beta emission (half life 2.1x105 years)

radiopharmaceuticals
to make sure the radioisotope reaches the correct organ, it must be converted into a pharmaceutical. This means it is chemically combined with other elements to produce a substance that can be taken in by the tissue of interest. I.e. Injections (tc-99m). They are also referred to as tracers because they are designed to target particular organs and tissues in the body,


ok guys i really hope these notes do help :awesome:
Good luck for tomorow lets show ocr what we're made of. Much love :h :h
you're amazing
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scotzbhoy
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(Original post by balloon92)
Is the quark model for Beta - decay:
udd to uud ?

As some confusion as text book saying different to mark scheme of June 2010
Yes, that is correct.
Think of it this way. In Beta- decay, the proton number goes up, so this tells you that a neutron is converted into a proton. A neutron is UDD, so this conversion is UDD-->UUD.
In Beta+ decay, the proton number goes down, as a proton is converted into a neutron. Since a proton is UUD, this conversion is UUD-->UDD.
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MarieLyon
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(Original post by sulexk)
This is actually EMF against time I believe, since in the initial position we have maximum flux linkage
oops sorry I contridicted myself from earlier. I thought we were still on about from the vertical postion (not cutting any lines)
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yokabasha
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(Original post by Oh my Ms. Coffey)
(Original post by MarieLyon)
thats magnetic flux against time.
Nh its voltage against time aint it, cause the area of the coil is perpendicular to the field, so flux is max, therefore voltage is zero, cause the rate of change of flux linkage is = voltage
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Captain Hob
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I'm a little confused by the mark scheme for the February 11 paper, question 6(b).

The present mass of the Sun is 2.0x10^30 kg. The Sun emits radiation at a rate of 3.8x10^26 J/s. Calculate the time in years for the mass of the Sun to decrease by one millionth of its present mass.

In the mark scheme, the change in mass is taken to be 2.0x10^24 kg. Surely if the mass were to decrease by one millionth, it the change in mass would be 2.0x10^6 kg? :confused:


In other news, I hate OCR.
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MarieLyon
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(Original post by yokabasha)
Nh its voltage against time aint it, cause the area of the coil is perpendicular to the field, so flux is max, therefore voltage is zero, cause the rate of change of flux linkage is = voltage
I know, I know - I mis-read it.
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TobeTheHero
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why morning!?
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MarieLyon
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(Original post by Captain Hob)
I'm a little confused by the mark scheme for the February 11 paper, question 6(b).

The present mass of the Sun is 2.0x10^30 kg. The Sun emits radiation at a rate of 3.8x10^26 J/s. Calculate the time in years for the mass of the Sun to decrease by one millionth of its present mass.

In the mark scheme, the change in mass is taken to be 2.0x10^24 kg. Surely if the mass were to decrease by one millionth, it the change in mass would be 2.0x10^6 kg? :confused:


In other news, I hate OCR.
see, i thought that. i tried to times by 1/million and got a stupid number.
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sulexk
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(Original post by Captain Hob)
I'm a little confused by the mark scheme for the February 11 paper, question 6(b).

The present mass of the Sun is 2.0x10^30 kg. The Sun emits radiation at a rate of 3.8x10^26 J/s. Calculate the time in years for the mass of the Sun to decrease by one millionth of its present mass.

In the mark scheme, the change in mass is taken to be 2.0x10^24 kg. Surely if the mass were to decrease by one millionth, it the change in mass would be 2.0x10^6 kg? :confused:


In other news, I hate OCR.
Actually I see what you mean, if it said "to one millionth of its present mass" then that would be correct, but saying "by one millionth" indicates subtracting a millionth from its initially mass, which would be almost insignificant.
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MarieLyon
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(Original post by sulexk)
Actually I see what you mean, if it said "to one millionth of its present mass" then that would be correct, but saying "by one millionth" indicates subtracting a millionth from its initially mass, which would be almost insignificant.
it does, i did both ways its pretty much negliable.
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susan23
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ok can someone explain this to me....

when you have a coil rotating in a magnetic field and you produce a voltgae against time graph

the MAximuM magnetic flux is when the voltage is zero? because the maximum cut is when the coil is just a bit over perpendicular and just a bit below perpendicular so the difference is large? does that make sense?

sorry for the **** picture, i had no idea how to explain it otherwise...Name:  mag.jpg
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Lengalicious
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(Original post by sulexk)
THE BIG QUESTION ON ELECTROMAGNETIC INDUCTION!

This is presumed to be a very challenging question- if you can do it, feel free to explain it to us!

3ai) Magnetic flux is flux density x the area through which it passes BA
aii) flux = BA where A = x^2 therefore flux = Bx^2, linkage is number of coils therefore flux linkage = NBx^2

bi) emf = rate of change in flux linkage
- therefore flux linkage over 0.2 s is 1250x0.032x(0.02)^2/0.2 = 0.08 v which translates to 80 mV
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balloon92
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Also how would you calculate the force at the midpoint between two charges?
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Captain Hob
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(Original post by MarieLyon)
see, i thought that. i tried to times by 1/million and got a stupid number.
(Original post by sulexk)
Actually I see what you mean, if it said "to one millionth of its present mass" then that would be correct, but saying "by one millionth" indicates subtracting a millionth from its initially mass, which would be almost insignificant.
Glad it's not just me then. Hopefully they're not that vague tomorrow.
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susan23
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(Original post by balloon92)
Also how would you calculate the force at the midpoint between two charges?
its usually zero i think.
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Lengalicious
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(Original post by susan23)
its usually zero i think.
wrong
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susan23
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(Original post by Lengalicious)
wrong
well it depends if its a neutral point or not. if not then its not zero. depends on the charges and what the fields are lke
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Captain Hob
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(Original post by balloon92)
Also how would you calculate the force at the midpoint between two charges?
Coulomb's law, using the product of both charges and the total distance between them.
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Lengalicious
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(Original post by susan23)
well it depends if its a neutral point or not. if not then its not zero. depends on the charges and what the fields are lke
think of how ridiculous that sounds if its a neutral point then its not between two charges.... the field is weakest at the centre between 2 charges but if they are interacting there is always a field strength as the definition of charge is 'not neutral'
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muffingg
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A 20 uF capacitor is charged up to 200V and then disconnected from the supply. It is then connected across a 5.0 uF capacitor. Calculate the energy dissipated when they are connected together.

(u = microFarad 10^-6)


Anyone please?
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