June 2011 G485-Fields, Particles and Frontiers of PhysicsWatch

7 years ago
#541
Now im confused

On this diagram

Is the EMF at maximum in its current position?
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Thread starter 7 years ago
#542
(Original post by Oh my Ms. Coffey)
Now im confused

On this diagram

Is the EMF at maximum in its current position?
Yes it is
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7 years ago
#543
(Original post by MarieLyon)
depending on where the coil is cutting the field, yeah.
your confusing lenz law and faradays law.
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7 years ago
#544
So EMF depends on the direction at which the wires are cutting the magnetic field, they are cutting at maximum as direction between wires and field is 90 degrees. At the top EMF is 0 because wires are moving in the same direction as the magnetic field so the angle between is 0.
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Thread starter 7 years ago
#545
(Original post by Oh my Ms. Coffey)
Now im confused

On this diagram

Is the EMF at maximum in its current position?
I know it seems confusing. But think about it like this:

Induced EMF=rate of change of magnetic flux linkage

When the flux linkage is maximum the rate of change of magnetic flux linkage will be zero. It is maximum when the plane of the coil is perpendicular to the field lines(i.e. from the diagram imagine the ocil in the vertical position-"As if lying on one side")

Otherwise, when the plane is parallel to the field lines, then the flux linkage is zero since the plane of coil is parallel to the field-not cutting any field lines, and so you will get maximum rate of change of flux linkage here.

Its sort of like SHM, the maximum acceleration occurs when the pendulum bob is at one endpoint of its oscillation, the speed is zero but the acceleration is maximum.
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7 years ago
#546
(Original post by muffingg)
Hey, thanks for sharing. Do you have any more notes like these?
Yh i did some for like each unit. Ill try and upload them as attachments now if not ill ust copy n paste my work lol x
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7 years ago
#547
(Original post by Oh my Ms. Coffey)
Now im confused

On this diagram

Is the EMF at maximum in its current position?
No the emf will be 0 if it is at time 0, because the area of the coil is parallel to the lines. But if time has passed, there has been a change in magentic flux linkage, so it will have an emf, so it depends.
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7 years ago
#548
(Original post by yokabasha)
Yh I agree with that, but the coil needs to be perpendicular to the field for maximum magnetic flux/ linkage.
actually sulex is right. The definition of magnetic flux relies on the dot product between the vector perpendicular to the area and vector magnetic flux density. When parallel, the perpendicular area vector is exactly at 90 degrees to the vector representing the B field. Now the dot product is zero when these two quantities are at 90 degrees therefore the magentic flux is zero when parallel. Now if the coil is rotated at a constant angular frequency it can be shown using differentiation and faraday's law that if the coil starts in the perpendicular position so its flux linkage follows a cosine graph, then the emf will follow a -sin graph with the same frequency. Now sin is maximum when theta=90 degrees which represents when the coil is parallel to the magnetic field.

Another way of looking at is comparing it to SHM.

Yet another way of looking at it us using the cross product: F=qB x v where B and v are the vectors that represent magnetic flux density and velocity x represents the cross product. B x v=Bvsin(theta) Where here B and v are the magntitudes of B and v. So the force on the charge is maximum when B and v are at 90 degrees to each other. When the force is maximum on the charge, the emf must also be maximum. Equivalently, when the coil is moving parallel to the B theta=0 i.e. the force on the charge is zero, which means the emf is also zero.

The above is a more mathematical explanation of it, but the basic out come is still the same.
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7 years ago
#549
(Original post by anshul95)
well actually that is an incorrect definition of the photoelectric effect as applied to X-rays. Don't worry I know the CGP book says this, but the extra energy of the photon (which of course has to above the work function) becomes the energy of the electron. http://www.upscale.utoronto.ca/PVB/H...yInteract.html
You are explaining the characteristic wavelengths which occur when the electrons hit the tungsten target (this is not the photoelectric effect as it does not release an electron but an x-ray photon).
but for the purpose of the exam tomorrow its correct.
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7 years ago
#550
Is the quark model for Beta - decay:
udd to uud ?

As some confusion as text book saying different to mark scheme of June 2010
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7 years ago
#551
Whats the estimated wordy/calculation based content of this module?
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7 years ago
#552
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7 years ago
#553
(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
from neutron to proton its beta minus.
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7 years ago
#554
(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
this is correct however there is also proton decay which is the opposite incase it is talking about that =/
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7 years ago
#555
(Original post by Oh my Ms. Coffey)
thats magnetic flux against time.
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7 years ago
#556
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
Good luck for tomorow lets show OCR what we're made of. Much love
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7 years ago
#557
(Original post by MarieLyon)
from neutron to proton its beta minus.
So that would be right?
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Thread starter 7 years ago
#558
(Original post by MarieLyon)
thats magnetic flux against time.
This is actually EMF against time I believe, since in the initial position we have maximum flux linkage
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7 years ago
#559
(Original post by balloon92)
So that would be right?
yeah - dont forget you also get an antineutrino.
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Thread starter 7 years ago
#560
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!

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