Physics Module 8 Flashcards
(91 cards)
1
Q
Hubble’s Law Gradient
A
steeper the gradient, younger universe
2
Q
What is the age of the universe
A
1/Hubble’s Constant
3
Q
How did Hubble create the constant
A
Using work of other scientists, he graphed recessional velocity (Vesto Slipher) on Y axis, and distance (Henrietta Leavitt) on the X axis.
4
Q
What did the Hubble Constant provide evidence for?
A
An expanding universe
5
Q
2 opposing theories
A
Big Bang (matter dilutes as the universe expands)
Steady-State (matter is constantly created as the universe expands)
6
Q
What are stars made of?
A
Hydrogen
- protons moving so fast because of how hot they are
- when they fuse they release energy
VERY hot temp in core pushes stars outwards
- this is known as radiation pressure (OUT)
Due to the mass of the star
- gravitational pressure (IN)
These two are normally balanced but when they aren’t
- red giant expands as radiation increases
- white dwarf collapses as gravitation increases
7
Q
Luminosity and Brightness
A
Luminosity is the total energy emitted by a star per second (POWER) - affected by size and temperature
Brightness is the energy received per squared metre on earth (INTENSITY) - affected with distance, also known as radiation flux
8
Q
Why was apparent magnitude based on brightness not fair?
A
As distance impacts brightness - may actually be more luminous just a long way away
9
Q
Stellar Classification System
A
Oh Be A Fine Girl Kiss Me
O - hottest (blue)
white
yellow
orange
M - coldest (red)
10
Q
Hertzsprung Russel Diagram
A
Surface Temp (X) against Luminosity (Y)
Main sequence stars along the middle diagonal
Red giants top right
White dwarves bottom left
OBAFGKM long bottom
11
Q
Stellar Evolution
A
(determined by mass)
Molecular Cloud > protostar > main sequence > red giant
red giant > planetary nebula > white dwarf
red giant > supernova > neutron star or black hole
12
Q
What can you estimate about a star
A
You can estimate a star’s age based on where it is in the evolutionary path
13
Q
What is nucleosynthesis?
A
The formation of hydrogen and helium from quarks after the Big Bang
14
Q
Conditions for Stellar Nucleosynthesis to occur?
A
- high temperature (around 10/15 million K) due to high potential energy turning into high kinetic energy
- high density (lots of particles in a relatively small space)
15
Q
Proton-Proton chain
A
(fusing hydrogen to helium)
Stars with the same or smaller mass then our sun will produce energy by fusing protons to obtain Helium nuclei in the chain reaction
total process releases about 20MeV
mainly used for main sequence
- 2 protons fuse to produce a diproton
- the diproton in unstable and decays to deuterium by beta decay (2/2He > 2/1He + e(+) + V(E)
- The deuterium nuclide undergoes fusion to Helium-3 by incorporating another proton (energy is released here as a gamma ray)
(2/1 H + 1/1H > 3/2He + gamma) - 2 helium-3 nuclides undergo fusion to Helium-4, which is stable and 2 protons are released back to the star.
16
Q
Carbon-Nitrogen Cycle
A
(used in bigger stars)
top half of main sequence
17
Q
PP Chain and CNO Cycle
A
similarities
- both produce helium from 4 hydrogens
- 2 positrons + 2 neutrons produced
- similar energy produced
- some fuel source (hydrogen)
differences
- CNO relies of C-12 as a catalyst
- 1 extra gamma ray produced in CNO
- CNO in larger stars only
18
Q
Triple Alpha Process
A
Will occur in red giants that have left the main sequence
19
Q
why is gravity important in fusion reactions inside the sun
A
Fusion reactions require high temperatures
- particles must be moving at high speeds (H, He)
- Gravitational forces are responsible for the inwards pressure that causes a protostar to shrink
- As it shrinks, GPE is converted to KE, until T is high enough
20
Q
Who discovered the electron
A
JJ Thomson
(big shift in understanding of matter)
He proposed the plum pudding model - electrons were distributed throughout the atom, with their charge balanced by the presence of a uniform sea of positive charge.
21
Q
How did JJ Thomson discover the electron
A
Cathode Rays
- Applied a potential difference to a pair of electrodes placed inside a glass vacuum tube.
- At low pressure/high voltage, a stream of energy flowed (neg to pos).
- The same cathode ray was emitted from different metal cathodes, therefore, ray was common to all substances.
Thomson believed they were particles
22
Q
Debate for cathode rays
A
Waves
- Rays pass through thin metal sheets without interacting
- Rays are not deflected by electric fields
Particles
- Maltese Cross (rays cast sharp shadows - no diffraction), meaning they travel in straight lines and can be blocked by metal
- Rays are deflected by magnetic fields
- Rays have momentum (caused paddlewheel to move)
23
Q
What did Hertz observe with cathode rays
A
No deflection of the cathode ray when it passed through an external electric field
- the two fields cancelled each other out and the ray went straight
24
Q
How did JJ Thomson calculate
A
- used electric field (electron field turned on, electrons deviated up) E = V/d could calc E
- B turned on, the electron beam re-centred (no deviation) FE = FB
Eq = qVB
V = E/B velocity could be determined - Electric field turned off and deviation of the beam was measured (deviated down in circle)
FB = FC
qvB = mv²
q/m = v/Br
(could calc charge to mass ratio)
25
Conclusion from JJ Thomson Experiment
Charge to mass ratio was the same for all metals, therefore, cathode rays were common to all types of metals.
This was one piece of evidence that led him to believe that electrons were subatomic particles.
26
What did Millikan's Oil Drop Experiment Do
indirectly calculated the charge of the electron
- sprayed small droplets of oil into an electric field between two parallel plates (each had a different mass when they left the atomiser)
- lost electrons when they came out of atomiser but became negatively charged at the plates
- measured under microscope to find the radius of the droplets
- then used the volume formula (V = 4/3pir³)
- then used mass = density x volume
- then calculated E = v/d
- then calculate q
27
How did Millikan indirectly calculate it
He measured the 'gaps' - not a direct measurement. He could see the increments of how many electrons were on the drop
Found that the charge was in discrete packets - all multiples of one value
he found the value to be very close to the known value of an electron
28
Rutherford Model
Geiger and Marsden experiment under direction of Rutherford
- gold foil with alpha particles
- atom was mostly empty space with positive nucleus
when alpha particle got close enough to nucleus, it deflected away.
- showed the atom had a heavy nucleus, surrounded by empty space and then electrons
- showed mostly empty space (had to wait a long time for deflection)
(alpha positive, nucleus positive, the closer to the nucleus the more deflection)
29
Chadwick
Discovered the neutron (alpha at paraffin wax)
When alpha particles strike beryllium, an invisible, neutral radiation is released.
Rutherford proposed this neutral radiation was a subatomic particle (neutron)
BUT being neutral, it was difficult to measure.
-> SO Chadwick fired them at a block of paraffin wax and protons were released.
He investigated the speed of the protons and used the laws of conservation of momentum and energy to determine the properties of the incident radiation.
He discovered the neutral radiation was made up of particles with an almost identical mass to the proton but with no charge... NEUTRON
30
Rutherford VS Thomson models
similarities
- both included electrons and had some form of positive charge
differences
- thomson's model had positively charge distributed uniformly, whereas Rutherfords model had a small central positive nucleus with surrounding empty space
31
Why was it so hard to discover the neutron
- no charge, therefore they don't deflect in uniform magnetic fields.
- electrons and protons were observed via magnetic fields
32
Bohr's Postulates
1. Electrons orbit the nucleus in clearly defined energy states called orbitals or shells.
2. Any transition between energy states involves the absorption or emission or a discrete amount of energy (E=hf)
3. The angular momentum of electrons in each orbital is quantised
33
Who is Bohr
Bohr revised Rutherford's model by suggesting that electrons were confined into clearly defined orbits, and could jump between these, but could not freely spiral inward or outward in intermediate states.
- an electron must absorb or emit specific amounts of energy in order to transition between these fixed orbits
34
Emission of a photon
When an electron goes from an excited state to a ground state, a photon is emitted (up to bottom)
35
Electric Potential Energy
The electron when it is at the upper level (most excited state) has the most electric potential energy
When the electron is on the lower level, it has the least electric potential energy
This transition has to be done in one jump, cannot be done in stages.
36
What happens to this potential energy
It is given out as a photon
(energy jump corresponds to a specific frequency or wavelength)
THIS OUTLINES THE EVIDENCE FOR THE EXISTENCE OF ATOMIC ENERGY LEVELS
37
Why is the emitted energy a specific amount
The emitted light consists of photons of specific wavelengths (E=hf)
energy levels of the atom are discrete
38
Less Bohr Please
Lyman - 2 or higher down to 1 (UV)
Balmer - 3 or higher down to 2 (visible)
Paschen - 4 or higher down to 3 (IR)
39
What happens if an electron has more energy than required
Will be absorbed but will have KE and will move off faster
40
Limitations of Bohr
- only worked for single electron atoms (e.g., H, He)
- relative intensities couldn't be explained (lines in Balmer aren't equally as bright)
- zeeman effect couldn't be explained
- hyperfine splitting (lots of lines instead of one - Bohr could only predict main spectral lines)
- mixture of classical and quantum
- did not explain structure of nucleus
41
De Broglie
wave-particle duality
Suggested if light can behave as a particle then particles could have waves associated to them
De Broglie hypothesis
wavelength = h/p , wavelength = h/mv
all particles have a wavelength
42
Electron as a wave evidence
when electrons are made to pass through crystals, they do diffract, thus providing evidence for its wave nature
e.g., X Ray diffraction > electrons work in the same way
Experiment: Davisson and Germer
- scattering on the surface of nickel where a single crystal had been grown
- allowed for the determination of the wavelength (due to interference pattern on the screen from the path difference), which then agrees with De Broglie
43
What are electron waves
The electrons in atoms behave like standing waves, which wrap around the nucleus in an integral number of wavelengths.
(standing waves don't dissipate energy) therefore, electrons don't radiate energy
44
Schrodinger
wave function/probability wave
- proposed a much more complex equation for electron waves
- his wave function allowed an accurate determination of the atomic energy levels for electrons
probability clouds for electrons
- you don't know exactly where the electron is but the wave equation shows the most probable spot
the squared amp value finds the probability of finding the electron in that position
45
Heisenberg
uncertainty principle
- accurately describes the undefined, probabilistic nature of small particles/waves
46
Pauli
exclusion principle
- each electron is described by a combination of 4 quantum numbers
47
Why would an electron confined in a nucleus have very high energy
to contain an electron in the nucleus, it would need a wavelength comparable to the size of the nucleus (very small wavelength due to very small nucleus size)
As wavelength is inversely proportional to momentum, a decrease in wavelength would cause increased momentum, thus higher energy.
48
what is the concept of matter waves
All FUNDAMENTAL particles being associated with a wave
49
4 fundamental forces
gravity - acts between objects with mass (weakest)
weak force - governs particle decay (between quarks and leptons - via W and Z bosons) - quite weak, short range
electromagnetism - acts between electrically charged particles (via photon) infinite range
strong force - binds quarks together (quarks and gluons via gluons) short range, strongest --> need to be in correct range
50
forces within nucleus
electric repulsion of protons strains the nucleus (electrostatic force), but the strong nuclear force holds the nucleus together
51
why do protons push out
with very big nuclei, protons push outwards as the strong nuclear force is no longer in range for all the protons (only a small section), but the electrostatic force is.
Therefore, they get pushed out
-> a single proton doesnt leave, it takes others with it (alpha particle)
this is why smaller nuclei are more stable than larger nuclei
52
when do nuclei become unstable?
when they do not have the right balance between protons and neutrons (all elements above Z=82 are unstable and radioactive)
and you get more massive, you need more neutrons compared to protons
53
nuclear stability graph
middle diagonal (belt of stability)
if it is to the left (they have too many neutrons - want to drop neutrons and gain protons)
if it is to the right (they have too many protons - want to drop protons and gain neutrons)
these processes are done to get back to the belt of stability
54
what is an alpha particle
a helium nucleus, positive +2 charge
poor penetrating ability (can't go through paper)
good ionising ability (absorbed)
55
what is a beta particle
an electron, negative -1 charge
medium penetrating ability (can't go through aluminium)
medium ionising ability
56
what is a gamma ray
electromagnetic radiation, no charge
good penetrating ability (can go through lead)
poor ionising ability (is not absorbed)
57
alpha decay
big unstable > something more stable + alpha
the top number is subtracted by 4, bottom number is subtracted by 2,
as a helium nucleus is 4/2He
58
beta minus decay
emission of an electron from the nucleus (and anti-electron neutrino) - happens when SF is too big
(one of the neutrons change into a proton, and an electron is emitted)
this means proton number (bottom) increases by 1, while total number remains the same
59
beta plus decay
emission of a positron (and a neutrino)
(proton turns into a neutron) - when electrostatic is too big
60
gamma decay
gamma emission does not change the structure of the nucleus, but it does make the nucleus more stable because it reduces the energy of the nucleus
61
why do you need the anti-electron neutrino in beta minus decay
to conserve linear momentum
62
what do alpha particles have
discrete energies
63
discovery of the neutrino
there was 'missing energy', as the reactants were heavier than the products.
Pauli and Fermi hypothesised the existence of a third particle in the products of beta decay (the neutrino)
64
why are neutrino's so hard to detect
1. no charge (don't ionise)
2. no mass (don't collide)
3. invisible
65
What is half-life
the average time taken for half the nuclei present in any given sample to decay
66
the decay process is...
random (you can never predict which individual nuclei will decay next)
spontaneous (meaning we can't influence when it will decay)
67
the radioactive decay law
the decay law states that the number of nuclei that will decay per second is proportional to the number of atoms present that have not decayed yet
68
decay constant and half-life need to be in the same what?
UNITS e.g., minutes, seconds
69
what is 1 atomic mass unit
1/12 of the mass of a carbon-12 nucleus
931.5 MeVc-2
70
What is binding energy
the energy needed to separate an atom into its separate parts (have to do work to separate, which gives it more mass)
binding energy = mass defect x 1 mass unit
71
what is mass defect
the loss in mass when the mass of an atom as a whole is compared to the mass of its made up components individually
72
are particles heavier when they are together or apart
apart (as doing work to separate gives more mass)
coming together (decrease their mass to give off energy for fusion)
73
binding energy per nucleon graph
- nuclides in middle of graph have the highest binding energy per nucleon and thus are most stable
- going from bottom left up: aim is fusion (adding)
- going from right to left: aim is for fission (disintegration of heavier mass)
tells you how tightly bound the nucleons are for different isotopes
74
Fusion and Fission
Fusion - coming together
H + H > He
heavier > lighter
Fission - splitting
U > Ba + Kr + 3n
heavier > lighter
they become more stable due to a higher binding energy per nucleon
fusion gives out more energy per nucleon (as seen through steeper slope on graph)
75
why does a neutron knock the uranium out of shape
because if the nucleus deforms enough, the electrostatic repulsion between the protons in each half becomes greater than the strong force, and it then splits into two
76
Chain Reactions
if each ejected neutron causes another uranium nucleus to undergo fission, we get a chain reaction
- number of fissions increases rapidly and a huge amount of energy is released
there can be controlled fission reactions, control rods absorb some of the neutrons
77
fission reactors
fuel rods - uranium (energy is released when the parent nucleus of the fissionable material breaks down into two daughter nuclei and several neutrons
moderator - water/graphite (slows neutrons to increase chance of fission events)
control rods - cadmium (absorbs excess neutrons to control the rate of thermal energy production)
coolant - water (absorb thermal heat produced in fission reaction, prevent overheating)
radiation shield - thin lead/thick concrete (absorb potentially harmful radiation from escaping reactor core)
78
Artificial Transmutations
Transmutate: change chemical identity
Fermi was able to synthesis transuranic elements through neutron bombardment
- he induced and observed nuclear fission
79
standard model
split into:
FERMIONS take up space
- LEPTONS (electron, muon, tau, electron neutrino, muon neutrino, tau neutrino) FUNDAMENTAL, no SF
- QUARKS (up, down, charm, strange, top, bottom) FUNDAMENTAL, is SF
BOSONS dont take up space
- HIGGS BOSON non-force mediating
- GAUGE BOSON (strong nuclear - photon, electrostatic - photo, weak nuclear W/Z Boson, gravitational - graviton)
force mediating
80
2 quark and 3 quark combinations
2 quark - mesons (quark + antiquark), e.g., pion
3 quark - baryons e.g., proton (uud), neutron (dud)
called hadrons
81
the positron
1930 - predicted the existence of electrons with a positive charge
1932 - discovered a positive electron in an image from a cloud chamber, identified because its curvature in a magnetic field was in the opposite direction to an electron
SAME RADIUS, DIFFERENT DIRECTION
82
muon
found a new particle in the tracks left in a could chamber (1937)
now found it is a muon, belongs to the same family as electrons and neutrinos
83
the neutrino
Pauli first predicted neutrino in 1931 when studying beta decays
- 1956 first detected an antineutrino by studying beta decay (reacted with protons to produce neutrons and positrons)
used cloud chambers > what particles must have been present based on what we detect after annihilation
84
evidence protons and neutrons aren't fundamental
1964 postulated that all mesons and baryons consisted of quarks
realised there were a set of fundamental things which made up more particles
so many particles were being discovered, they realised not all of them could be fundamental
EVIDENCE
- similar to gold foil experiment
electrons were scattered when they collided with smaller dense objects inside the nucleon
(due to electrostatic reaction between quark and electron)
85
what is an accelerator
a machine used to accelerate particles to high speed thus high energy
basic elements:
- a beam in an evacuated tube, a target, and a detector
86
how are particles detected in accelerators
observing an ionising path of a charged particle
(arcs of the paths) and combining with conservation laws
87
how accelerators test theories
"work backwards"
the conservation of momentum and energy is used to work backwards to deduce the particle originally created in the collision
88
3 types of accelerators (1)
1. linear
series of cylinders separated by a disk with a hole to allow a beam to pass through.
- an electric field is applied across the gap, linked to AC.
- particles bunch up into packets (if they come out at the wrong time, they aren't passed on)
- the cylinders get longer as the particle is getting faster
- the polarity switches as the particles go through so it is repelled from the one it is in and attracted to the next one
89
3 types of accelerators (2)
2. cyclotrons (circle that gets wider each time)
charged particles in the centre of the machine are accelerated across an electric field which is produced between two hollow metal plates (DEES)
- particles move in circular motion
- the time spent in the dee is the same each time (even tho curve is getting bigger, they are getting faster)
- eventually it will escape the loop
90
3 types of accelerators (3)
3. synchrotrons (circle stays same size)
circular accelerators, travels in an evacuated pipe (stays focused by the magnetic field)
- work done by the electric field to speed them up
- after a while, relativistic effects kick on (moment dilates)
- the particles pass through the rings millions of times
91
speed limiting factors for accelerators
- accelerating charged particles emit radiation and lose energy in the form of photons
- superconductors used to create strong magnetic fields have upper limits (heating effects)
