M8

Question 1

What were the two competing theories for the origins of the universe?

Answer 1

Big Bang and Steady state

Question 2

What was the Big Bang theory?

Answer 2

George Gamow suggested that if the universe was expanding then at some point, it must have occupied a very small place from which the universe expanded and cooled.
i.e. the universe originated from a single point.

Question 3

What was the steady state theory?

Answer 3

Fred Hoyle suggested that the universe is:
Infinite — the ‘outer’ stars would never reach infinity and so could go on moving away from us forever.
Expanding — Matter is being created all the time at just the right rate to keep the density of the universe constant.

Question 4

What was the key piece of evidence for the Big Bang theory?

Answer 4

Physicists from Princeton in the 1960s theorised that if the big bang theory were true, then that small condensed spot from which everything expanded, must have been very hot. Any hot object would have created large amounts of electromagnetic radiation. As the universe expanded, the wavelength of this radiation must have also. Over time, it should have also then cooled. Calculations revealed that the wavelength would have stretched to around a millimetre and cooled to a few degrees warmer than absolute zero. Two other scientists discovered that the radiation was close to the expected wavelength and corresponded to heat radiation at 2.7K. The radiation was called “cosmic microwave background”. Whilst the CMB radiation was quite uniform from all directions, it wasn’t perfectly uniform and this is evident in small variations in temperature created by this CMB. This is key as it meant that the early universe was not fully uniform, The small variations meant that matter was slightly clumped together in some places and so its unbalanced gravity could gradually pull it together to form the stars and, on a larger scale, galaxies.

Question 5

Describe the lifecycle of a star of mass < 8 solar masses

Answer 5

1. Protostar (gravity -> accretion of matter until thermal equilibrium)
2. Main sequence (fuses hydrogen in core to form helium nuclei)
3. Red Giant (fuses helium to carbon in SHELL when h fuel depletes)
4. Death - Planetary nebula (clouds of gas that are blown away outer layers of a star)
5. White dwarf left behind after nebula (dense stars do not fuse)

Question 6

Describe the lifecycle of a star of mass 8-20 solar masses

Answer 6

1. Protostar (gravity -> accretion of matter until thermal equilibrium)
2. Main sequence (fuses hydrogen in core to form helium nuclei)
3. Red Giant (fuses helium to carbon in SHELL when h fuel depletes)
4. Death - Supernova (violent explosions of uncontrolled nuclear reactions)
5. Neutron star - remnants of core after the supernova, electrons and protons forced together to form a sea of neutrons, densest objects, possess intense B fields which emit an intense beam of radio waves.

Question 7

Describe the lifecycle of a star of mass > 20 solar masses

Answer 7

1. Protostar (gravity -> accretion of matter until thermal equilibrium)
2. Main sequence (fuses hydrogen in core to form helium nuclei)
3. Red Giant (fuses helium to carbon in SHELL when h fuel depletes)
4. Death - Supernova (violent explosions of uncontrolled nuclear reactions)
5. Black holes - crushed remnants of core after supernova

Question 8

What are the similarities between CNO and PP reactions in main sequence stars?

Answer 8

Both fuse hydrogen to helium and have the same overall net reaction
Both have a mass defect, indicating energy is released at the completion of the cycles.

Question 9

What are the differences between CNO and PP reactions in main sequence stars?

Answer 9

CNO is a catalytic cycle while PP is a linear process,
CNO requires the presence of carbon as a catalyst and high temperatures (dominates when T > 1.8 x 10^7 K) & PP dominates in smaller stars with temperature < 1.8 x 10^7 K

Question 10

Thomson’s charge to mass experiment

Answer 10

The setup consisted of a highly evacuated cathode ray tube, with a pair of charged parallel plates (e-field) and Helmholtz coils (B-field) outside the tube.
The electric and magnetic fields were orientated at right angles to each other such that they exerted opposing vertical forces on the beam of electrons
The experiment was then done in 2 stages:
Balancing E-Field and B-field:
Using the electric and magnetic fields simultaneously, he adjusted the strength of both until the electron beam passed through both fields undeflected. This showed that both fields cancelled each other out, i.e. v = E/B
B-Field only:
At this stage, the Efield was turned off and the beam of electrons was deflected under the influence of the magnetic field ONLY
Causing the beam to undergo circular motion, allowing for the radius of curvature to be measured
FB=FC
qvB=mv2/r
q/m=E/vB^2 (sub v = E/B)
As Thompson, knew the value for E, B and radius, he found the charge-to-mass ratio to be: q/m=1.761011C/kg

Question 11

Milikan’s oil drop experiment

Answer 11

Millikan devised the following experiment where a spray of oil drops was introduced from above, into an electric field created by two charged parallel plates.
At certain electric field strengths, the oil drop would become suspended between the charged parallel plates. This indicated that the oil drop must be charged (to experience a force in the E-field)
The force by E-Field must balance the weight force i.e. qE=mg. Some of these drops would be charged by either; friction with the atomiser gun or friction with the air. He varied the field and found ultimately, the more charged the oil drop was, the weaker the electric field needed to be, to balance the weight force F=Eq, so increased q meant decreased E-field. Milikan then introduced X-rays to the chamber to cause more ionisation of the oil drops, In altering the E-field, he could estimate what the charges of the various oil drops were at various degrees of ionisation. Regardless of the degree of ionisation, he found that the charge on a drop was always a multiple of: 1.6 x 10-19 C. NOTE: at the time, electrons and protons were considered fundamental particles. Being fundamental meant that these two particles carried the smallest possible quantity of charge (q). All charged bodies must therefore carry an integer multiple of q. Millikan used this concept in determining the charge of a single electron.

Question 12

Why was the Geiger Marsden experiment undertaken?

Answer 12

The experiment was designed by Rutherford’s associate; Hans Geiger and his student Ernest Marsden to test out the structure of the atom.
They wanted to test out the plum pudding model but later found that the results of the experiment could be explained by Rutherford’s later proposed model of the atom.

Question 13

Explain how it was conducted and the significance of the Geiger Marsden experiment.

Answer 13

The experiments were performed by firing alpha particles at a thin sheet of metal foil
The source of alpha particles was encased in a lead box with a small hole out of which a focused beam of alpha particles could emerge, Gold foil was used as it could be beaten into a very thin slice whilst still maintaining its shape and integrity. As per the plum pudding model, which depicts the atom as a ‘diffuse ball’ of positive charges with electrons interspersed in it, the expectation was that ALL alpha particles (which are as small as an atom) would not collide with anything and pass straight through, no deflections were expected to occur. However, results showed deflection at large angles in the centre of the ball. Showed that the plum model was incorrect. The results showed that alpha particles got through for the most part except in the central region of the atom. This could only make sense if the central region of the gold atom was small, dense and positively charged and would repel the alpha particles (also, small, dense and positively charged).This led Rutherford to propose an atomic model of a charged nucleus surrounded by electrons that orbited the nucleus

Question 14

How did Chadwick ultimately discover the neutron?

Answer 14

In 1931, two German scientists Walther Bothe and Herbert Becker bombarded the light elements lithium, beryllium, and boron with alpha particles. From these elements, a new type of radiation was thought to have emerged upon bombardment. This radiation was highly penetrating, hence they assumed it was gamma rays. However, this was proven to be wrong by Frederic Joiliot and Irene Curie, These two were interested in the radiation made in Germany, so they fired alpha particles at a block of paraffin wax, managing to knock some of the protons out of the wax. They were not only able to reproduce Bothe’s radiation but also found that this radiation caused high energy (5MeV) protons to be ejected from other substances. Later, James Chadwick applied the law of conservation of momentum to understand that protons were too heavy to be dislodged by mere gamma rays (have no mass but carry energy). He was able to show that the mass of Bothe’s radiation was very similar to the mass of a proton but just without the charge (as it was un-deflected by electric or magnetic fields). He named this new particle, the neutron.
With this, atomic models featuring a nucleus with both positive protons and neutrally charged neutrons emerged shortly after.

Question 15

What was Rutherford’s model?

Answer 15

Rutherford put forward the nuclear model of the atom in which a central positive nucleus was orbited by electrons in circular orbits

Question 16

What were the limitations of Rutherford’s model?

Answer 16

Accelerating charged particles emit EMR and according to the LOCOE electrons would emit energy due to their circular motion (an accelerating charge radiates EMR)
This means net energy would decrease as they would not be able to maintain orbit, the electron spirals until it hits the nucleus
As it does this, it was understood that the electron should be emitting energy in a continuous fashion, producing a continuous spectrum
Instead, the spectra produced by certain species showed discrete wavelengths of light
In direct contradiction with Rutherford’s model. Bohr also couldn’t explain why each element emitted a different set of wavelengths.

Question 17

What was the Bohr model of the atom?

Answer 17

Bohr introduced a model in which, electrons move in circular orbits around the nucleus of an atom, there are only several discrete orbits of different radii that can exist for each atom, these are numbered (n=1,2,3). An electron can ‘jump’ or transition to a higher energy orbit if it absorbs an amount of energy that is exactly equivalent to the difference between the electron’s initial and final energy levels. The quanta of energy, thus the frequency of light absorbed is unique for each element (unique nucleus). An electron once excited will always return to the ground state and emit electromagnetic radiation of a quantity equal to that which was absorbed initially.

Question 18

Limitations of Bohr model

Answer 18

Could only accurately explain the behaviour of atoms with one electron (e.g. hydrogen and ionised helium), It could not accurately predict the behaviour of higher energy orbits of multi-electron atoms
Could not explain the varying intensity and thickness of spectral lines in general
Could not explain the splitting of spectral lines due to the influence of magnetic field (Zeeman effect)
Could not explain why electrons can remain in their stable orbits without spiralling into the nucleus due to electrostatic attraction.
Couldn’t explain why the electrons did not give off EMR within their stable orbits.

Question 19

How was De Broglie able to justify Bohr’s model?

Answer 19

De Broglie introduced the wave nature of electrons, electrons have a ‘wavelength’ as do all matter.
He stated that matter waves must exist as standing waves if they are to exhibit any form of stability, if particles can be thought of as matter waves, then these matter waves must be able to maintain steady energy values to be considered stable. Broglie stated: The stable orbits of the hydrogen atom are those where the circumference is exactly equal to a whole number of wavelengths. Broglie was able to justify Bohr’s stable orbits as he:
Treats an electron within an atom as a wave that can interfere with itself
If the orbit has a circumference equal to an integer multiple of the De Broglie wavelength, constructive interference occurs and a standing wave is set up, resulting in a stable orbit.

Question 20

Outline Davisson and Gerner’s experiment

Answer 20

De Broglie’s theory was just a theory until American scientists Davisson and Germer were able to experimentally verify these findings in 1927. When they observed the diffraction patterns of electrons.
Set up:
The Electron gun- provided electrons released via thermionic emission and were accelerated towards the Nickel metal target and Electrons were scattered off the lattice of the Nickel target.
The movable detector was mobile and scientists found that as they moved their detector through the different scattering angles, they observed a series of max and min intensities (interference pattern). The results showed that the electron waves were being diffracted as they passed through the metal lattice and then subsequently interfered with each other.
Analysing the diffraction pattern, Davisson and Gerner calculated a value of 0.14nm for the wavelength of an electron which was consistent with Broglie’s hypothesis for the wavelength of a matter wave. Supporting De Broglie’s hypothesis of the wave nature of electrons.

Question 21

What was Heisenberg’s uncertainty principle?

Answer 21

Heisenberg’s uncertainty principle states that the position and velocity of an object cannot be accurately known at the same time.

Question 22

Describe Schrodinger’s model of the atom

Answer 22

Schrodinger built on the work of both Heisenberg and de Broglie
He suggested Heisenberg’s principle but rather in probabilistic clouds of space where electrons are likely to be found, these are now known as orbitals rather than orbits because they are actually complex 3D shapes. Because electrons are not concrete particles, they don’t follow particular paths in space and time at all. Rather, because of the wave nature of an electron, the paths can be thought of more as clouds, where the particular location of an electron at any point in time is based on probability since to measure a precise point, according to Heisenberg, only introduces uncertainty, preventing you from knowing where the electron would be at the next moment in time

Question 23

Beta minus decay occurs when

Answer 23

n:p, too many neutrons
A neutron turns into a proton and emits a beta minus particle and an antineutrino

Question 24

Beta plus decay occurs when

Answer 24

n:p, too many protons
A proton turns into a neutron and emits a positron (beta+) particle and a neutrino and energy

Question 25

What are the 4 fundamental forces?

Answer 25

1. Strong nuclear force
2. Electromagnetic
3. Weak nuclear force
4. Gravitational

Question 26

Fermions can be divided into

Answer 26

leptons and quarks

Question 27

What are particle accelerators?

Answer 27

Particle accelerators allow for high-energy collisions between particles like protons. Due to their like-charge protons required high energies for a collision to occur. These were provided by Particle accelerators, as they became key pieces of equipment in the discovery of particles.

Question 28

Describe the observations Hubble made and how he used these to confirm universe was expanding

Answer 28

Hubble showed that many nebulae thought to be in the Milky Way were actually separate galaxies outside the Milky way. He then measured the redshift of the light emitted from these galaxies, he found all galaxies exhibited a redshift and hence were moving away from Earth. Also found that the further away a galaxy was, the more its light was redshifted and the faster it was moving away from Earth. Therefore he concluded that because galaxies were moving away from one another, the universe must be expanding.

Question 29

All naturally occurring elements have been synthesised within stars, Explain how this occurs

Answer 29

When stars form within nebulae, they are initially composed of material of the nebula, 80% hydrogen and 20% helium, Once fusion reactions have begun in the cores of stars, hydrogen nuclei fuse to form helium and release large amounts of energy. When H fuel depletes, the star evolves, and many main sequence stars become red giants fusing helium to carbon in the core and hydrogen to helium in the surrounding shell. Further exothermic shell-burning reactions occur in successively deeper shells within the star converting, carbon to neon and magnesium, oxygen to silicon & sulphur, silicon & sulphur to iron. Iron is the last element to be fused, any further reactions have greater energy IN than energy OUT hence they are inefficient and do not take place in nature.

Question 30

How do supernovas allow for the formation of new elements?

Answer 30

When the nuclear fuel of stars with large masses is exhausted, outward radiation pressure cannot balance inward g pressure, so the star contracts and implodes. Atoms collapse into nuclei and the gigantic loss of GPE is converted into heat resulting in a supernova. This massive explosion synthesises all naturally occurring elements as well as a number of highly unstable isotopes, blasting them in all directions to become the metals scattered like fine dust in the nebula where new stars may form eventually. Supernova are thought to be the only way these elements can ever become available.

Question 31

Explain how carbon and other heavier elements are available on the Earth with reference to their stellar origins

Answer 31

Planets form in much the same way as stars (i.e. the accretion of matter in a gas cloud) If the gas cloud the planet accreted from were the remnants of another dying star, it would have elements fused in that star, within the crust of that planet. Carbon would have been possible from a dying red giant that used a triple alpha reaction (3He -> 12C ). Elements as heavy as Iron could have come from the supernova of a dying supergiant. Elements heavier than iron cannot be fused within a supergiant as it is energetically inefficient, instead these can only be fused in supernovas (star death). Hence the Earth must have been formed within the remnants of another dying star containing such elements.

Question 32

Describe the discovery of the electron by J.J Thompson

Answer 32

In 1897 J.J Thomson was studying electric discharges from cathode ray tubes. Thomson found that the tube glowed more when the gas was at a lower pressure. This showed that something was exciting the gas. When the tube was painted with phosphorescent paint, the paint opposite the negatively charged cathode sparked and glowed. Thomson placed two electrically charged plates above and below the cathode ray tube, and the cathode ray deflected towards the positively charged plate, indicating the beam was negatively charged. He also changed the material of the cathode, and the same beam emerged. Thomson concluded that the beam was composed of a stream of negatively charged particles. Thomson was later able to demonstrate, that cathode ray particles are fundamental constituents of every atom. To indicate their importance, cathode rays were renamed electrons.

Question 33

What is the binding energy? What does it indicate

Answer 33

the binding energy of the nucleus is the energy required to separate the nucleus into individual nucleons. It pertains to the stability of the nucleus, as a greater binding energy indicates a more stable nucleus which requires more energy to separate into individual nucleons.

Question 34

True or false: protons and neutrons are considered fundamental particles as per the standard model of matter

Answer 34

false, only the electron is considered fundamental as it is a lepton that cannot be broken up into smaller components, while proton and neutron can be broken into their quark compositions.

Question 35

What are the flaws of the standard model of matter?

Answer 35

One major problem of the Standard Model is that it does not include gravity, one of the four fundamental forces. The model also fails to explain why gravity is so much weaker than electromagnetic or nuclear forces.

Question 36

Compare the spectrums produced by discharge tubes and incandescent filaments

Answer 36

The discharge tube will produce a spectrum composed of lines at discrete wavelengths (Which will depend on the energy levels of the gas in the tube). An incandescent lamp produces a continuous spectrum, with the peak emission wavelength dependent on its temperature.

Question 37

What is the difference between the binding energy of reactants and the binding energy of products?

Answer 37

The binding energy of reactants is the amount of energy required (to be absorbed) to break apart the nuclides. The binding energy of products is the amount of energy released when the nuclides are formed.

Question 38

When total binding energy or products is greater than reactants, this results in an____

Answer 38

exothermic reaction (energy is released). This is because less binding energy is absorbed to break apart the reactants and more binding energy is released when products are formed.

Question 39

When total binding energy or products is less than reactants, this results in an____

Answer 39

endothermic reaction (energy is absorbed). This is because more binding energy is absorbed to break apart the reactants and less binding energy is released when products are formed.

Question 40

Describe inflation that took place at the beginning of the universe

Answer 40

A rapid period of inflation was necessary to create the universe we have today. This period is thought to have lasted about 10^-24 seconds and allowed the universe to expand to 10^50 times its initial size. Necessary to prevent the early universe from collapsing into a black hole. A black hole is a point of infinite density, very high mass and a very small volume. This period, allowed for the cooling and formation of matter.

Question 41

Explain the significance of pair production in the origins of the universe

Answer 41

To introduce the origins of matter, we must introduce Heisenberg’s uncertainty principle which states that:
When the velocity of a moving particle is known its position is uncertain and vice versa. In the context of the Big Bang, this can be reinterpreted as an uncertainty between mass and time such that: The shorter the time frame we are referring to the greater the uncertainty regarding the mass. The real meaning of this is that in a small period, mass can exist and cease to exist and vice versa. This can be understood by exploring matter and antimatter: For every particle of matter that is created, there is an antimatter particle that also exists. For example: Electrons and positrons. When matter and antimatter particles meet they annihilate each other and convert to energy (E=mc2). This explains how mass would exist and cease to exist in this short period of time. As two opposite particles are always produced in this creation, the process is called pair production. Due to annihilation, normally pair production doesn’t result in the creation of any lasting matter. However, during the period of inflation, because of the extremely rapid expansion of space, pairs of particles rapidly became separated and didn’t get a chance to annihilate. And so in that tiny fraction of a second of inflation, huge amounts of matter were created. The annihilation also produced an enormous increase in the amount of radiation. This radiation filled all of space and dominated the universe for the next few hundred thousand years. It was, of course, the origin of the CMB radiation seen today.

Question 42

Following pair production, how were elements such as helium formed

Answer 42

In the first few seconds of the universe, while the temperature remained over a billion degrees or so, elementary particles known as quarks combine to form protons and neutrons.
Protons and neutrons were forced close enough to fuse together, forming hydrogen, helium and lithium nuclei. After a few more minutes the temperature dropped below that needed for this fusion and no further nuclei were formed. Examining the spectra of some of the oldest stars and galaxies showed that they were approx. 25% helium and the rest hydrogen. The rest of the elements were formed within stars