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OCR Physics A2 > Nuclear and Particle Physics (DONE) > Flashcards

Nuclear and Particle Physics (DONE) Flashcards

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1
Q

How was the concept of the atom first discovered?

A
  • various greek philosophers reasoned if you continuously cut a material in half, there will be a point where it becomes indivisible.
  • this is therefore the fundamental building block called an atom.
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2
Q

What did British scientists discover about the structure of the atom?

A
  • they discovered that we have small particles of negative charge called electrons.
  • they also realised that if there are negative charges in the atom, there must also be positive charges to balance them out.
  • This was called the plum pudding model
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3
Q

Explain the features of the plum pudding model.

A
  • British scientists assumed the atom consisted of negatively charged electrons and positively charged protons.
  • They thought that electrons and protons were distributed evenly throughout the atom.
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4
Q

Explain the Rutherford scattering experiment.

A
  • Rutherford placed an alpha source within a box with a hole through one side, creating a stream of alpha particles through this hole.
  • The alpha particles were fired at a gold leaf only a few hundred atoms thick.
  • The experiment was carried out in a vacuum
  • a scintillator made from zinc sulphate was placed behind the gold leaf, it illuminates each time a particle hits it.
  • The frequency of which the alpha radiation hits the scintillator was observed at different angles of theta.
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5
Q

What does the Rutherford scattering experiment prove about the structure of the atom?

A
  • showed that most of the atom consists of empty space as most alpha radiation was able to pass through the gold leaf.
  • some alpha particles were deflected slightly and some rebounded at large angles of theta, this means that there must be a dense, positively charged nucleus in the centre of the atom where electrostatic repulsive forces cause the particles to change direction.
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6
Q

Why is the plum pudding model untrue?

A
  • the protons are not evenly distributed across the atom as shown in the plum pudding model which would allow all alpha particles to pass through.
  • The protons are held in a dense, positively charged nucleus surrounded by empty space and electrons in electron shells.
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7
Q

How were neutrons within the atom discovered?

A
  • it wasn’t until 1930 that neutrons are found to be in the nucleus.
  • it was discovered as the mass of the protons within the nucleus couldn’t account for the mass of the element.
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8
Q

What determines the element an atom represents?

A
  • the number of protons determines the element.
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9
Q

What do the numbers next to each elements symbol show about the atom?

A
  • the top number is the ‘A’ Nucleon number which shows the number of nucleons within the atom.
  • the bottom number is the ‘Z’ Proton/atomic number which shows the number of protons within the atom.
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10
Q

What is a nucleon?

A
  • a particle which exists inside the nucleus of an atom.
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11
Q

What isotopes of hydrogen can we have?

A
  • the standard hydrogen atom contains 1 proton and 0 neutrons.
  • we can also have a deuterium hydrogen atom with 1 proton and 1 neutron
  • and finally a tritium atom with 1 proton and 2 neutrons
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12
Q

What is an isotope?

A
  • two or more forms of the same element with the same number of protons and different numbers of neutrons.
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13
Q

What are the isotopes of Helium?

A
  • Helium atoms contain 2 protons:
    Helium-3 contains 1 neutron, it is very rare and has potential to be used in the future for nuclear fusion.
    Helium-4 contains 2 neutrons and is most common, used in balloons.
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14
Q

What is a Helium-4 nucleus also known as?

A
  • alpha radiation
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15
Q

What are the masses of each particle in the atom?

A
  • The mass of protons and neutrons are slightly different with neutrons being slightly heavier than protons, (formula booklet).
  • The mass of an electron is about 2000 times less than the proton and neutron, (formula booklet).
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16
Q

What can we use to represent the rest masses of particles within the atom?

A
  • Atomic Mass Unit, u.

- The value of 1u is 1.661x10^-27 and is found in the formula booklet.

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17
Q

What are the masses of the particles in the atom using the Atomic mass unit?

A
  • protons and neutrons are roughly equal to 1u.

- While an electron is roughly equal to 1/2000u.

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18
Q

Other than the mass what else can we find out about the size of the nucleus?

A
  • we can consider the size of the nucleus for example the diameter.
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19
Q

What did Rutherford discover about the size of the nucleus and atom?

A
  • he found that the diameter of a nucleus is around 10^-15 metres.
  • he also found the diameter of the atom is 10^-10
  • meaning that the diameter of the nucleus is 10^5 times smaller than the atom.
  • this explains why during the alpha scattering experiment most particles passed through the gold leaf.
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20
Q

What is the radius of the nucleus dependent on and therefore what is the equation used?

A
  • dependent on the number of nucleons in the nucleus.
    Equation:
    R = r0 x A^(1/3)

where R = radius
r0 = constant (1.2 fm)
A = nucleon/atomic mass number

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21
Q

What is the fentometre prefix?

A

10^-15

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22
Q

What can the radius of the nucleus be used to find out?

A
  • the radius can be used to find the volume of the nucleus.
  • by also using the mass of the nucleons, the density of the nucleus can be find using:
    density = volume/mass
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23
Q

What should the density of the nucleus be roughly equal to and what does it show about the nucleus of an atom?

A
  • The density should be equal to around 10^17

- it shows that if you had 1 cubic metre volume of nucleus it would be very heavy.

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24
Q

What are the 2 assumed problems with the model of an atom such as Helium with 2 protons, 2 neutrons and 2 electrons?

A
  • if you have a negative electron and positive proton then why don’t they attract.
  • if you have 2 positive protons in the nucleus why don’t they repel.
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25
Q

Explain why electrons in an atom are not attracted to the protons in the nucleus.

A
  • because the electrons have a horizontal velocity.
  • the force of attraction causes the electron to change direction and orbit the nucleus.
  • the force of attraction acts as a centripetal force, similar to how a satellite orbits the earth.
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26
Q

Explain all of the forces acting between 2 protons.

A
  • both protons will experience equal and opposite electrostatic forces causing them to repel.
  • as both protons have a mass they will experience a negligible attractive gravitational force which will increase as the protons get closer (its not enough to balance the electrostatic force as the mass is so small).
  • The strong nuclear force acts over a very short distance and allows 2 protons to exist next to each other.
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27
Q

How does the strong force reach the protons?

A
  • a gluon, which has a very short half life transmits the strong force to the protons .
  • they can only transmit the force over very short distances due the short half life.
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28
Q

Explain how the strong force varies as the distance between 2 protons r, decreases.

A
  • When the distance between 2 protons is large the strong force is negligible.
  • As the distance r, decreases the attractive force becomes stronger as gluons transmit the strong force.
  • When the protons reach a perfect distance apart the maximum attractive force is reached which holds the protons together.
  • At this point the strong force is equal to the electrostatic force.
  • When the distance r, gets too small and the protons start to merge into a singularity we get a strong repulsive force separating the protons.
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29
Q

What is an important thing to remember when looking at the graph of the strong force against the distance r?

A
  • the attractive force is always a negative symbol.
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30
Q

Why do a lot of heavier elements tend to have more neutrons than protons?

A
  • when we look at heavier elements with a lot of protons they need more neutrons to dilute the electrostatic repulsion.
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31
Q

Why do a lot of heavier elements tend to be unstable and radioactive?

A
  • this is because the gluons which transmit the strong force have a very short half life.
  • this means that with heavier elements with lots of protons the gluons are unable to transmit the strong force across all protons without decaying.
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32
Q

In the early part of the 20th century, what particles were known and what couldn’t be explained about the structure of an atom?

A
  • They could explain that all elements are made out of protons, neutrons and electrons.
  • They found that there were particles like electrons but with a positive charge.
  • They also couldn’t explain beta radiation looking at electrons emitted from the nucleus.
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33
Q

What did scientists find out about protons and neutrons in the early part of the 20th century?

A
  • They found new particles and realised that protons and neutrons are not fundamental particles.
  • They found protons and neutrons are made out of 3 quarks and we have up and down quarks.
  • They found up quarks, down quarks and electrons were fundamentals.
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34
Q

What quarks are protons and neutrons made from?

A
  • Protons uud charge = 1e

- Neutrons ddu charge = 0e

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35
Q

What new leptons did scientists discover?

A
  • They found particles similar to electrons but positively charged and heavier called Muons.
  • They also found particles similar to electrons but with no charge called Nutrino’s.
  • They found electron neutrino’s and therefore found Muon Nutrino’s.
  • They then discovered further Hadrons called Charm and Strange Quarks.
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36
Q

What are leptons?

A
  • elementary particles that interact through the weak force.

- electrons, muons, neutrino’s.

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37
Q

What are Hadrons?

A
  • Particles made from a combination of quarks.

- Have 2 families Baryons and Mesons

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38
Q

What are Baryons?

A
  • particles made from a combination of 3 quarks.
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39
Q

What are Mesons?

A
  • Particles made from 1 quark and 1 antiquark.
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40
Q

What particle carries the electromagnetic force?

A
  • Photons
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41
Q

How was the weak force discovered?

A
  • Inside a proton or neutron there are quark inside which have a given charge.
  • This means in order to keep 2 quarks of the same charge close together there must be a force, called the weak force.
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42
Q

What is the weak force carried by?

A
  • W and Z Bosons.
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43
Q

What are the charges of each of the quarks?

A
  • up quarks have a charge of +2/3e

- down quarks have a charge of -1/3e

44
Q

What are the properties of anti-quarks?

A
  • have the same mass as regular quarks.
  • the charges of the quarks are reversed
  • If we have an anti uud quark combination then we get an anti-proton with a charge of -1e.
  • we show anti quarks by putting a line above the quark symbol.
45
Q

What is the Baryon number of a Baryon and a quark?

A
  • Baryons have a baryon number of 1.

- Quarks have a baryon number of +1/3

46
Q

Why do nuclei decay?

A
  • Nuclei decay when they are unstable.

- They decay in order to become more stable.

47
Q

What are the 2 particles released from nuclear decay?

A
  • Beta minus/electron

- Beta plus/positron

48
Q

What is a Beta plus/positron particle?

A
  • The antimatter equivalent of a Beta minus particle/electron.
49
Q

What happens when Beta minus decay occurs?

A

neutron –> proton + Beta minus + anti neutrino

50
Q

What quantities are conserved in beta decay?

A
  • Charge
  • Baryon number
  • Lepton number
  • nucleon number
51
Q

How were neutrino’s discovered using the Beta decay equation?

A
  • Using the Beta minus decay equation they found during the 1930’s that the lepton number was not conserved.
  • Therefore there had to be a particle that isn’t a baryon and has no charge.
  • Using an anti- neutrino in the Beta minus decay equation they could conserve the lepton number.
52
Q

What are the characteristics of a neutrino?

A
  • virtually no mass
  • move at very high speeds
  • do not interact with matter
  • have a lepton number of 1/-1
53
Q

What happens when Beta plus decay occurs?

A

proton –> Neutron + Beta plus + neutrino

54
Q

What are the lepton numbers of neutrino’s/anti-neutrino’s?

A
  • neutrino = 1

- anti neutrino = -1

55
Q

How can you break down the Beta minus decay equation into quarks?

A

udd –> uud + Beta minus + anti neutrino.

  • Which simplifies into:

d –> u + Beta minus + anti neutrino

  • Charge, Baryon number and Lepton number is also conserved.
56
Q

Why do lighter atoms such as helium have the same number of protons as neutrons in the nucleus?

A
  • In helium there are 2 protons and 2 neutrons.
  • The protons experience electrostatic repulsion however the strong force is able to hold the nucleons together in the nucleus.
57
Q

What happens to the number of neutrons compared to protons in large atoms such as iron?

A
  • Iron is a very heavy atom with large amounts of protons.
  • This means more neutrons than protons are needed to help to dilute the electrostatic repulsive force between protons.
  • Allows the atom to become stable.
58
Q

What is the relationship between the numbers of neutrons and protons?

A
  • for light atoms we see a linear relationship.
  • however as atoms get heavier we start to see more neutrons than protons.
  • the curve shown on graphs of this relationship shows all stable elements.
  • isotopes of the same element will have different numbers of neutrons.
  • isotopes below and above the stable line will aim to emit particles in order to achieve stability.
  • The particles emitted include alpha particles (heavier atoms), Beta minus and Beta plus (lighter atoms).
  • The atoms that do emit radiation to become stable are called radioisotopes.
  • This process is unpredictable as it occurs in the nucleus.
59
Q

What are the 4 types of radiation?

A
  • alpha
  • beta minus
  • beta plus
  • gamma
60
Q

Where does ionising background radiation come from?

A
  • all around us e.g. rocks, air, space.
61
Q

What is ionising radiation?

A
  • radiation that is emitted from the nucleus of an atom that carries enough energy to remove electrons from the outer shells of atoms, turning them into ions.
62
Q

properties of alpha radiation

A
  • helium nuclei (2 protons 2 neutrons)
  • range in air = few cm due to the large nucleus which interacts with air.
  • stopped by 0.2mm paper
63
Q

properties of beta radiation.

A
  • fast moving electron
  • range in air = several m
  • stopped by 1cm aluminium
64
Q

properties of gamma radiation.

A
  • em wave
  • range in air = several km
  • stopped by 10cm of lead
65
Q

Why does beta plus radiation have a short range?

A
  • when a beta plus particle is emitted it will soon find another electron.
  • when a pair of matter and the antimatter equivalent come into contact with each other they annihilate and release energy
  • this is the reason why PET scanners work as they have a short range.
66
Q

How does the strength of gamma rays change as the thickness of lead blocking it increases?

A
  • lead is a very dense material used to stop gamma rays.
  • however thin lead does not completely block all gamma rays only a proportion are blocked.
  • this creates an exponential graph until it reaches 10cm where gamma is completely absorbed.
67
Q

What piece of equipment is used to measure radiation safely?

A
  • geiger muller tube
68
Q

Explain how a Geiger muller tube works?

A
  • has a thin mica window at the which allows ionising particles to move into the tube.
  • the gas within the tube is ionised and this produces a current which is detected by another piece of equipment called a digital counter.
  • the digital counter detects the current and using this signal calculates the amount of radiation detected.
  • it displays a continuous radiation count over time.
69
Q

How do you make sure that the radiation count for a source of radiation is accurate when using a geiger muller tube?

A
  • measure the background radiation over a certain time period e.g. 1 minute.
  • then subtract this value from the radiation count for radioactive source over the same time period.
  • this excludes background radiation from results.
70
Q

What safety precautions can be used when measuring radiation using a geiger muller tube?

A
  • use tongs as long as possible to handle radioactive source.
  • do not point source directly at face.
  • use a source holder in order to prevent dropping the source with the tongs.
71
Q

Explain the process of alpha particle decay.

A
  • in order for an element to become more stable it may emit an alpha particle meaning 2 protons and 2 neutrons are emitted from the nucleus.
  • this means element X gives out an alpha particle and creates a new element Y.
  • energy is also given out in the form of a gamma photon or kinetic energy of the alpha particle.
  • mass and proton numbers must be conserved in the equation.
72
Q

Describe beta minus decay.

A
  • in order for a particle to become more stable it could turn a neutron into a proton, releasing a beta minus particle.
  • therefore element X will give out a beta minus particle and form a new element Y which will have a nucleon bigger than X by 1 due to the neutron turning into a proton.
  • energy is also given out in the form of kinetic energy of the beta particle.
  • an anti neutrino is also given out, balancing the lepton number.
73
Q

Describe beta plus decay.

A
  • in order for a particle to become more stable it could change a proton into a neutron, releasing a beta plus particle.
  • this means element X will give out a beta plus particle and create a new element Y with one less proton causing the nucleon number to be less than X by 1.
  • energy in the form of kinetic energy of the beta plus particle is also given out.
  • a neutrino is also given out.
74
Q

Can you get gamma decay and how does this work?

A
  • an unstable nucleus can give out a gamma ray photon in order to become more stable.
  • however this does not change the protons or neutrons in the nucleus meaning it is the same element it has just given out energy to become more stable.
75
Q

Describe an experiment which can be used to demonstrate exponential decay.

A
  • through using skittles and counting the number of skittles faced down after randomly throwing them and recording the number of skittles left facing upwards we can obtain data which shows a similar pattern to how the number of un-decayed nuclei will change over time.
  • we can plot the data on a graph with N (the number of skittles left) on the y-axis and T (number of throws) on the x-axis.
  • Initially you should start with a fairly large number at 0 throws, then after each throw the number should decrease by roughly 50%.
  • The graph will be a curve showing exponential decay, in the form of y = e^-x
76
Q

What is exponential decay?

A
  • where for the same time interval, the proportion change in undecayed nuclei will also be the same.
77
Q

What form does the graph of exponential decay have and what does this show?

A
  • The graph of exponential decay has the form y = e^-x

- This means that over the same time interval it will decay by the same proportion.

78
Q

How can we use the exponential decay graph to create an equation for the number of un-decayed atoms left, N. ?

A
  • it allows us to understand that the number of un-decayed nuclei left is dependant on the original number of un-decayed nuclei.
  • therefore using the form of the graph y = e^-x we can create an equation as time goes on the number of un-decayed atoms will be equal to N = N0e^-t
  • However different nuclei will decay faster than others giving different curves, this means we need to insert a decay constant (lambda) into the equation.
  • The final equation is N=N0e^lambdat
79
Q

Explain why a decay constant is needed to create an equation for un-decayed nuclei.

A
  • Some nuclei will however decay faster or slower than others giving a different curve, this means we need to insert a decay constant (lambda) into the equation.
  • The final equation is N=N0e^lambdat
  • The quicker something decays, the larger the value of the decay constant.
80
Q

How can we use the equation for un-decayed nuclei to find an equation for the activity of a radioactive substance?

A
  • The activity of a substance is proportional to the number of un-decayed nuclei N as A = lambda*N
  • We can therefore also have a similar graph for the activity as time increases.
  • This graph will also have an exponential curve and the equation for activity will be A= A0e^-lambdat
81
Q

What happens when a radioisotope emits a radioactive particle and what 2 rules must the nature of the decay abide by?

A
  • In order to become more stable a radioactive isotope will emit a radioactive particle, it doesn’t matter if it is a particle of alpha radiation or a beta minus/plus particle.
  • The important thing is that the nature of the radiation that is given off follows 2 rules,
    1. Completely random (you cannot tell when they atom is going to emit a particle)
    2. Spontaneous (the fact it happens without an external stimulus and we cannot influence it).
82
Q

What is the nuclear activity of an isotope and what is it proportional to?

A
  • the number of particles emitted from the nucleus per second.
  • Activity has the symbol A, if you have a bigger sample with more radioisotopes there is going to be a higher activity.
  • We can therefore say that activity is proportional to the number of radioactive isotopes.
83
Q

How do we create our equation for the activity of a substance?

A
  • the activity A, is proportional to the number of radioactive isotopes N.
  • this means we can use a constant of proportionality to turn it into an equation.
  • as each isotope will have a different level of activity, we use the decay constant to represent how active the substance is.
  • something with a high decay constant will give off a large amount of radiation per second.
  • therefore the equation is
    A = lambda*N
  • N is a unit-less quantity and the decay constant has units of s^-1
  • this means activity must also have units of s^-1 but in order to distinguish this we use the unit Becquerel, Bq.
84
Q

What is a Becquerel?

A
  • the unit for activity of a radioactive isotope.

- 1Bq is the total number of things that decay in one second.

85
Q

What is the half life?

A
  • the half-life of a radioactive isotope is the time it takes for half of the original number of nuclei to decay.
86
Q

How would the half life be represented on an exponential decay graph?

A
  • The symbol for half-life as used on an exponential graph on the x-axis would be t1/2.
  • The value on the y-axis would therefore be Noriginal/2.
87
Q

How can we derive an equation for the half life of a radioisotope using the exponential decay equation?

A
  • the half life on an exponential graph would be t1/2 on the x - axis which is the point where the number of un-decayed nuclei = Noriginal/2
  • By substituting this into the equation
    N = Noriginale^-lambdat
    we get
    Noriginal/2 = Noriginale^-lambdat1/2
  • Through cancelling we then get
    ½ = e^-lambda*t1/2
  • then take the reciprocal of both sides
    2 = e^ lambda*t1/2
  • then take the natural log ln of both sides ln(2) = ln(e^ lambda*t1/2)
  • then simplify to get
    ln(2) = lambda*t1/2
  • we can then rearrange this equation to find the half life t1/2 = (ln2)/lambda
  • the half life is measured in s^-1
88
Q

What is one use of radioactivity?

A
  • carbon dating
  • This relies on the fact there are different isotopes of carbon.
  • Carbon always has 6 protons in it, but sometimes it has either 6 or 8 neutrons and these are the 2 main types of carbon we have.
  • the carbon with 6 neutrons is the stable form which will exist for a long time, but the carbon with 8 neutrons which we often call carbon 14 is an unstable radioisotope and as this decays we can use it to work out the age of archaeological specimens.
89
Q

How is carbon - 14 formed?

A
  • Carbon 14 is formed in the upper atmosphere, we start with nitrogen that has 7 protons and 7 neutrons and it is bombarded by a neutron, as the nucleus is bombarded by the neutron it makes carbon 14 + a proton.
  • The neutron which bombards the nitrogen nucleus is formed through interactions in the upper atmosphere with cosmic radiation.
  • Carbon 14 is not a stable isotope and has a half life of 5730 years.
90
Q

How can carbon dating show the age of archaeological specimens?

A
  • carbon 14 in the atmosphere is used by plants for photosynthesis .
  • this is where there is Co2 added to water, and inside the plant sugars C6 H12 O6 are made and then it gives out oxygen which is where we get the oxygen that we breath.
  • the important thing is, inside the carbon there is going to be a certain proportion of carbon 14 and if we know the fixed ratio of carbon 12 : carbon 14, we can work out how long ago something was living by the change in ratio.
  • when a specimen is alive, this ratio is a fixed ratio of around 10^12 : 1
  • however when a specimen dies and stops respiring the carbon 14 within the body will decay as it is unstable, this will cause the amount of carbon 14 to decrease over time, we can use the half life of carbon to estimate how long ago the specimen lived.
  • the equipment used to analyse this is a mass spectrometer.
91
Q

What are the advantages and disadvantages of carbon dating?

A
  • For samples that are around 15000 years old this is very useful method as you can see a significant change in the ratio of carbon 12 to carbon 14.
  • However for anything that has died recently carbon dating is not ideal, this is because the half life of carbon 14 is so long that there will be no difference in carbon 12 : 14 ratio.
  • Furthermore for things that are really old there will be so little carbon 14 left that it will be too difficult to get a reading and so is therefore not useful.
92
Q

What is Einsteins famous equation and what does it show?

A
  • About 90 years ago Einstein came up with the equation E = mc^2
  • Effectively it is the equivalence of mass and energy.
  • This means that if you have some energy, it can turn into mass or if you have a mass, it can turn into energy.
  • Mass is measured in kg
  • C stands for the speed of light = 3 x 10^8 ms-1
  • Energy is measured in joules.
93
Q

What is mass defect and how does it occur?

A
  • the mass defect is the difference between the mass of an isotope and its mass number.
  • mass defect occurs as in order to break apart an atom, binding energy must be supplied firstly to separate the electrons from the nucleus and then to separate neutrons and protons in the nucleus.
  • once the nucleons are separated they will have more energy than if they were still held in the nucleus and because they have more energy they also have more mass.
  • this principle shows a defect in mass when the particles are held in the atom as the energy of the system is less than it is separately and the particles therefore lose their energy through mass.
94
Q

How would you calculate the mass defect of an atom?

A
  • calculate the sum of the mass of each individual particle using the formula booklet and the relative atomic mass constant, u.
  • subtract the mass number of the element from the sum of the individual particles.
  • convert the figure to kg by dividing by u.
95
Q

What is the binding energy?

A
  • the energy that holds a nucleus together or the energy needed to separate the parts of the atom.
  • equal to the mass defect.
  • if you have an atom which is heavier the total binding energy will be greater.
96
Q

What does the binding energy graph show and what are the key points on the graph?

A
  • shows the binding energy per nucleon against the mass number A.
  • the points will only begin from mass number 2, this is because if you only have 1 nucleon no binding energy is needed.
  • the heaviest atom will be uranium and the lightest a deuterium hydrogen atom.
  • iron has the highest binding energy per nucleon meaning it is the most stable element.
  • Most elements follow the curve however helium, carbon and oxygen do not, they sit higher above the line.
  • these elements are particularly stable and this is because they are made out of alpha particles which are the most stable nuclei.
  • the reason you do not get an element with 4 neutrons and 4 protons is because it is more stable in its separate form and would decay into 2 alpha particles.
97
Q

Describe why fusion and fission occurs using the binding energy per nucleon graph.

A
  • Lighter elements can become more stable and give out more energy by joining together, so up until the point of iron, atoms want to join together and it can do that through fusion.
  • Fusion doesn’t occur past iron as it is the most stable state.
  • However heavier elements past iron such as uranium can become more stable by splitting apart through fission.
  • Fusion and Fission occurs as every element wants to be as stable as possible and iron is the most stable element.
98
Q

What is induced nuclear fission?

A
  • Induced nuclear fission is where we cause a massive nucleus to split apart and in doing so it gives out energy.
99
Q

How does the process of nuclear fission work?

A
  • a slow moving neutron is fired at a uranium-235 atom causing it to fission.
  • When the slow moving neutron is fired at the uranium-235 the neutron is absorbed, this then makes uranium-236 which is not a stable atom.
  • therefore the atom decays into 2 daughter nuclei of different sizes, usually a form of Barium and kryptonite.
  • The mass number of the products of this reaction has to add to 236 therefore neutrons are given off to reach the mass number.
  • This reaction doesn’t just give off the particles shown in the equation, it also gives off energy in the form of kinetic energy from the particles and gamma radiation. It is this energy which is used in nuclear power stations.
100
Q

Explain the chain reactions that occur during nuclear fission.

A
  • if there are not enough neutrons to carry on the reaction, the reaction will stop meaning it is a subcritical reaction.
  • however what you tend to have is one reaction causing another as during a fission event a neutron is released and eventually absorbed by another uranium atom, this is a critical reaction.
  • you can also have a super critical event where 1 reaction cause 2 or 3 other reactions, similar to what you would have in an atomic bomb.
  • the mass defect of the atoms releases energy which in turn generates electricity.
101
Q

What are the 4 main parts of the fission reactor?

A
  • the moderator which is usually made of graphite or heavy water slows down the fast neutrons so they can be absorbed.
  • Fuel rods in an aluminium casing often go into the reactor vertically and they contain enriched uranium, the fuel we actually need in the fuel rod is uranium 235 however there is not a huge amount of it naturally occurring in the uranium that we mine underground, this means we have to enrich it. over time the fuel within the fuel rods will decrease.
  • Control rods sit vertically meaning that if there is an emergency they will fall under gravity into the reactor meaning they will get in between the fuel rods allowing the control rods to control the rate of reaction. They are often made from Boron coated steel, and absorb any excess neutrons.
  • The coolant which is usually water stops the reactor from overheating but it also allows the reactor to create hot water. Co2 or liquid sodium can also be used. The mass defect of the atoms in the reactor cause energy to be released which can heat the coolant providing hot water and steam which turns a turbine generating electricity.
102
Q

How can we work out how much energy is released during fission?

A
  • we can use Einsteins equation E =mc^2

- we know the mass of the fuel and the speed of light therefore we can calculate the energy released.

103
Q

What are the advantages and disadvantages of nuclear fission?

A
  • An advantage of this is no Co2 as there is no combustion.
  • Little fuel needed to produce lots of energy so not transporting tonnes of coal every week.
  • Disadvantages include that it is expensive
  • Waste products have a very long half life t1/2
  • Disasters can have massive impacts
104
Q

What is nuclear fusion?

A
  • Fusion is the process of joining light nuclei together to make heavier nuclei, whilst releasing energy in the process.
  • It’s not just limited to hydrogen making helium but it happens with many of these lighter elements inside stars
105
Q

What 2 things are needed for fusion to occur?

A
  • There needs to be high pressure and high temperature.
  • The higher pressure you have the more chance of collisions and the hotter it is the more chance each Collison has enough energy for the atoms to bond together.
106
Q

How does the process of nuclear fusion occur?

A
  • Step one is for 2 hydrogen nuclei to collide.
  • When the two hydrogen nuclei (1 proton each) join together they make a deuterium hydrogen atom which needs to have 1 proton and 1 neutron meaning a proton has to turn into a neutron and in doing this it releases a beta minus particle.
  • A neutrino is also emitted to conserve lepton number.
  • Stage 2 is when a hydrogen nuclei collides with the deuterium hydrogen nuclei, forming an isotope of helium with mass number 3 and proton number 2.
  • As well as this it gives out energy in the form of gamma photons due to mass changes.
  • The final stage is where you have 2 of the helium – 3 nuclei which collide, however 2 of the hydrogen nuclei are given off, this means we are left with Helium – 4 with 2 neutrons and 2 protons.
  • So at the start we begin with 6 hydrogen nuclei and by the end we have a helium - 4 nucleus and 2 hydrogen nuclei.
107
Q

How can nuclear fission provide energy?

A
  • in a fission reactor there is a decrease in the mass.
  • according to e = mc^2 the mass is converted to energy.
  • binding energy of products is greater than binding energy of original nucleus
  • the difference in the binding energies is released as energy.

OCR Physics A2 (15 decks)

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