26 - Nuclear Physics

Question 1

einsteins mass energy equation + interpretations

Answer 1

ΔE=Δmc^2

• mass is a form of energy (in annihilation, entire mass of positron and electron are converted into two gamma photons - energy)
• energy has mass (e.g. moving ball has greater mass than its rest mass, mug of tea losing heat is losing mass)

Question 2

pair production

Answer 2

replacement of a single photon with a particle and its corresponding antiparticle of same total energy

Question 3

energy is nuclear reactions

Answer 3

energy is RELEASED
total rest mass of particles after > before
increase of Δm x c^2 is equal to kinetic energy of colliding photons

Question 4

define binding energy

Answer 4

minimum energy required to completely separate a nucleus into its constituent protons and neutrons

Question 5

define mass defect

Answer 5

the difference between the mass of a nucleus and its completely separated protons and neutrons

Question 6

measure mass accurately using a..

Answer 6

mass spectrometer

Question 7

define induced fusion

Answer 7

thermal neutron splits nucleus into two smaller nuclei and fast neutrons

Question 8

what is a thermal neutron?

Answer 8

thermal/slow neutron
neutron is fission reactor with mean kinetic energy similar to thermal energy of particles in reactor core

Question 9

process of induced fission

Answer 9

uranium-235 nucleus captures thermal neutron and becomes unstable uranium-236 nucleus so rapidly splits to produce daughter nuclei (barium-141 and krypton-92) and three fast neutrons

Question 10

energy is nuclear fission

Answer 10

mass of particles after is less than mass of particles before. the change in mass corresponds to energy released as ΔE=Δmc^2

binding energy after is greater than binding energy before (daughter nuclei more stable) and the difference between these binding energies is equal to the energy released
(most of energy released is kinetic energy of daughter nuclei)

Question 11

slowing down of 3 fast neutrons

Answer 11

slowing down the 3 fast neutrons produced in a fission reaction causes a CHAIN FISSION REACTION as the fast neutrons combine to other uranium nuclei to produce further fission reactions (exponential growth of neutron numbers)

Question 12

parts of a fission reactor

Answer 12

(water heated, evaporates, steam turns a turbine)

uranium fuel rods/elements - contain fissionable uranium nuclei

moderator - slows down the neutrons to cause chain reaction (usually water) - neutrons collide with nuclei in moderator, transferring KE

control rods - control rate of fission reactions by absorbing excess neutrons to prevent further fission reactions if rate of reaction needs to be decreased. rods pulled out to increase rate - less neutrons absorbed

concrete walls - stop radiation as all radiation is stopped by concrete

Question 13

environmental impact of fission reactors

Answer 13

main issue is radioactive waste - plutonium-239 which is toxic, radioactive and has a long half life
storage and disposal of radioactive waster is an issue - fuel rods have to be buried underground and prevent entering food and water supplies

… so governments are pushing more cleaner and more renewable energy resources e.g. wind and solar power

Question 14

fusion reactions

Answer 14

bringing nuclei close together within 10^-15m so short range nuclear force can attract them into a large nucleus (overcome strong electrostatic force at short range)

therefore temperature needs to be high so nuclei can move fast enough to get closer to trigger fusion - overcome electrostatic force of repulsion

Question 15

energy in fusion reactions

Answer 15

mass of nucleus after is less than mass of nuclei before. the change in mass corresponds to BINDING ENERGY released as ΔE=Δmc^2

binding energy after is greater than binding energy before (daughter nuclei more stable) and the difference between these binding energies is equal to the energy released

Question 16

example of fusion

Answer 16

two protons fusing to create deuterium nucleus, positron and neutrino

energy released

Question 17

fusion on earth

Answer 17

clean process but no power stations on earth use fusion yet as problems revolve around maintaining such high temperatures and pressure for long enough to sustain fusion
(present experimental fusion reactors produce energy for short time and not enough needed to start reactions)

Question 18

binding energy per nucleon curve

Answer 18

iron56 - highest binding energy per nucleon so is most stable element (around 8.5MeV)

A<56 - lower binding energies per nucleon so weak electrostatic forces, undergo FUSION
gradient steeper, release greater binding energy

A>56 - binding energies per nucleon but decreases with A, so heavier elements undergo FISSION
gradient less steep so release less binding energy

Helium does not fit trend and peaks as it is more stable so has higher binding energy per nucleon
(which is why nuclear reactions tend to emit alpha radiation)