chapter 26 - nuclear physics Flashcards
einsteins eq
E = mc²
- mass is a form of energy (demonstrated by how opposite particles eg photon electron, destroy each other - annihilation - and mass is turned to gamma photons)
- energy has mass - moving ball has KE so has energy implying it has a greater mass then at rest
pair production
a single photon vanishes and its energy creates a particle and corresponding antiparticle eg electron-positron pair
deuterium nucleus
one proton one neutron
- to separate have to do work
energy required = change in mass
therefore the mass of separated nucleons is greater than of the nucleus
mass of separated nucleons vs nucleus
mass of separated nucleons is greater than of the nucleus
- to separate have to do work
- energy required = change in mass
mass defect
difference between the mass of the separated nucleons and the mass of the nucleus
binding energy
minimum energy required to completely separate a nucleus into its constituent protons and neutrons
binding energy = mass defect*c²
how binding energy changes with no. nucleons
as the nucleus has more nucleons the external energy required to split this nucleus is greater - more strong nuclear bonds to break
binding energy per nucleon
the minimum energy required to completely separate a nucleus into its constituent protons and neutrons
/ the number of nucleons
graph of binding energy per nucleon and nucleon number
as nucleon no (M) increases as does BE up to iron then decreases
- more BE = more stable - iron most stable
to the left BE increases as M increases - fusion
to the right BE decreases as M increases - fission
why low BE is fusion
two low M nuclei join to produce a larger M nucleus so has greater BE than the initial nuclei so energy is releases
why high BE is fusion
a high M nucleus splits into 2 low M nuclei
- 2 nuclei have higher BE so energy is released
induced fission
when fission is caused by slow moving neutrons
- slow moving neutrons absorbed by a nucleus making it unstable and splitting into 2 daughter nuclei plus fast neutrons
thermal neutrons
- slow moving neutrons that cause fission
KE is similar to thermal energy of particles
fission energy
total mass after is always less than before
the loss in mass corresponds to the energy released
BE after is greater then before
difference in BE = energy released
released as KE
chain reaction of fission
if the fast neutrons produced are slowed down they can go on to cause more fission reactions (eg 3 is there are 3 produced)
after n fission events there would be 3^n neutrons - grows exponentially
- steady power from a fission reactor is due to ensuring 1 slow neutron survives between successive fission reactions
fission reactor
fuel rods spaced evenly within a concrete vessel (reactor core)
coolant used to remove thermal energy produced
fuel rods surrounded by moderator and control rods can be moved in/ out the core
fuel rods
contain enriched uranium
which consists of U-238 with 2/3% of U-235
moderator
slows down fast neutrons produced in fission reactors
- must be cheap, available and not absorb neutrons in the reactor
- chance of fast neutrons being absorbed is small whereas thermal neutrons have a greater chance
- fast neutrons dont lose much KE from collisions with nuclei but collide elastically with protons in water or carbon they transfer KE and slow down
eg water carbon
normally also the coolant
mean KE of thermal neutrons
3/2kT
control rods
made up of a material whose nuclei absorb neutrons
eg boron or cadmium
position is automatically adjusted to ensure that exactly one slow neutron survives per fission reaction
to slow down/ stop fission rods pushed further into the core
environmental impact of fission
neutrons are absorbed by U238 which decays into plutonium 239 which is extremely toxic and radioactive with a long half life
- the daughter nuclei produced from its fission are also radioactive
radioactive waste disposal
- burried deep underground - as isotopes with long half lives cant enter out food and water supplies
- locations must be geologically stable, secure from attack and safe
fusion reactors
have to bring nuclei very close so strong nuclear force attracts them
all nuclei have + charge so repel
have to be at high temps to move fast and get close enough
eg in the sun
fusion examples
proton + proton -> ²H (deuterium) + positron + neutrino
deuterium + proton -> He-3
He3 + He3 -> He4 + 2 protons
fusion on earth
no power stations using fusion
due to problems in
- maintaining high temp long enough
- confining hot fuel within a reactor
now all fusion reactors only produce energy for a short time in small quantities
