Matter And Radiation BROWN Flashcards
What do we know about atoms from Rutherfords alpha scattering investigations?
Every atom contains:
- A positively charged nucleus composed of protons and neutrons
- Electrons that surround the nucleus
What is a nucleon?
Proton or neutron in the nucleus
Give information about electrons and the nucleus
Each electron has a negative charge
Nucleus is positively charged
Electrons are held in the atom by electrostatic force of attraction between them and the nucleus
What does an uncharged atom have?
Equal number of protons and electrons
Becomes an ion if it gains or loses electrons
What is an isotope?
Atoms with the same number of protons and different numbers of neutrons
What is the nucleon number?
Total number of protons and neutrons in an atom or sometimes the mass number of the atom
Neutron or proton in nucleus
What is each type of nucleus called?
Nuclide
What is the specific charge of a charged particle?
It’s charge divided by mass
A stable isotope has nuclei that do not disintegrate so what force must there be holding them together?
Strong nuclear force
Called this because it overcomes the electrostatic force of repulsion between the protons in the nucleus and keeps the protons and neutrons together
What is the range of a strong nuclear force?
No more than about 3-4 femtometres
1 fm = 10 (to the power of)15 m
What is the range of the electrostatic force between two protons?
Infinite range (although the strength of the force decreases as the range increases)
What effect does a strong nuclear force have between two protons or two neutrons or a proton and a neutron?
It has the same effect between two protons as it does between two neutrons or a proton and a neutron
What is the attractive force of a strong nuclear force? What happens at separations small than than this?
Has an attractive force from 3-4 fm down to 0.5 fm
It is a repulsive force that acts to prevent neutrons and protons being pushed into each other
Naturally occurring radioactive isotopes release which 3 types of radiation?
Alpha
Beta
Gamma
What does alpha radiation consist of?
Consists of alpha particles which each comprise two protons and two neutrons
What is the symbol for an alpha particle?
4
a
2
Proton number is 2 and it’s mass number is 4
After alpha radiation is takes place, what happens?
As a result of the change, the product nucleus belongs to a different element
What does beta radiation consist of?
Consists of fast moving electrons
What is the symbol for an electron as a beta particle?
0
B
-1
OR
B- (minus up at top)
Because it’s charge is equal and opposite to that of the proton and it’s mass is much smaller than the protons mass
What happens during alpha radiation in an unstable nucleus?
The nucleus emits an a particle and forms a new nucleus
Nucleon number decreases by 4 and atomic number increases by 2
What happens during beta radiation in an unstable nucleus?
A B particle is created in the nucleus and is instantly emitted
A neutron in the nucleus changes into a proton
An antineutrino is also emitted
This happens as a result of a neutron in the nucleus changing into a proton
What is an antineutrino?
An antiparticle with no charge
What happens after beta radiation has taken place?
As a result of the change the product nucleus belongs to a different element
This type of change happens to a nuclei that have too many neutrons
What happens to the atomic number after beta radiation and why does this happen?
Atomic number increases by 1 but the nucleon number stays the same
This is because a neutron changes into a proton in the nucleus
What is gamma radiation?
Is electromagnetic radiation emitted by an unstable nucleus
Give information about gamma radiation
Can pass through thick metal plates
It has no mass and no charge
When is gamma radiation emitted?
Emitted by a nucleus with too much energy, following an alpha or beta emission
When the energy spectrum of beta particles were first measured, what were the beta particles found to release?
(When each unstable nucleus lost a certain amount of everything in the process)
They were found to release kinetic energies ip to a maximum that depended on the isotope
In a vacuum, what do all electromagnetic waves travel at.
The speed of light
C
3 x 108 ms1
What is the equation for the wavelength of electromagnetic radiation of frequency f in a vacuum?
Wavelength = speed of light (3 x 108 ms1) / frequency
Note light wavelengths are often expressed in nm 1nm=10*9m
What do electromagnetic waves consist of?
An electric wave and a magnetic wave which travel together and vibrate
- at right angles to each other and to the direction in which they are travelling
- in phase with each other
What are electromagnetic waves emitted BY?
Emitted by a charged particle when it loses energy
This can happen when
- a fast moving electron is stopped or slows down or changes direction
- an electron in a shell of an atom moves to a different shell of lower energy
What are electromagnetic waves emitted AS?
Emitted as short bursts of waves, each burst leaving the source in a different direction
Each burst is a packet of electromagnetic waves referred to as a proton
What is a photon and how was it established?
A packet of energy/electromagnetic waves
It was established by Einstein when he used his ideas to explain the photoelectric effect
He believed the energy of a photon depended on its frequency in accordance to the equation E=hf where h is the Planck constant and the value of h is 6.63x10*-34 Js
What is the photoelectric effect?
The emission of electrons from a metal surface when light is directed at the surface
What does a laser beam consist of and how is the power of a laser beam worked out?
Consists of photons with the same frequency
The power of a laser beam is the energy per second transferred by the photons
For a beam consisting of photons of frequency f
The power of the beam = nhf
Where n is the number of photons in the beam passing a fixed point each second - this is bc each photon has energy of hf
What happens when matter and anti matter meet?
They destroy each other and radiation is released
What is a positron?
Antiparticle of the electron and so carries a positive charge
Each positron that is emitted travels no further than a few mm before it meets an electron and they annihilate each other - two gamma photons may be produced as a result
Has the same symbol as beta particles
0
B
+1
\+ B
When does positron emission take place?
Positron emission takes place when a proton changes into a neutron in an unstable nucleus with too many protons
A neutrino (symbol v) which is uncharged is emitted
How do positron emitting isotopes occur?
Do not occur naturally
They are manufactured by placing a stable isotope, in solid or liquid form, in the path of a beam of protons
Some of the nuclei I’m the substance absorb extra protons and become unstable positron emitters
What did Einstein show about the mass of a particle?
The mass of a particle increases the faster it travels and his equation E=mc*2 related the energy supplied to the particle to increase in mass
He showed the rest energy must be included in the conservation of energy
What did Dirac predict?
He predicted the existence of antiparticles that would unlock rest energy whenever a particle and a corresponding antiparticle meet and annihilate each other
Predicted the opposite process of pair production
Diracs theory of antiparticles predicted that for every type of particle there is a corresponding antiparticle that….
- annihilates the particle and itself if they meet, converting their total mass into photons
- has exactly the same rest mass as the particle
- has exactly opposite charge to the particle if the particle has charge
What did Dirac predict would happen when he predicted the opposite process of pair production?
He predicted that a photon with sufficient energy passing near a nucleus of an electron can suddenly change into a particle-antiparticle pair, which would then separate from each other
What is the energy of a particle or antiparticle often expressed in?
Millions of electron volts MeV
Where 1 MeV = 1.60 x 10*-13 J
1 eV is defined as the energy transferred when an electron is moved through a potential difference of 1V
Given the rest mass of a particle or anti particle, how can it’s rest energy in MeV be calculated?
Using E=mc*2
What is E = mc*2 used to calculate?
The rest energy of a particle or antiparticle after being given the rest mass
What is annihilation?
Occurs when a particle and a corresponding antiparticle meet and their mass is converted into radiation energy
Two photons are produced in the process - as a single photon cannot ensure total momentum of zero after the collision
How do you calculate the minimum energy of each photon produced?
Minimum energy of each photon produced, hfmin = E*(bottom)0
What happens during pair production?
In pair production a photon creates a particle and a corresponding antiparticle, and vanishes in the process
For a particle and antiparticle, each of rest energy E*(bottom)0 we can calculate the minimum energy and minimum frequency that the photon must have to produce this particle anti particle pair
Minimum energy of each photon produced hfmin = E*(bottom)0
When discovering the positron, what happens when a magnetic field is applied to the chamber?
Anderson (physicist who discovered it) knew the trail of a charged particle would bend in the field
A positive particle would be deflected by the magnetic field in the opposite to a negative particle travelling in the same direction
The slower it went the more it would bend
What happens when a single force acts on an object?
It changes the momentum of the object
What is the momentum of an object?
Mass multiplied by its velocity
What happens when two objects interact?
Exert equal and opposite forces on each other
Momentum is transferred between the objects by these forces if no other forces act on them
Eg/ if two protons approach each other they repel and move away from each other
For example, two protons approach each other they repel and move away from each other if two protons approach each other they repel and move away from each other, why does this happen?
The electromagnetic force between two charged objects is due to the exchange of virtual photons (can’t detect them directly)
If we intercepted them (eg/using a detector) we would stop the force acting
What does a strong nuclear force do?
Holds the neutrons and protons in a nucleus together
But it doesn’t cause a neutron go change into a proton in beta minus decay or a proton to change into a neutron in beta plus decay
What is the weak nuclear force?
Force responsible for beta decay
In both B- decay and B+ what is created?
A new particle and anti particle are created in each type of decay but they are NOT CORRESPONDING particle- antiparticle pairs as one is an electron or a positron and the other is a neutrino or antineutrino
Neutrons and anti neutrinos hardly interact with other particles but such interactions do sometimes happen, for example:
- a neutrino can interact with a neutron and make it change into a proton. A B- particle (an electron) is created and emitted as a result of the change
- an antineutrino can interact with a proton and make it change into a neutron. A B+ particle (a positron) is created and emitted as a result of the change
Neutrinos and anti neutrinos hardly interact but such interactions sometimes do happen, these interactions are due to the exchange of particles referred to as W bosons. Unlike photons these exchange particles:
Have a non zero rest mass
Have a very short range of no more than about 0.001fm
Are positively changed (the W+ boson) or negatively charged (the W- boson)
What role does a W boson play in beta decay?
W boson meets a neutrino or antineutrino changing them into a B- particle (an electron) or a B+ particle (a positron)
But if no neutrino or antineutrino is present:
- the W- boson decays into a B- particle and an antineutrino
- the W+ boson decays into a B+ particle and a neutrino
Charge is conserved in both processes
What is electron capture?
Sometimes a proton in a proton-rich nucleus turns into a neutron as result of interacting through the weak interaction with an inner-shell electron from outside the nucleus
The same change can happen when a proton and an electron collide at very high speed
Photons and W bosons are known as force carriers, why?
Because they are exchanged when th electromagnetic force and the weak nuclear force act respectively
What is the exchange particle of the strong nuclear force?
Pions
