physics unit one revision notes.docx

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Physics Unit One Revision Notes 1.1 Inside the Atom The nucleus is positively charged composed of neutrons and protons Electrons surround the nucleus The electrons are held in the atom by the electrostatic force of attraction between them and the nucleus. Electron has a smaller mass than both the proton and the neutron. Therefore it has the highest charge to mass ratio. Specific charge. The proton and the neutron have almost equal mass Electron has equal and opposite charge to the proton Neutron is uncharged Isotopes: Atoms with the same number of electrons and protons but different number of neutrons A --- Mass/Nucleon Number X --- Element Z --- Proton/Atomic Number This isotope notation shows the neutrons and protons inside the nucleus Specific Charge: Charge of the particle/Mass Unit: Ckg^-1 Electron has the highest specific charge out of the three particles.

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Page 1: Physics Unit One Revision Notes.docx

Physics Unit One Revision Notes1.1 Inside the Atom

The nucleus is positively charged composed of neutrons and protons

Electrons surround the nucleus

The electrons are held in the atom by the electrostatic force of attraction between them and the nucleus.

Electron has a smaller mass than both the proton and the neutron. Therefore it has the highest charge to mass ratio. Specific charge.

The proton and the neutron have almost equal mass

Electron has equal and opposite charge to the proton

Neutron is uncharged

Isotopes:Atoms with the same number of electrons and protons but different number of neutrons

A --- Mass/Nucleon Number X --- ElementZ --- Proton/Atomic Number

This isotope notation shows the neutrons and protons inside the nucleus

Specific Charge:

Charge of the particle/Mass

Unit: Ckg^-1

Electron has the highest specific charge out of the three particles.

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1.2 Stable and unstable nuclei

The force that holds an isotope’s nuclei together so it doesn’t disintegrate, stable, is called the strong nuclear force.

Strong nuclear force overcomes the electrostatic force of repulsion between the protons in a stable nucleus.

Range is no more than 4 fm, femtometers, about the same as the diameter of a small nucleus.

Electrostatic force between two charged particles has an infinite range. (Decreases as the range increases)

Attractive force from 4 fm down to 0.5 fm. (0.5 – 4 fm.)

At separations smaller than 0.5 fm, it is a repulsive force, preventing neutrons and protons being pushed into each other.

Radioactive decay:1) Alpha radiation:Alpha particles: Two protons and two neutrons.4 A A-4 4 X α Y + α2 Z Z-2 2

This happens to a large nuclei because strong nucleus force does not have enough range to hold it together therefore it is unstable.

2) Beta- radiation: 0

Fast moving electrons. ( -)β β -1This happens when a neutron in the nucleus changes into a proton.

Beta particle is created when the change happens and is emitted instantly.

A particle with no charge, anti electron neutrino ̅νe is also emitted.

A A 0 X Y + β- +  ̅νe

Z Z+1 -1

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3) Beta+ radiation:This occurs when a Beta+ particle (positron) is created when a proton in the nucleus changes it to a neutron.

This happens in an unstable nucleus with too many protons.

This does not occur naturally, manufactured.

A particle with no charge, electron neutrino Ve is also emitted.

A A 0 X Y + β+ +  Ve

Z Z-1 1

4) Gamma radiation ( ):γ

Is an electromagnetic radiation. It has no mass or charge

Emitted by a nucleus with too much energy.

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1.3 Photons

Electromagnetic waves

In a vacuum, all EM waves travel at a speed of light, c, which is 3.0x108 ms-1.

Speed of wave (c) = Wave length ( ) x frequency of wave (f)λ

List of the EM s waves from longest wavelength to shortest wave lengthRadio Longest wavelengthMicrowaveInfraredVisibleUltravioletX-raysGamma rays Shortest wavelength

EM wave is an electric wave and magnetic wave.

They travel together

They vibrate at right angles to each other and to the direction they are travelling.

In phase with each other, i.e. reaches a peak together.

PhotonsEM waves are emitted by a charge particle when it loses energy.

- A fast moving electron stopped, slows down, or changes direction- An electron in a shell of an atom moves to a different shell of lower

energy. This is why photons are emitted when atoms de-excite.

EM waves are emitted in short bursts of waves in different directions.

Referred as photon.

Photon theory can be used to explain photoelectricity when the old wave theory cant

This shows the wave particle duality of EM waves

Photon energy E = hf

Power of laser beam = nhf

N is the number of photons in the beam passing a fixed point each second.

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1.4 Particles and antiparticles

Antiparticles have the same mass as the particle but opposite charge.

Annihilation:When antimatter and matter particles meet, they destroy, annihilation each other, and converts their total mass into two photons, radiation energy.

Pair production:A photon with sufficient energy could suddenly change into a particle – antiparticle pair which would then separate from each other.

The minimum energy of photon needed is 2 x minimum energy of photon (rest energy)

A photon with less energy could not therefore create a positron and an electron.

1 eV = 1.60 x 10-19 J

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1.5 How particles interact

Particles interact through an exchange of an exchange particle

Momentum is transferred between the objects.

When two objects interact, they exert equal and opposite forces on each other.

The strong nuclear force holds the neutrons and protons in a nucleus together

Weak nuclear force causes the decay of unstable nuclei.

In both β+ and β- a new particle and a new antiparticle are created, but they are not corresponding particle antiparticle pair.

In β- an electron and anti neutrino is created

In β+ a positron and neutrino is created.

Interaction: Exchange ParticleGravity Graviton?Strong Nuclear Force Gluon (between quarks)

Mesons (between hadrons)Electromagnetic force Virtual photonsWeak nuclear force W+, W-,Z0

Everything but photons have:A non zero rest massA very short range, while photon is infiniteAre charged

Neutrino – neutron interaction. It could be W- going to the other direction Neutron (all hadrons) all decay to proton. UDD to UUD, down changed to Up. Neutrino changes to an electron, as lepton number must be conserved.

Proton – antineutrino interaction Proton changes to neutron. UUD to UDD, up quark changed to down. Antineutrino changes to a positron, as lepton number must be

conserved.

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Beta decays:

β+ decay Proton decays into a neutron

W+ boson decays into a positron and a neutrino, as lepton number must be conserved.

β- decayNeutron decays into a protonW- boson decays into an electron and antineutrino, as lepton number must be conserved

Electron capture:Proton decays into a neutronElectron is decayed in to an electron neutrino, as lepton number has to be conserved

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Chapter 2

Mesons are produced through the strong interaction and decay through the weak interaction

Decay modes:K mesons decay in to mesons, muons and antineutrinos/ antimuons and πneutrinos.

Particle Charge Classification Antiparticle Charge Classification Interaction

Proton +1 Baryon (part of hadrons)

Antiproton -1 Antibaryon Strong, Weak decay, EM

Neutron 0 Baryon (part of hadrons)

Antineutron 0 Antibaryon Strong, Weak decay, EM

Electron -1 Lepton Positron +1 Antilepton Weak decay, EMNeutrino 0 Lepton Antineutrino 0 Antilepton Weak decay, EMMuon (heavy electron)

-1 Lepton Antimuon +1 Antilepton Weak decay, EM

Pi meson (+,0,-)

+1,0,-1 Mesons (part of hadrons)

+ for – and vice versa0 for 0

+1,0,-1 Antimeson Strong, EM

K meson(+,0,-)

+1,0,-1 Meson + for –0 for anti 0

+1,0,-1 Antimeson Strong, EM

Charged pi mesons decay into muons and antineutrinos OR antimuons and neutrinos.

Muons and antimuons decay into electrons and antineutrinos or positron and neutrinos

Decays always obey conservation rules.

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Hadrons:

Created through the strong interaction

They are split in to two groups, BARYONS AND MESONS

Hadrons are particles and antiparticles that can interact through the strong interaction.

Baryons: All hadrons also decay through the weak interaction apart from proton, the only stable baryon

Baryons have 3 quarks

Typical examples of baryons: Protons and neutrons

Protons (UUD) Neutrons (UDD)

Anti particles of them are just all anti quarks.

Mesons:

Do not include protons in their decay products

Mesons have 2 quarks each consisting of a quark and an antiquark

K-mesons have strangeness, they decay only to pi mesons.

Strangeness is always conserved in strong interactions but not conserved in weak interactions.

If the meson contains one strange quark, it has strangeness of -1.

There are up down and straight quarks, and their antiparticles. Only strange quark and anti quark has strangeness.

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Leptons:

They are the fundamental elements of matter, fundamental particles, meaning that they cannot be broken down, are not composed of smaller particles.

Leptons do not interact through the strong interaction, only weak interaction and EM if charged

Lepton number is conserved for any reaction. +1 to a lepton and -1 to an antilepton, 0 for any non-lepton.

Sometimes we need to apply the rule separately to each branch of leptons, the electron branch (electrons, electron neutrinos and their anti particles) and the muon branch (muons, muon neutrinos and their anti particles)

2.5 Conservation rulesParticles and antiparticles obey certain conservation rules when they interact.

1) Energy and charge --- ALL REACTION2) Conservation of lepton numbers --- All particle antiparticle interaction3) Conservation of strangeness in STRONG INTERACTIONS4) Baryon Number

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3.1 Photoelectricity In this model EM radiation is represented by stream of a particles called photons, not wave, the energy of each photon E=hf

Metal contains conduction electrons that move about freely inside the metal

Electrons are emitted from the surface of a metal with EM RADIATION ABOVE a certain frequency was directed at the metal. This is known as photoelectric effect.

If below threshold frequency, no electrons are emitted no matter what the intensity.

Photoelectrons are emitted INSTANTANEOUSLY no matter what the intensity of radiation is at or above threshold frequency.

There is a definite minimum frequency of radiation which the photoelectric effect cannot take place if it was below. Threshold frequency, this depends on the substance.

Each photon is absorbed by a SINGLE conduction electron, one to one interaction

If this gives the electron enough energy to escape from the metal lattice it does so immediately.

Emission of electrons also called photoelectrons

Above threshold frequency the rate of emission depends on the INTENSITY of the light. Intensity of radiation determines the RATE at which photons fall per unit surface area of the metal

Greater number of photons incident on metal the greater number of escaping electrons

The kinetic energy of emitted electrons depends on the frequency of the incident radiation.

Kinetic energy of the emitted electron when it leaves the metal is the energy GIVEN to it by the absorbed photon MINUS the work it has to do to leave the surface

Minimum amount of work done is work function.

Kinetic energy varies from zero up to a maximum energy due to some photoelectrons coming from slightly below the metal surface, they make collisions and have to do more work than the work function to escape.

Work function is the minimum energy that a surface electron can lose.Therefore KEmax= hf – work function. (baisically hfmin)

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How did the wave model of EM radiation fail to account for the photoelectric effect?

1) Kinetic energy of photoelectron does not depend on the intensity of the EM radiation.According to wave model, em radiation energy should be distributed uniformly over the wave front. Increase intensity of radiation would increase the energy stored and energy of photoelectrons will be increasing. So an increase of intensity would increase KE, this is not what was observed.

2) For a given metal there is a threshold frequency where below no electrons are emitted despite the intensity. The wave model would suggest that any frequency of em radiation would emit electrons if shone on long enough.

3) Photoelectrons emitted INSTANTANEOUSLY no matter the intensity. Wave model would have suggested that electrons would take time to collect necessary energy for their emissions.

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3.3 Collisions of electrons with atomsAtoms are normally at the ground state, the lowest energy level.

Ions are created from removing or adding electrons to an uncharged atom.

Removing makes it positive while adding makes it negative.

Any process of creating ions is called ionization

Electron volt: unit of energy equal to work done when an electron is moved through a pd of 1V.Work done when an electron moves through a PD of 1 volt is 1.6 x10-19 J1eV= 1.6 x10-19 J (the charge of one electron)

Excitation by collision:When atoms gain additional energy but the energy is not enough to ionize it, excitation occurs instead.

Atoms can be excited by giving them additional energy e.g. electron, photon.

When excitation occurs colliding c=electron makes an electron INSIDE the atom, atomic electron, move from an inner shell to an outer shell.

Energy needed as the atomic electron moves away from the nucleus.

The incident electron does not have to have exactly energy equal to the energy difference between two energy levels in order to produce excitation. An incident photon, however, if it is going to be absorbed entirely by the atom, it has to have energy exactly equal to the energy difference between two energy levels.

Each atom has a UNIQUE set of energy levels

After being excited it goes back to the ground state by loosing energy by emitting photons with exact amount of energy for them to land on an energy level, the energy difference between two energy levels. It can de-excite via different routes

They de-excite as they do not retain the absorbed energy permanently. An excited atom is UNSTABLE because an electron that moves to an outer shell leaves a vacancy in the shell it moved from. It has to be filled by an electron from an outer shell transferring to it. The energy of the photon is equal to the energy lost by the electron and therefore by the atom

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Absorption spectrum: Photon energy is related to frequency therefore wavelength and color. As the atom will only absorb photons of certain energies, it will only absorb certain colors in the continuous spectrum, ignoring the others. Therefore the absorbed wavelengths are missing. Black lines.

Emission spectrum: Atoms will only emit photons corresponding to their energy levels, therefore they will only a few colors.

Every element produces a different pattern oflight wavelength, as the energy levels of each type of atom are UNIQUE to that atom.

Energy of emitted photon hf= E1-E2

Fluorescence:The process of excitation and de exciting directly or indirectly explains why certain substances fluoresce, glow with visible light when they absorb UV radiation. Atoms in the substance absorb UV photons and become excited when they de-excite they emit visible photons. IF the source of UV radiation is removed the substance stops glowing.Fluorescent tube is a glass tube with fluorescent coating (phosphor) on its inner surface.

1) Inside the tube it contains mercury vapor at low pressure and with the electrons in the tube due to the current.

2) Mercury atom emit UV photons as well as visible photons and other photons as they de excite.

3) UV photons are absorbed by the fluorescent coating, they also de excite and emit visible photons

Fluorescence lamp is a lot more efficient than filament lamp as it waste less as heat energy

Why coating: Mercury atoms emit UV radiation that is not visible and can be harmful. Coating absorbs UV radiation and emits lower frequencies photons in the visible region

Why low pressure: The fluorescence lamp must have a large distance between collisions to allow electrons to gain enough energy, or the vapor will not completely absorb the electrons. Also the current need to pass, if too many gas atoms it won’t.

Wave Particle Duality

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Electrons Electromagnetic radiationWave: Diffraction Wave: Interference/diffractionParticle: Deflection in magnetic field Particle: Photoelectric effect

Matter particles have a dual wave particle nature

Wave like behavior is characterized by a wavelength, de Broglie wavelength.

Wavelength = planks constant (h) / momentum (p)Momentum = Mass x Velocity