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AQA Physics A
Transcript of AQA Physics A
Unit 1 Particle
Physics Atom models/structure Plum pudding model Rutherford presents nuclear model of atom:
Protons + neutrons in the nucleus
Surrounded my electrons Bohr found that electrons
orbit nucleus De Broglie -
Wave particle duality Geiger and Marsden fired beams of positively charged alpha particles at thin metal foils. Schrodinger - electron modelled as a wave rather than a particle Chadwick obtains evidence for
existance of neutrons Einstein - light may act like a particle; photoelectric effect Planck- atoms emit light at certain frequencies Photons Emitted by charged particles Beam Power = nhf
n= no. of electrons
h= Plancks constant - 6.63x10^-34
f = frequency Fast moving electrons are stopped, slowed down or they change direction An electron moves to a shell of lower energy Photon Energy = hf c = f λ
c = speed of light
f = frequency
λ = wavelength Radioactive Decay Alpha particles α Beta particles β Gamma Radiation γ X --> Y + α A Z A - 4 Z - 2 4 2 X --> Y + β + V A Z A Z + 1 0 -1 - Antimatter Annihilation
Matter + Antimatter = produces 2 photons Min Energy produced:
2hf = 2E min 0 (Minimum E of 2 electrons) (Rest E of electron =
0.511 MeV) Min E of Photon needed =
hf = 2E Min 0 (less E cannot create matter and antimatter) Cloud Chamber tracks:
Magnetic field + lead plate
-Slower = more bend
This produced an e+ track in the opposite direction to an e- Interactions Feynman diagrams Quarks and Leptons Quantum Phenomena! Electric Current Direct Current
Currents Classification of Particles Matter/Antimatter Hadrons Leptons Baryons Mesons Strong + heavy
Particles and antiparticles that can interact through strong interactions and electromagnetic interactions if charged
Decays through weak interactions apart from protons Particles exist on their own
Light and weak
Interact through weak interactions + through electromagnetic interactions if charged Examples:
Electron, Muon, Tau, Neutrinos Are protons + all other hadrons that decay into protons
Proton is the only stable one Examples:
Protons, Neutrons, Σ Hadrons that don't include protons in their decay Examples:
K mesons, π mesons Increasing mass Other Particles: Muons Pions (π - mesons) Kaons (K - mesons) Leptons In an interaction between a lepton + a hadron
Lepton number must be conserved +1 for a Lepton
-1 for an Antilepton
0 for a non lepton Neutrinos:
Muon Decay -> muon neutrino - ν
Beta Decay -> electron neutrino - ν
μ e (These are different to each other) Lepton + Hadron Muon Decay Quarks These cannot exist on their own, in mesons there is always a quark - antiquark pair. Baryons are made of three quarks; antibaryons of three antiquarks.
Mesons are made up of one quark and one antiquark.
Gluons bind quarks together; they are subject to the strong interaction. There are four numbers that must be conserved:
Charge, shown as the fraction of the electronic charge. e.g - 1/3 of e
Strangeness number if there are strange quarks
Each antiquark has equal and opposite values of charge, baryon number and strangeness. Charged π-mesons decay into:
muons and antineutrinos
antimuons and neutrinos
π decays into high energy photons 0 Muons and antimuons decay into:
electrons and antineutrinos
positrons and neutrinos K mesons decay into:
muons and antineutrinos
antimuons and neutrinos Photoelectricity Emission of electrons from a metal surface when surface is illuminated by light of frequency that is greater than the minimum value (threshold value) 1 photon is absorbed by 1 electron to produce 1 photoelectron. E = hf = Ø + ½mv² max E - Energy of Photon (small package of light)
h - Planck's constant
f - threshold frequency
Ø - work function (energy for electron to get out of metal)
m - mass of photoelectron
v - max velocity of electron No. of Photons is proportional to intensity
½mv²max = hf - Ø y = mx + c Greater frequency of incoming rays = greater velocity of photoelectric effect If hf = Ø
then the electrons are emitted with no kinetic energy, this also shows the threshold (min) frequency to produce photoelectrons. Vacuum photocell Collisions and Energy Levels Wave Particle
Duality Electrons in atoms can only occupy certain energy levels.
The groud state is the lowest energy state of the atom.
For an atom to emit photons, first the atom at ground state must absorb enough energy, by other electrons colliding with the orbiting electrons, for one of the atom's electrons to move to a higher shell, this is known as excitation. When the electron loses some of it's energy, it moves down to a lower energy shell, known as de-excitation, this emits a photon of a certain frequency - depending on which energy level it falls to. When an electron moves from energy level e.g E3, to a lower energy level E2,
the energy of emitted photon hf = E3 - E2 x = frequency
y = max Kinetic Energy
f = threshold (min) frequency
-Ø = work function
gradient = h (planck's constant) Electrons Light Wave-like nature: light can be diffracted
Particle-like nature: photoelectric effect Particle-like nature - can be deflected by a magnetic field
Wave-like nature - narrow beam of electrons directed at thin metal foil, electrons seem to be diffracted onto a screen creating a pattern of rings The electric current is the rate of flow of charge in the wire/component due to charged particles called charge carriers In metals charge is carried by conducting electrons which collide with other electrons/atoms
In a salt solution, charge is carried by ions (charged atoms or molecules) Current - Ampere (A)
Charge - Coulomb (C)
Q = I x t In an insulator, the electron is attached to the atom and so cannot move away to conduct electricity Current and Charge Current flows from + to -
Electrons flow from - to + Potential difference (voltage) is the work done per unit charge
1v = 1J per C V = W
Q Potential Difference and Power - The emf is the 'electrical energy produced per unit charge passing through the source'
(The volts going through the battery when it is not connected to anything else). Electric current has a heating effect as well as a magnetic effect. W = I t V
W = work done
I = current
t = change in time
V = voltage P = I V
P = electrical power
I = current
V = voltage Resistance Ω R = V
I - The difficulty of making current pass through a component caused by the charge carriers colliding with each other and fixed atoms. Ohms law - pd is proportional to current through a metal conductor, when all physical conditions are the same (e.g temp.) Calculating resistivity ρ (Ωm) Super Conductivity:
A wire/device made of a material with zero resistivity at and below a critical temperature.
When a current passes through it, there is no p.d because resistance is zero (p.d is proportional to resistance), so the current has no heating effect. Use a potential divider to vary p.d from zero
Use a variable resistor to vary the current to a minimum Components Resistance increases as the lamp becomes hotter Thermistor: resistance decreases with an increase of temperature At a junction, total current leaving a circuit is equal to the current entering a certain. Current entering a component is equal to current leaving the component in a series circuit Current passing through 2 or more components in series is the same through each component For 2 or more components in series, the total pd across all the components is equal to the sum of the potential difference across each component The pd across components in parallel is the same Circuit Rules Emf and Resistance Potential Divider Other Calculations Alternating current and power Oscilloscope V = IR The rate if heat transfer = I R 2 A source of e.m.f has some resistance to electrical current within it called its internal resistance. It results in the voltage across the terminals of the source dropping as currents is drawn from it.
It results in the source being less than 100% efficient as energy is dissipated in the internal resistance as current flows through it E.m.f (electromotive force) is measured at 0 current, this can be done by connecting just a voltmeter (high resistance) across the terminals. The gradient has the unit Ω - it is the internal resistance r Some voltage is needed across r because energy is needed for charge to flow through it - known as 'lost volts' Terminal potential difference is the p.d at the cell's terminals ε = IR + Ir Maximum power is delivered to the load when the load resistance is equal to the internal resistance of the source Two or more resistors in series and a source of fixed potential difference.
The pd is divided between the components. It is used to:
supply a pd which is fixed at any value between zero and the source pd
supply a variable pd
supply a pd that varies with a physics condition such as temperature or pressure.
A current that repeatedly reverses its direction
Frequency is number of cycles it passes through each second
The peak current is the maximum current (or pd) Use an oscilloscope to display the waveform (i.e variation with time) of the alternating pd from a signal generator
Connect the signal generator to a torch lamp and make the frequency low enough so you can see the brightness of the lamp vary The root square mean value of an alternating current is the value of direct current that would give the same heating effect as the a.c in the same resistor. Used to make high powered electromagnets, e.g used in magnetic levitation trains
Used for MRI scanners for whole body imaging
f = 1