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Physics: Key ideas and definitions

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Physics (Physics HL) Note on Physics: Key ideas and definitions, created by kirsten w on 08/01/2017.
kirsten w
Note by kirsten w, updated more than 1 year ago
kirsten w
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Page 1

T2: Mechanics

Key ideas:Vector quantities have a magnitude and a directionVectors:- Displacement- Velocity- AccelerationScalars: - Distance- SpeedInstantaneous; at one particular moment vs. average; the average in a time intervalNote: In graphs, the instantaneous value of a quantity is the gradient of the tangent to a particular point in time, while the average is the gradient of the line drawn between two points.Frames of reference → Relative velocity (vector addition/subtraction)Graphical representations of motion→ S-T graphs: gradient = velocity→ V-T graphs: gradient = acceleration area under graph = displacement→ A-T graphs: area under graph = change in velocityUniform accelerated motion can be described using the SUVAT-equations→ Note: can only be used in case of constant accelerationFree fall→ Neglecting air resistance; all objects have same acceleration due to an uniform gravitational field→ Not neglecting air resistance, object will reach a terminal velocityMeasuring uniform accelerated motion can be done with- Light gates - device that senses when an object cuts through a beam of light- Strobe photography - gives out brief flashes of light at fixed time intervals so that a picture taken shows the object's motion- Ticker timer - Can be arranged to make dots on a strip of paper at regular time intervals on a piece of paper attached to an falling objectProjectile motion→ The path of an object in projectile motion is shaped like a parabola→ Motion has a horizontal and a vertical component.Horizontal: No acceleration, constant velocity Vertical: Constant acceleration due to gravity → Use angle of launch to determine horizontal/vertical component (greatest range at a 45 degree angle)→ Range is determined by horizontal component and time, maximum height by the vertical component.----------------------------------------------------------------------------------------------------------Forces are vector quantities and can be divided in components→ Different types: gravitational/weight , electrostatic, magnetic, normal, friction, tension, lift.→ Acceleration occurs in the same direction of resultant forceNewton's Laws of Motion1: In the absence of a resultant force, an object remains at rest or moves with a constant velocity2: (F = ma) The resultant force on a body is equal to the rate of change of its momentum / equal to the product of the mass of the body and its acceleration.3: If body A exerts a force on body B then body B exerts an equal and opposite force on body A. Translational equilibrium→ Resultant force is equal to zeroFriction can be divided in two types; static and dynamic→ up until a maximum force, resultant force is zero due to friction i.e. the object remains at rest→ coefficient of friction (ratio: no units) dynamic coefficient < static coefficientAn object in free fall not in a vacuum (like a parachutist) will experience a force of friction due to air → Fluid resistance ---------------------------------------------------------------------------------------------------------Work is done when a force moves an object in the direction of the force over a distance. → i.e. work done lifting = mgh work done compressing a spring = 1/2 kx^2→ amount of energy transferred is equal to the work doneLaw of Conservation of Energy: the total energy of a closed system remains constant; energy cannot be created or destroyed. → Types: kinetic, potential, radiant, nuclear, electrical, gravitational, internal, electrostatic, chemical, thermal, etc. ----------------------------------------------------------------------------------------------------------Momentum is the product of mass and velocity, change in momentum is called impulse. Law of Conservation of Momentum: During any interaction between two bodies, the total momentum before is equal to the total momentum afterwards, provided no external forces act. In an elastic collision no mechanical energy is lost.

IB definitions: Displacement→ change in positionVelocity→ the rate of change of displacementSpeed→ the rate of change of distanceAcceleration→ The rate of change of velocityWork done→ Product of force and distance over which the force acted(Linear) Momentum→ The product of mass and velocityImpulse→ The product of force and timePower→ The rate of energy transferEfficiency → The ratio of useful energy output per energy input

Important formulas:

Page 2

T3: Thermal Physics

Key ideas:In a closed system, 2 objects will reach a thermal equilibrium. → Two systems in thermal contact are said to be in thermal equilibrium if there is no net transfer of energy between them.→ Temperature is a property that determines the direction of thermal energy transfer between two systems in thermal contact.The internal energy of an object consists of two components;→ KE: depends on temperature / average velocity of the molecules→ PE: Intermolecular electrostatic attractionsKinetic theory→ Temperature is a measure of the average kinetic energy of an object→ Substances: Solid: Particles vibrate around fixed position, strong attraction; particles are close together Liquid: Particles able to 'flow', less strong attractions: higher KE, lower PE Gaseous: Weak attractions; particles are far apartPhase changes occur when the potential energy of a substance changes (flat line in temp graph)→ Latent heat represents E needed for phase change→ Melting, boiling, sublimation, Evaporation is different from boiling→ Atoms/molecules gain enough KE to enter gaseous state. This may occur at any temperature.→ Rate dependent on: concentration, temperature, intermolecular forces, surface area

An gas is considered ideal under the conditions that→ Newton's laws apply to molecular behavious→ There are no intermolecular forces except during collision→ The molecules behave as elastic spheres → The molecules are in random motion→ The total volume of its molecules must be negligible compared with the volume occupied by the gasThe ideal gas laws outline the relationship between volume, pressure and temperature influence on an ideal gas→ At constant V, P/T = constant (The pressure law)→ At constant P, V/T = constant (Charles' law)→ At constant T, PV = constant (Boyle's law)The gas laws can be combined to produce one mathematical relationship: (PV)/(nT) = R, the molar gas constant.Pressure exerted by a gas on container walls can be explained as a result of collisions between the molecules and the container walls.

IB definitions:Thermal energy→ The non-mechanical transfer of energy between a system and surroundingsInternal energy→ the sum of the kinetic and potential energies of its moleculesHeat capacity → Amount of heat required to raise an object's temperature by 1 unitSpecific heat capacity (c)→ The amount of heat required to raise the temperature of an unit mass of a substance by one unit of temperature.Specific latent heat capacity→ The energy needed when changing the phase of a unit mass of substance at constant temperature Pressure→ The force normal to an area per unit areaMole→ The number of molecules as there is in 12 grams of carbon-12Molar mass→ The mass of one mole of substance

Page 3

T12. Quantum and Nuclear Physics

Key ideas:1. THE PHOTOELECTRIC EFFECTElectrons can be liberated from metals under the influence of radiation.→ This is called the photoelectric effect.→ Emitted electrons are referred to as photo electrons. Laws of Photo-Electric Emissioni. The number of electrons emitted per unit time is directly proportional to the intensity of the radiation.ii. The maximum kinetic energy of the electrons emitted increases with thefrequency of the radiationiii. There is a minimum frequency below which no emission occurs.→ ii. and iii. cannot be explained by the conventional wave theory of light.Photoelectric emission has a minimum frequency called the thresholdfrequency, f0.→ A certain minimum quantity of energy is needed to liberate an electronfrom a metal. This quantity is the work function of the metal, denoted by phi. → The minimum energy is proportional to the minimum frequency, f0.

Planck suggested the quantization of electromagnetic radiation to explain the photoelectric effect. → Light travels in 'energy packets' called photons.Einstein's photo-electric equation → The kinetic energy of an electron is proportional to its frequency minus the work function of the metal. → Evidence to support quantum theory, method to find Planck's constant.→ All extra energy above the threshold frequency will turn into the kinetic energy of the electron.Testing the photoelectric effect→ Photoelectric effect causes a current: voltage supply connected to oppose this current→ Voltage increased until current induced is reduced to 0; this is the stopping voltage→ Stopping voltage is proportional to the kinetic energy of the electrons2. MATTER WAVES AND WAVE PARTICLE DUALITYMax Planck suggested the basics of quantum mechanics: 1. Electromagnetic radiation is quantised 2. The amount of energy carried by each quantum is directly proportional to the frequency of the radiation (E = hf)Wave-particle duality describes how light and matter exhibit properties of both waves and particlesdepending upon circumstances. → De Broglie hypothesis: wavelength = h/p = h/(mv)→ De Broglie's hypothesis was confirmed by the Davisson-Germer Experiment:refraction of electrons3. ATOMIC SPECTRA AND ATOMIC ENERGY STATESClose to an atomic nucleus, a photon of the right energy can turn into particles→ Conservation of lepton number, baryon number, strangeness, etc.: a pair of an particle and antiparticle is produced.→ Minimum energy required: E=2 x (mass of electron) x c^2→ The pair production may affect an orbiting electron, which would receive a lot of kinetic energy and move out of its orbit→ The reverse process: particle and antiparticle come together to produce a photon of pure energy - annihilationIn the Bohr model of atomic energy levels, electron waves can be visualizedas standing waves 'wrapping around' the circumference of an electron orbit.→ standing waves of wavelength 2L/n→ Model of 'electron in a box', confined to move in one dimension.→ K.E. of electron found with (-13.6/n^2) eV; only certain energies 'allowed' for electrons in orbits.→ Angular momentum of the electron is also quantized: mvr=nh/(2π)Electrons can move between energy levels by emitting or absorbing a quantum of electromagnetic radiation.→ Energy possessed by quantum is equal to the energy difference. hf = delta EThe Schrodinger Model assumes that electrons in the atom can be describedby wave functions. → e has undefined position, but square of the amplitude of the wave function gives the probability of finding it at a distance r from the nucleus per unit volume of space. → Wave function Ψ is a complex numberP(r) = (Ψ^2) * delta VIn the quantum mechanical world, we cannot locate objects exactly.→ There is always an uncertainty connected to position and momentum.→ These uncertainties are related through the Heisenberg Uncertainty Principle. → The smaller the uncertainty in p, the greater the uncertainty in x and vice versa. Another such pair is time-energy→ t is the lifetime of a pair of particles produced by a photon with energy E→ The lifetime may be so short that it is practically undetectableQuantum tunneling→ The wave function gives the probability that a particle will be found in a particular region. This means there is a small but finite probability it is found where it is highly improbable to find it. → This means that physical or potential barriers do not necessarily prevent a particle moving from one place to another. → This effect might be responsible for relatively low temperature fusion taking place in stars. It is also used in the scanning tunneling microscope.4. NUCLEAR RADIUS AND NUCLEAR ENERGY LEVELS5. RADIOACTIVE DECAYThe energy spectrum of beta decay is continuous.→ Contradicts the law of conservation of energy→ 1933: new, virtually undetectable particle proposed to account for the missing energy and momentum: the neutrino→ Beta- : antineutrino Beta+: neutrinoLaw of radioactive decay→ The number of atoms that will decay per second(i.e. the activity of the sample) is proportional to thenumber present that have not yet decayed

IB definitions:Decay constant (lambda)→ The probability of decay of a nucleus per unit time→ decays per second / BequerelsHalf life→ The time taken for a number of original nuclei to decline to half of the initial valueActivity→ Number of decays per second in a sample

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