The WHOLE of A Level Physics AQA(relevant for other boards)

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in interactions: ▼ Feynman Diagrams ▼ Electron capture Charge Baryon number • Lepton number • Energy and momentum Strangeness(only if strong interaction) 2 Physics Revision p+en+ve ▼ Electron-proton collision p+en+ve ▼ Beta-plus decay pn+e+ + ve fünf р ▾ Beta-minus decay n W р Jürf that Р e 3 np+e+ AntiVe Physics Revision Jak n ▼ Electromagnetic Radiation and Quantum Phenomena ▼ Photoelectric effect This is where photoelectrons are emitted from the surface of a metal after light above the threshold frequency is shone on it • The number of photoelectrons released depends on the intensity of light not the incident photon energy • The higher the incident photon energy, the higher the KE of the freed photoelectrons Threshold frequency The minimum frequency of light required to release an electron • Can't be explained by wave theory as Each electron absorbs a single photon • If the intensity of light is increased then more photoelectrons will be emitted per second Work Function The minimum energy required for electrons to be emitted from the surface of a metal ▾ Stopping potential The potential difference required to stop the photons with maximum KE Ek (max) = eV, 4 ▼ Fluorescent tubes • A potential difference is applied to accelerate free electrons through the tube causing them to become ionised and releasing more free electrons • These free electrons collide with the mercury atoms causing them to become excited • When the mercury atoms de-excite they release photons in the UV range Physics Revision ● The phosphorous coating absorbs the UV photons and electrons in the atoms of the coating become excited When these atoms de-excite they release photons in the Visible light frequency range ▾ Emission spectrum Passing photons from a de-excited (hot) gas through a diffraction grating will produce a line spectrum, each line represents a discrete energy level - this proves that electrons can only transition between discrete energy levels ▾ Absorption spectrum Passing white light through a cool gas will produce an absorption spectrum, there will be black lines at certain wavelengths which represent the different energy levels ▼ Wave-Particle Duality X = h/mv ▾ Wave properties Diffraction and interference of electrons e.g. fluorescent tube will produce electron diffraction interference rings ▾ Particle properties e.g. the photoelectric effect shows light travelling as discrete packets of energy interacting with electrons in one-to-one interactions as the intensity of the light doesn't affect the photoelectron KE ▼ Waves 5 ▾ Progressive waves A progressive wave transfers energy without transferring material ▼ Longitudinal waves Oscillations of particles are parallel to the direction of energy transfer • made up of compressions and rarefractions • e.g sound ▼ transverse waves Oscillations are perpendicular to the direction of energy transfer all EM waves ● Physics Revision Only transverse waves can be polarised o Polarisation reduces the oscillations down to one plane - used in sunglasses and TV antenna o If a wave is polarised by one filter and then another at 90 degrees to the first one, no light will get through ▼ stationary waves Formed from the superposition of 2 progressive waves travelling in opposite directions in the same plane with the same frequency, wavelength and amplitude • No energy is transferred by a stationary wave ▼ Coherence A coherent wave has the same frequency and wavelength and a constant phase difference ✓ Youngs Double slit • Shine a coherent source through 2 slits about the same size as the wavelength of the light • each slit acts as a coherent source therefore a pattern of light and dark fringes is produced 6 o light when the path difference is nλ (the waves meet in phase) and constructive interference occurs Physics Revision o dark when the path difference is (n + 1/2) (the waves meet out of phase) and destructive interference occurs HI S2 $1 If white light is used instead of a monochromatic laser source: o Wider maxima o a less intense diffraction pattern o Central white fringe o Spectra of colours in other fringes (violet closest to the central maxima, red furthest away) ▼ Single slit A proportional to central maxima width and fringe spacing • e.g. red light will have a wider central maximum and greater distance between maxima compared to green light as it has a higher wavelength This forms a diffraction pattern with a bright central maximum f which is double the width of other maxima, the intensity of the maxima decreases from the centre 7 Intensity Physics Revision If white light is used instead of a monochromatic laser source: • the bright white central maximum • alternating bright fringes which are spectra of colours with violet closest to the central maxima ▾ Diffraction grating Much sharper and brighter interference pattern compared to double-slit interference 1X d Single Slit Diffraction Pattern Bright Fringe: 0 2x λπ 0 Normal 8 ● Physics Revision Air n₂ m₂ Water 0 ▾ Total internal reflection Occurs when the angle of incidence is greater than the critical angle and the ray is travelling from a denser to a less dense material ident ray N-50 Refracted ray N-20 the higher the refractive index the more optically dense a material is 2nd order (n=2) 1st order (n = 1) 0₁ Zero order Critical angle 1st order (n=1) 2nd order (n=2) Total internal reflection 0₁ 0₂ ▾ Signal degradation in optical fibres ▼ Absorption Part of the signals energy is absorbed by the fibre, this reduces the amplitude of the signal which can lead to a loss of information ▼ Pulse broadening 9 The received signal is broader than the transmitted signal causing a loss of information ▾ Modal Dispersion This is caused by the light rays entering the fibre at different angles meaning that they take different paths through the fibre and arrive at the receiver at different times causing pulse broadening ▾ Material dispersion This is caused by not using a monochromatic light source meaning that light will travel at different speeds leading to pulse broadening ▼ Mechanics and materials ▾ Newtons 1st law An object will remain at rest or travel at a constant velocity until it experiences a resultant force ▾ Newtons 2nd law The acceleration of an object is proportional to the resultant force experienced by the object (f=ma) ▾ Newtons 3rd law For each force experienced by an object, the object exerts an equal and opposite force back ▼ Elastic collisions • Momentum and kinetic energy are conserved ▼ Inelastic collisions Physics Revision • Only momentum is conserved • KE converted into other forms and lost such as head sound, gpe) ▼ Density Mass per unit volume measures how compact a substance is ▾ Tensile stress Force applied per cross-sectional area 10 ▾ Tensile strain caused by tensile stress, defined as the extension over the original lengtth Paper 2 ▾ Thermal Physics ▾ Thermal energy transfer 1st: The internal energy of a body = sum of all KE and PE of all its particles and can be increased by: doing work on the system to transfer energy to it Increasing the temperature of the system Physics Revision ● ● Temperature /°C Boiling point ● Melting point Solid Liquid Internal energy /J Gas Oth: If there are 2 objects at the same temperature there is no net flow of energy between them ▾ Specific heat capacity Amount of energy required to increase the temperature of 1kg of a substance by 1K without changing its state Q = mcA0, where Q is the energy required, m is the mass, c is the specific heat capacity and 0 is the change in temp • When an object's temperature is increasing: o The mean/average kinetic energy of the molecules is increasing 11 o KE remains constant, potential energy is increasing ▾ Specific latent heat The amount of energy required to change the state of 1kg of a material without changing its temperature Q = ml, where Q is the energy required, m is the mass and I is the specific latent heat • When an object is changing state between a solid and a liquid: o The number of nearest atomic neighbours are reduced o Allows the atoms to move their centre of vibration o Breaks some of the atomic bonds ▾ Boyle's Law When the temperature is constant, pressure and volume are inversely proportional P₁V₁ = P₂V₂ Physics Revision • pV = k If the volume of a fixed mass of gas is increased and the temperature is constant then the pressure will decrease as there are less frequent collisions between molecules ▼ Charles' Law When pressure is constant, Volume is directly proportional to absolute temperature • V₁/T₁= V₂/T₂ • V/T = k If the temperature of a fixed mass of gas is increased at a constant pressure then the volume of the gas will increase as the molecules gain KE and as pressure is constant(frequency of collisions is constant) so the molecules will move further apart 12 ▼ Pressure Law When volume is constant, pressure is directly proportional to the absolute temperature • P₁/T₁= P₂/T2 ● • P/T = k When the temperature of a fixed mass of gas is increased at a constant volume, molecules gain KE so they will move more quickly and the frequency of collisions will increase therefore pressure will increase ▾ Gas Laws pV = NkT, where: • N is the number of gas molecules, k is the Boltzmann constant and T is the temp in kelvin PV = nRT, where: ● n is the number of moles of the gas, R is the Molar gas constant, T is the temp in kelvin To switch between n and N: ΝΑ Physics Revision N/n, where: NA is the avagadros constant (6.02x10^23), N is the number of gas molecules and n is the number of moles Work =pAV (area under pressure-volume graph) ▼ Kinetic Theory model Assumptions for an ideal gas: • All molecules of the gas are identical • No intermolecular forces act on the molecules • The duration of collisions is negligible in comparison to the time between collisions = • The collisions are perfectly elastic • The molecules have negligible volume compared to the container 13 • Newtonian mechanics apply • Molecules move in straight lines between collisions Physics Revision Brownian Motion is the random motion of larger particles in a fluid caused by collisions with surrounding particles and can be observed by looking at smoke particles under a microscope. Brownian motion contributed to the evidence for the existence of atoms and molecules ▾ Deriving the KE gas equation WA m 11 1. Assuming the molecule collides with the wall elastically its change in momentum is mu − (−mu) = 2mu 2. Before it can collide with the same wall again it must travel a distance of 21 so the time between collisions is t = 21/u = Amv/At: 3. Find the force using F F = 2mu/21/u= mu² / 1 14 4. p = F/A therefore p = mu²/1A = mu²/V and for all particles moving in 3d this becomes p = Nmu²/3V 5. As the gas molecules move with random motion and different speeds, the root mean square of the speed needs to be found p = Nm(cms)²/3V and this rearranges to pV = 1/3(Nm(c,ms)² ▾ Gravitational Fields ▾ Newton's Law of Gravitation Gravity acts on any objects which have mass and is always attractive • The magnitude of the gravitational force between 2 masses is directly proportional to the product of the masses and inversely proportional to the square of the distances between them F = Gm₁m₂/r² ▾ Gravitational field strength Can be uniform: • The same gravitational force on a mass everywhere in the field • g = F/m Physics Revision Or radial: • Force exerted depends on the position of the object in the field = GM/p² ▾ Gravitational potential (V) • g = The work done per unit mass when moving an object from infinity to that point Gravitational potential is 0 at infinity • As an object is moved from infinity energy is released and gravitational potential energy reduces therefore it is always negative V = -GM/r (for a radial field) ▾ Gravitational potential difference (AV) The energy needed to move a unit mass between two points ● 15 Work = mAV o same as GPE = GMm/r g=-AV/Ar ▼ gravitational potential (V) against distance graph (r) • The gradient of the tangent to curve = -g ▼ gravitational field strength (g) against distance graph (r) • the area under the curve = AV ● ▼ Equipotential surfaces Equipotential lines are at 90 degrees to field lines These are surfaces which are created through joining points of equal potential together • Gravitational potential on an equipotential surface is constant everywhere • so Change in V is 0 so no work is done ▼ Escape velocity The velocity at which an object's kinetic energy is equal to its gravitational potential energy • v = √2GM/r o NEED TO REMEMBER THIS, can derive by setting KE equation to GPE equation ▾ Kepler's third law The square of the orbital period (T) is directly proportional to the cube of the radius (r) T² a r³ Physics Revision • Derived by setting centripetal force to equal gravitational force and replacing v with 2πr/T as 2πr = circumference and v = d/t ▾ Orbiting satellites 16 The total energy of a satellite = KE + PE • This is constant as if height decreases then GPE decreases but KE increases as it starts moving faster and vice versa ▼ Synchronous orbit The orbital period of the satellite is equal to the rotational period of the object its orbiting ● Physics Revision • One orbiting the earth would have an orbital period of 24 hrs ▾ Geostationary satellites • Type of synchronous satellite • Always stay above the same point as they orbit directly above the equator good for TV and phone signals ▼ Low-orbit satellites • Have significantly lower orbital radius than geostationary satellites therefore they travel much faster • The orbital period is much smaller (a few hours) • Earth moves relative to orbit . Good for monitoring weather and imagery ▼ Electric Fields ▼ Coulomb's Law The magnitude of the force between 2 point charges in a vacuum is directly proportional to the product of their charges and inversely proportional to the square of the distance between the charges F = Q1Q2/4Teor² ▼ Electric Field strength (NC-1) Force per unit charge experienced by an object in an electric field . Constant in a uniform field 17 • E = V/d • E = F/Q • Varies in a radial field • E = Q/4π€or² • Field Lines go from positive to negative ▼ Electric potential (V) The potential energy per unit charge of a positive point charge at that point in the field • Greatest at the surface of the charge Decreases as the distance from the charge increases ● Physics Revision • 0 at infinity • V = Q/4π€or If the potential difference is positive, the charge is repulsive If the potential difference is negative, the charge is attractive • The gradient of the tangent to a Potential difference(V) against distance(r) graph gives the electric field strength at that point(E) ▼ Electric potential difference(AV) The energy needed to move a unit charge between two points • AW = QAV Fields have an equipotential surface, when a charge moves along an equipotential surface, no work is done • Area under a electric field strength(E) against distance(r) graph is potential difference (AV) ▼ Electric potential energy = Q1Q2/4π€or W = • not given but you know electric potential is work done per unit charge therefore eclectic potential energy = electric potential x charge 18 • OR W=Fd, multiply force between 2 point charges by r to get the same equation ▼ Capacitance The charge stored (Q) by a capacitor per unit potential difference C = =Q/V ▾ Parallel plate capacitors Capacitor stores charge Can be separated by a dielectric(an insulating material) o Formed by polar molecules (one end positive, one end negative) which align themselves with the field ● Physics Revision o Each polar molecule will have its own electric field which opposes the electric field between the parallel plates therefore reducing that field o So, when a dielectric is added, the potential difference required to charge the capacitor decreases so capacitance increases 0 €₁= € / €0 • When connected to power, opposite charges build up on the two parallel plates forming an electric field • C = A€0€r/d ▼ Energy stored by capacitor Current (1) Potential difference (V) Charge (Q) Charging 1 = 1₂ e RC V = V₁(1 - eRC) Q=Q₁(1-e c) е Discharging 1 = 1₂ e RC V = V₁₂ e RC Q=Q₂e™ RC You can calculate the time constant(product of resistance and capacitance) from graphs of current charge and voltage against time 19 Time constant is the value of time taken to: • Discharge a capacitor to 1/e (0.37) of its initial value (of charge, current or voltage) Charge a capacitor to 1-1/e (0.63) of its initial value (of charge or voltage) Physics Revision 0.371 RC V₂ ● 0.63V RC Time to half is the time taken for the current, charge or potential difference of a capacitor to discharge to half the inital value •T(1/2) = 0.69 RC ▼ Magnetic Fields ▼ Cyclotron • used to produce radioactive tracers or high energy beams of radiation used in radiotherapy Made up of 2 hollow semecircular electrodes with a uniform magnetic field applied perpendicular to the plane of the electrodes and an alternating potential difference applied between the electrodes Charged particle fired into one of the electrodes - magnetic field makes them follow a circular path • At the end of that electrode the particle is accelerated by the potential difference between the electrodes • The velocity of the particle has increased so now it will have a larger radius 20 • Potential difference reversed so the particle is accelerated again before entering the next electrode ▾ Faradays law The induced EMF is equal to the rate of change of flux linkage • A change of 1Wb per second will induce a EMF of one volt ▼ Lenzs law The induced EMF is always in such a direction as to oppose the change in flux linkage that caused it (the motion that caused it) ▼ Magnetic Flux density (B in Tesla) The force on 1m of wire carrying a current of 1A at right angles to the magnetic field • The measure of the strength of a magnetic field number of field lines per unit area ▼ Magnetic flux (Wb) Total number of field lines • EMF induced if a conductor cuts the field lines as they experience a change in flux ▾ Alternator (AC) A generator of alternating current - convert KE to electrical energy by rotating coil in magnetic field Alternating current s always changing • Needed in transformers ▼ UK MAINS VOLT 230V RMS ▼ Transformers Used to change the size of a voltage for an alternating current • AC flowing in primary coil produces magnetic flux Physics Revision 21 ● o this is passed through(directed by) the iron core to the secondary coil therefore a EMF is induced in both primary and secondary coils (from faradays law) STEP UP - low voltage input to high voltage (more turns on secondary coil) STEP DOWN - high voltage input to low voltage (more turns on primary coil) ▼ Transformer efficiency issues Energy lost due to: Physics Revision As current is alternating the field is changing so change in flux linkage ● Eddy currents (looping currents inside the core) as they act against the field that is inducing them reducing the field strength so dissipate energy by generating heat o Can be reduced by a laminated core with layers of insulation Resistance in coils as this generates heat o Thick copper wires minimise this as they have low resistance o Power wasted from resistance is P = I²R • Work done magnetising and demagnetiseing the core o Reduced by having a soft iron core Nuclear Physics ▾ Gamma radiation uses As a detector o Radioactive source with short half life in order to reduce exposure injected into a patient and then gamma radiation detected to help diagnose patients • To sterislise surgical equipment • Gamma radiation will kill any bacteria present in the equipment, the ionising radiation doesn't effect the nucleus so the equipment wont get radioactive • In radiation therapy 22 • Used to kill cancerous cells in a targeted region of the body ▼ Handling radioactive sources • Use long handled tongs to move the source • Store source in a lead lined container keeping the source as far away as possible from yourself and others • Never pointing source towards others ▾ Sources of background radiation Radon gas released from rocks • Artificial sources caused by nuclear weapons testing and nuclear meltdowns • Cosmic rays which enter the earths atmosphere from space • Rocks containing naturally occurring radioactive isotopes ▼ Decay constant The probability of a nucleus decaying per unit time ▾ Activity The number of nuclei that decay per second ▾ Nuclear instability Physics Revision 23 126 Physics Revision 82 50 28 14 6 N (Number of Neutrons) 6 14 28 50 ▾ Distance of closest approach Type of Decay B+ B Da Fission Proton Neutron Stable Nuclide Unknown ▾ Too many neutrons Decays through B- emission ▾ Too many protons Decays through B+ emissions or electron capture ▾ Too many nucleons Decays through alpha emission Too much energy Decays through gamma emission Stability is not uniform because more neutrons are needed to increase the distance between protons in order to decrease magnitude of EM repulsion to keep nucleus stable ▾ Nuclear radius 82 (Number of Protons) 24 ● Physics Revision Alpha particle fired at gold nucleus will have initial KE which can be measured • As it moves towards the positively charged nucleus electrostatic repulsion will slow it down o KE reduced and Electric Potential energy increased • The point at which particle stops and has no KE left is the distance of closest approach o Can calculate the distance as initial KE = final electric potential energy and then use O Ee = Q1Q2/4πeor ▼ Electron diffraction ● • Electrons are leptons so they will not interact with nucleons in nucleus through SNF • They are accelerated to high speeds so their De Brogilie wavelength is around 1fm • Directed to thin film of material in front of the screen causing them to diffract through the gaps Diffraction pattern forms a set of concentric circles with central bright spot and get dimmer as you move away from centre • Plot graph of intensity against diffraction angle from which you can find the angle of the first minimum • Then use sine = 0.61X/R to estimate the nuclear radius ▼ Mass defect Mass that is "lost" is converted into energy and released when nucleons fuse to form a nucleus ▼ Binding energy Energy required to separate the nucleus into its constituents ▼ Atomic mass unit (u) 1/12 of the mass of a carbon-12 atom 25 ▾ Nuclear Reactors ▾ Moderator slows down the neutrons released in fission reaction to thermal speeds through elastic collisions between nuclei of moderator atoms and neutrons • Water or graphite is often used • The larger the atomic mass of the moderator particles, the more collisions are needed to slow down the neutrons as less momentum is transferred if they have a larger mass ▼ Control rods Absorbs neutrons in the reactor in order to control the chain reactions • Cadmium or boron Physics Revision ▼ Coolant absorbs the heat released during fission reactions in the core of the reactor • Water, molten salt or gasses can be used as a coolant • Nuclear power stations are used as they produce no polluting gasses, they are a reliable source of power, and they need far less fuel however they produce radioactive waste and nuclear meltdowns can be catastrophic • The benefits of nuclear power outweigh the risks • U-235 is needed for fission, U-238 just absorbs neutrons and becomes unstable but doesn't split Paper 3 ▼ Astrophysics ▼ Telescopes ▼ Ray diagram for a refractive telescope in normal adjustment 26 Physics Revision objective lens ▼ Ray diagram for a Cassegrain reflecting telescope ● light rays fo principal focus convex A secondary mirror concave primary → mirror image formed at infinity eyepiece lens eyepiece lens ▼ Chromatic aberration For a given lens the focal length of red light is greater than blue therefore they are focused at different points since blue is refracted more than red o Has little effect on reflecting telescopes as this is caused by refraction 27 Physics Revision ▼ Spherical aberration The curvature of a lens/mirror can cause light rays at the edge to be focused in a different position which causes image blurring and distortion ▼ Achromatic doublet • To minimise spherical and chromatic aberration you can use a convex lens made of crown glass and a concave lens made of flint glass cemented together to bring the rays of light to focus at the same position ▼ Disadvantages of refracting telescopes 28 Physics Revision ● The glass must be pure and free from defects which can be difficult for a large-diameter lens • large lenses can bend and distort under their own weight due to how heavy they are • Chromatic and spherical aberration effect refracting lenses Incredibly heavy therefore difficult to manoeuvre • More magnification = larger diameter objective lenses with long focal lengths ● • Lenses can only be supported from the edges which can be difficult if they are large and heavy ▾ Advantages of reflecting telescopes • Mirrors which are just a few nanometres thick can be made which give excellent image quality • mirrors unaffected by chromatic aberration, and spherical aberration can be solved using parabolic mirrors • Not as heavy as lenses so they are easier to handle and manoeuvre Large primary mirrors are easier to support from behind as you don't need to be able to see through them Due to these reasons, reflective telescopes are preferred in modern telescopes ▼ Radio telescopes • Use radio waves to create images instead of light - the atmosphere is transparent to radiowaves so we can build ground-based radio telescopes • Need to be in isolated locations to avoid interference from nearby radio sources • Parabolic disc focuses radio waves onto a receiver 29 Physics Revision detector) parabolic dish ● amplifier O Simple Radio Telescope ▾ Similarities to optical telescopes • Both function in the same way (intercept and focus to detect intensity) brace • can be moved to focus on different sources or to track a moving source ● parabolic dish similar to objective mirror of reflecting telescope . Ground-based ▾ Differences between optical telescopes • Wavelengths of radiowaves are much larger than the wavelength of visible light so radio telescopes have to be much larger in diameter in order to achieve the same quality (larger diameter = larger collecting power) Cheaper and simpler to construct as a wire mesh is used instead of a mirror • Must move across an area to build up an image 30 Physics Revision ● Experience a large amount of man-made interference ▼ IR telescopes • Use infrared radiation to create images ● Large concave mirrors focus radiation onto the detector • The detector has to be cooled to almost absolute 0 by cryogenic fluids as IR radiation is emitted as heat • Must be well shielded to avoid thermal contamination from nearby objects as well as its own IR emissions • Must be launched into space as the atmosphere absorbs most IR radiation • Used to observe cooler regions in space ▼ UV telescopes • Use ultraviolet radiation to create images • The ozone layer blocks all UV rays with X < 300nm, therefore, must be positioned in space • Utilise the Cassegrain system to bring rays to focus which are detected by solid-state devices which use the photoelectric effect to convert UV photons to electrons • Used to observe the interstellar medium and star formation regions ▼ X-ray telescopes • Use X-rays to create images All X-rays absorbed by the atmosphere therefore must be positioned in space • Combination of parabolic and hyperbolic mirrors is used (must be smooth so x-rays don't pass through) • Used to observe high-energy events such as active galaxies, black holes and neutron stars ▾ Gamma telescopes 31 ● Physics Revision • • Use gamma radiation to create images use a detector made of layers of pixels as they would just pass through mirrors Have to be in space Observe events such as solar flares, gamma-ray bursts, quasars and black holes Advantages of larger diameter telescopes ▼ Collecting power A measure of the ability of a lens or a mirror to collect incident EM radiation • Increases with the size of the objective lens/mirror • Directly proportional to the area of the objective lens o collecting power a (objective diameter)^2 The greater the collecting power the brighter the image produced ▾ Resolving power The ability of a telescope to produce separate images of close-together objects • For an image to be resolved the angle between the straight lines from the earth to each object must be at least the minimum angular resolution (0 = X/D) • AKA Rayleigh Criterion o two objects will not be resolved if the central maximum of either of the images falls within the first minimum diffraction ring of the other ▼ CCDs - Charge-coupled devices Become charged when they are exposed to light by the photoelectric effect • CCDs are more useful for detecting finer details and producing images which can be shared and stored ▾ Quantum efficiency 32 The percentage of incident photons which are absorbed CCD - 80% Physics Revision Human eye- 4-5% ▾ Spectral range The detectable range of wavelength of light CCD - IR, UV, Visible Human eye - Visible only ▼ Pixel resolution Total number of pixels used to form an image on a screen, more, smaller pixels = more clear image CCD-50 megapixels Human eye - 500 megapixels ▼ Spatial resolution Minimum distance 2 objects must be apart in order to be distinguishable CCD-10 um Human eye- 100 um ▼ Convenience How easy images are to form and use CCD-Needs to be set up but images are digital Human eye - Simpler to use as there is no need for extra equipment Classification of stars ▼ Luminosity The rate of light energy released / power output of a star ▼ Intensity The power received from a star per unit area (luminosity per unit area) 33 Physics Revision • Follows an inverse square law - intensity is inversely proportional to the square of the distance between the stars ▾ Apparent magnitude (m) How bright the object appears in the sky • Depends on the star's luminosity and distance from Earth • classified as the Hipparcos scale (brightest 1, faintest 6) • the intensity of a magnitude 1 star is 100x the intensity of a magnitude 6 star Hipparcos scale is logarithmic, if the magnitude changes by 1, the intensity changes by a ratio of 2.51 (mag 5 star is 2.51x brighter than mag 6 star) ▾ Absolute magnitude (M) This is the apparent magnitude of the star if it were placed 10 parsecs away from the earth • therefore, doesn't depend on distance from the Earth M = = 5log(d/10) • where d is in parsecs m - ▾ Parallax The apparent change of position of a nearer star in comparison to a distant star in the background, as a result of the earth orbiting the sun • Measured by the angle of parallax (0) • The greater the angle of parallax the closer the star is to the earth 34 Physics Revision ▼ Units Distant stars ▾ Parsec "," Near star 1 AU Apparent parallax motion of near star Parallax angle = 1 arc second = {" 1 Parsec Earth's motion around Sun ▼ Astronomical Unit (AU) • The average distance between the centre of the earth and the centre of the sun • 1AU 1.50 × 10¹¹ m • The distance at which 1AU subtends an angle of 1 arcsecond ( 1/3600°) - can be written as 1" • Can also be said to be the distance at which the angle of parallax is one arcsecond 1pc = 2.06 × 105 AU = 3.08 × 10¹6 m = 3.26ly 35 Physics Revision Distant stars ▾ Black body radiation Near star 1 AU Apparent parallax motion of near star Parallax angle = 1 arc second asun d=tone toevo As small angle 7 Earth's motion around Sun :: d=1² 1 e T rad EARTH $its ⇒d=1 pc arcsec ▾ Light year The distance that an EM wave travels in a year in a vaccum 1ly = 9.46 × 10¹5 m A perfect emitter and absorber of all possible wavelengths of EM radiation • Stars can be approximated as black bodies ▾ Stefan's Law The power output (luminosity) of a star is directly proportional to surface area (A) and its absolute temperature^4 P = 0 AT4 • used to compare power output, temperature and sizes of stars ▼ Wein's Displacement Law 36 Physics Revision 37