Review Physic knowledge points οΌkey and difficult pointsοΌ Vector and scalar Vector: displacement, velocity, momentum, force and acceleration. (Both have magnitude and direction) Scalar: distance, speed, time and temperature. (Only have magnitude) Force and motionοΌkey and hard partοΌ Momentum: P=mv Impulse: I=Ft=m(v-u) v: final velocity u: initial velocity Conservation of momentum: total momentum before collision = total momentum after Hooke’s law: F=kx (k is spring constant, x is extension, not length of spring !!!) Boyle’s law: P1V1=P2V2 (take place when mass and temperature have no change) Sky diver Step 1: just jumps out of the plane, no air resistance, weight causes accelerates. Step 2: as velocity increases, air resistance increases, but resistance <weight, so velocity still increase. Step 3: air resistance is equal to weight. resultant force is zero, falling at constant velocity. Step 4: opens the parachute; air resistance increases dramatically. Now the resultant force is upward, (resistance > weight) so starts to decelerate. Step 5: as velocity decreases, air resistance decreases, acceleration decrease. Step 6: air resistance balances the weight again. smaller constant velocity. Turning effect of forces equilibrium Resultant force is zero. Resultant moment is zero. Centre of mass/gravity Mass concentrate on a point Gravity force acts on a point Brownian motion Gas molecules move rapidly and randomly. This is due to collisions with other lager molecules. Momentum changed, do random motion. Increase evaporation - Increase temperature. - Increase surface area, more particles close to the surface, easier to escape. - Draught the surface, escaped particles are blown away so that cannot fall back. pressure How does pressure exist Particles are moving freely and bouncing off the wall of the container (each other) and momentum is changed, exerting a force on the wall of the container (each other). Affects pressure - Mass, in a fixed volume, more particles and dense, more collisions, bigger force. - Temperature - Volume, as V increases, distance between particles is bigger, less collisions. In opposite, as V decreases, pressure increases. Thermal energy transfer Conduction Lattice: as heated, particles vibrate, jostle their neighbors and pass the heat. Metal: electrons and ions vibrate more, free electrons move freely, collide the ions and transfer the heat. Radiation - All objects emit radiation. - Black/dark matt surfaces are best absorber and emitters. - White/light and shiny surfaces are poorest absorber/ best reflectors and emitters. Sound (longitudinal wave) Speed Material Speed m/s air 330 water 1500 glass 5000 Sound travel fastest in solid, slowest in air. Light (transverse wave) Refractive index sin(lager angle) n=sin (π ππππππ πππππ) π£1 n=π£2 (light speed: 3*108) 1 n=sin π (c=critical angle) incidence angle (ε ₯ε°θ§) inflection angle(εε°θ§) incidence ray (ε ₯ε°ε ηΊΏ) inflection ray (εε°ε ηΊΏ) normal line (ζ³ηΊΏ) waves Formula Wave speed = π€ππ£ππππππ‘β ππππππ = wavelength × frequency Topic 15 spectra (ι»ζ¬) Magnetism Methods of magnetization Magnetized: - stroking them with a magnet, - hammering them in a magnetic field - putting them inside a coil with a direct current through it. Demagnetized: - hammering them, - heating them - putting them inside a coil with an alternating current through it. Electricity Electron Charge (C) = -1.6×10-19C Proton charge = 1.6×10-19C Electricity quantity I = Q/t e.m.f = E/Q (emf=voltage) p.d = E/Q P=UI=W/t=U2/R=I2R W=Pt=UIt=U2/R×t=I2Rt U=IR R=U/I R=U/I π 1×π 2×π 3…. R=π 1+π 2+π 3…. (in parallel current) Topic 19 electrical circuits Electromagnetic forces “ε·¦εε³η΅“ Identify the magnetic field in a wire. - As the coil rotates, the magnetic field through the coil changes, which induces an e.m.f. in the coil. - The magnitude of the e.m.f. is maximum when the coil is horizontal as the field lines are cut the fastest, and zero when vertical as no field lines are being cut. - The e.m.f. can be increased by increasing the number of turns on the coil, increasing the area of the coil, using a stronger magnet or increasing the speed of rotation. - Slip ring changes the direction of current, but magnetic field does not change, so the cause the situation as the diagram showed above. How a transformer work i) An a.c. in the primary coil produces a changing magnetic field in the core. ¡i) This changing magnetic field induces an a.c. in the secondary coil. iii) d.c input doesn't work. Calculation ππ ππ = ππ ππ When efficiency is 100% I1V1=I2V2 Energy lost on cables = I ×RοΌincrease voltage to reduce the energy loseοΌ 2 Atomic physics name mass speed charge alpha 4 faster +2 beta 0 slowest -1 gamma 0 fastest 0 Alpha can be reflected by thin air or 5cm skin Beta can be reflected by a sheet of aluminum of 2mm or 100cm in air Gamma can be reflected by the lead of 2 cm or 2m concrete Application of radiations i) Alpha--Smoke detectors ii) Medical treatment of beta radiation ill) Beta radiation--Thickness Control Mill iv) Medical Tracers (gamma). ) Gamma radiation---detecting cracks Using cell damage: i) gamma ray-- Radiotherapy (gamma knife) il) gamma ray--food irradiation iii gamma ray- - sterilization (medical equipment) Identify the particles movement 1.ζεΏζεζ―η΅ζ΅ζΉε/ζ£η΅θ·η§»ε¨ζΉεοΌbetaοΌοΌη΅εοΌθ΄η΅οΌεη΅ζ΅η移ε¨ζΉεηΈεγ Beta εΈ¦θ΄η΅οΌalpha εΈ¦ζ£η΅γ ζ ζεΏζεδΈΊ alpha η移ε¨ζΉεοΌηΈεζΉεδΈΊ beta 移ε¨ζΉεγ 2.εζζεδΈΊη£εΊζΉε 3.倧ζζζεδΈΊεεζΉε