unit 2 electron waves & photons

unit 2 electron waves & photons

1. CH8: currrent &charge

   • current
     • I = Q/T
       • Current I: The amount of charge passing a given point in a circuit per unit time 1A : One coulomb of charge passing a given point 1s
   • charge
     • electrical charge Q(C): a physical property,either be +ve or -ve
       • coulomb: the electrical charge flowing past a point in one second when current is 1A
       • Q = +- ne e=1.6x10^-19
         • charge carrier: proton:relative charge of +1e; electron: relative charge of -1e (e is elementary charge)
     • moving charges
       • most electrons are fixed in mental atoms ; some electrons from each atom are free to move
       • conventional current: a current from a positive terminal towards a negative terminanl
       • electrolytes: liquids that can carry an current(flow of ions)
         • cation(Na+) move towards cathode(-ve electrode) ; anion(Cl-) move towards anode(+ve electrode)
   • kirchhoff's first law: for any point in an electrical circuit, the sum of currents into that point is equal to the sum of currents out of that point ∑in = ∑out
     • conservation of charge: electric charge can either be created or destroyed,the total number of electric charge id constant
   • mean drift velocity
     • I = nAve
       • the number density: number of free charge carrier per unit volume (the higher it is, the better the electrical conductor)
       • mean drift velocity: the average velocity of electrons moving through a wire

2. CH9: energy power resistance

   • p.d. & e.m.f.
     • Ɛ = W/Q
       • electromotive force：the energy transferred from chemical energy to electrical energy per unit charge
     • V = W/Q
       • potential difference: the energy transferred from electrical energy to other forms per unit charge
   • the electron gun
     • electrical device used to produce a narrow beam of light, electrons gain energy to escape the surface of the mental(thermionic emission)
     • eV =1/2 mv²
       • work done on electron = gain in KE
   • resistance
     • R = V/I
       • Ohm's law: (for a metallic conductor) kept at a constant temperature, the current in the wire is directly proportional to the p.d. (across its ends)
         • As the wire gets hotter the positive ions vibrate more, increasing the R
   • resistivity
     • R =ρL/A R ∝ L 1/A
       • define resistivity: to describe the electrical property of a material, at a given temperature ρ = RA/L
   • graphs of I-V
     • resistor
       • 1. obey Ohm's law, ohmic conductor 2.resistance constant 3. behaves the same way regardless of the polarity
     • filament lamp
       • 1. not obey Ohm's law, non-ohmic component 2.resistance not constant 3. behaves the same way regardless of the polarity
   • special resistors
     • diode
       • only allow a current in one particular direction
         • 1. not obey Ohm's law, non-ohmic component 2.resistance not constant 3. behaviour depends on polarity
     • thermistor
       • because of the semi-conductor's negative temperature coefficient, temperature ↑R↓
     • LDR
       • darker, R ↑
   • power energy
     • (electrical)power: the rate of energy transfer by electrical component
       • P = W/t
         • P = VI P = V²/R P = I²R
     • energy
       • 1kWh = 3.6 x 10^6 J

3. CH10: electrical circuit

   • Kirchhhoff's second law: In any circuit, the sum of the e.m.f. is equal to p.d. around a closed loop ∑V = ∑ε
     • conservation of energy: energy can either be created or destroyed,the total number of energy is constant
   • internal resistance
     • internal resistance: the resistance of a source of e.m.f. due to its construction, which causes a loss in energy as the charge passes through the resource
       • current shouldn't e too high or it will raise the temperature of the cell, increasing its internal resistance
         • small internal resistance: rechargeable batteries-to be charged using high currents without overheating or wasting energy; big internal resistance: high-voltage power supplies-this acts as safety feature,preventing the power from delivering a fatal electric current
     • ε= V + Ir (= IR + Ir) emf = terminal pd + lost volts
   • potential divider
     • Vout = (R₂/R₁+R₂) x Vin V₁/V₂ = R₁/R₂
   • sensing circuit with variable resistor
     • LDR thermistor potentiometer

4. CH11: waves 1

   • progressive waves
     • transverse
       • the oscillations are perpendicular to the direction of energy transfer
         • 1.waves on the surface of water 2.electromagnetic waves 3.waves on stretched string 4.S-waves in earthquakes
     • longitudinal
       • the oscillations are parallel to the direction of energy transfer
         • 1.sound waves 2.P-waves in earthquakes
   • waves property
     • displacement ; amplitude ; period
     • c = fλ
       • wavelength ; frequency ; wave speed
     • phase difference
       • describes the difference between the displacement of particles along a wave or of particles on different waves
       • in phase
         • phase difference is zero . they both reach their max positive displacement at the same time
       • anti phase
         • phase difference is 180° . one reaches its max positive displacement at the same time as the other reaches its max negative displacement
   • reflection
     • law of reflection : angle of incidence = angle of reflection
   • refraction
     • refractive index
       • n=c/v
         • sin1/sin2 = c1/c2 = n2/n1
         • n air = 1
     • it occurs when a wave changes direction as it changes speed when it passes from one medium to another
     • the speed of water waves is affected by changes in the depth of water: when a water enters shallower water, it slows down and λgets shorter
   • diffraction
     • when waves pass through a gap or travel around an obstacle, they spread out
     • diffraction effect is most significant when the size of gap is about the same as wavelength. spacing between the wavefronts shows that there is no change in wavelength
   • Polarisation
     • the particles oscillate along one direction only
       • in longitudinal waves, the oscillation is always parallel to the direction of energy transfer, so it can't be plane polarized, their oscillations are already limited to one plane
       • sunglasses contain polarising filters, these only allow light oscillating in one plane to pass through them, reducing the glare reflected off flat surfaces like lakes
   • intensity
     • the radiant power passing through a surface per unit area
     • I = P/A = P/4πr²
     • I ∝ amplitude²
   • total internal reflection
     • n x sinc =n air x sin90

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