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Flashcards · YPH11 · Topic 2
Waves and Electricity flashcards
32 cards on Topic 2, subtopics 2.3 to 2.4. Pick a subtopic or study them all.
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2.3 Waves and Particle Nature of Light
- Wave equation and period
- v = fλ, T = 1/f
- Why does polarisation show that light is a transverse wave?
- Only transverse waves have oscillations in more than one plane perpendicular to the direction of travel, so only they can be polarised. Sound (longitudinal) cannot be polarised.
- Refractive index and Snell’s law
- n = c/v n₁ sin θ₁ = n₂ sin θ₂
- Two conditions for total internal reflection
- Light travels from a medium of higher refractive index to one of lower refractive index, and the angle of incidence is greater than the critical angle (sin C = n₂/n₁).
- Coherent sources
- Sources with the same frequency and a constant phase difference. Needed for a stable interference pattern.
- Path difference for constructive and destructive interference (sources in phase)
- Constructive: path difference = nλ. Destructive: path difference = (n + ½)λ.
- How does a stationary wave form, and how far apart are the nodes?
- Two waves of equal frequency and amplitude travel in opposite directions and superpose. Adjacent nodes are λ/2 apart.
- Speed of a transverse wave on a stretched string (Core Practical 5)
- v = √(T/μ), where T is the tension and μ the mass per unit length.
- Diffraction grating equation
- nλ = d sin θ, where d is the slit spacing and n the order.
- 589 nm light falls on a grating with 500 lines per mm. First-order angle?
- d = 1/(500 × 1000) = 2.0 × 10⁻⁶ m sin θ = 589 × 10⁻⁹ ÷ 2.0 × 10⁻⁶ = 0.2945 θ = 17.1°
- Huygens’ construction
- Every point on a wavefront acts as a source of secondary wavelets; the new wavefront is the envelope of these wavelets. Used to explain diffraction.
- Photon energy, and the photoelectric equation
- E = hf = hc/λ hf = φ + ½mv²max
- Work function and threshold frequency
- Work function φ: the minimum energy needed to remove an electron from the metal surface. Threshold frequency f₀: below it, no electrons are emitted at any intensity (hf₀ = φ).
- Why does the photoelectric effect support a particle model of light?
- One photon gives all its energy to one electron, and energy is not accumulated. So below f₀ there is no emission however intense the light; above it emission is instantaneous; intensity changes only the rate of emission.
- De Broglie wavelength, and the evidence for it
- λ = h/p = h/mv. Electrons passing through a thin graphite film produce diffraction rings, and diffraction is a wave property.
- Why are line spectra discrete?
- Electron energy levels are discrete, so only photons with energy hf = E₂ − E₁ can be emitted or absorbed.
- Intensity
- Power per unit area: I = P/A.
2.4 Electric Circuits
- Current in terms of charge carriers
- I = Q/t, and I = nAvq: number density × cross-sectional area × drift velocity × charge on each carrier.
- Why does a thinner section of wire heat up more, for the same current?
- From I = nAvq, a smaller area A needs a larger drift velocity v, so electrons collide more often with the lattice and transfer more energy there.
- Resistivity, and how it differs from resistance
- R = ρL/A. Resistivity ρ (Ω m) is a property of the material; resistance R is a property of the specimen.
- Three expressions for electrical power
- P = VI = I²R = V²/R
- Filament lamp I–V characteristic
- A curve that flattens: as the current rises the temperature rises, lattice vibrations increase and the resistance rises.
- Diode I–V characteristic
- Almost no current until about 0.6 V in forward bias, then a sharp rise; effectively no current in reverse.
- Why does the resistance of an NTC thermistor fall as it gets hotter?
- Heating releases more charge carriers, and this outweighs the increased lattice vibration.
- Kirchhoff’s first law
- The current into a junction equals the current out. A consequence of conservation of charge.
- Kirchhoff’s second law
- Around any closed loop, the sum of the e.m.f.s equals the sum of the p.d.s. A consequence of conservation of energy.
- Resistors in series and in parallel
- Series: R = R₁ + R₂ Parallel: 1/R = 1/R₁ + 1/R₂
- E.m.f. vs potential difference
- E.m.f.: energy supplied to each coulomb of charge by the source. P.d.: energy transferred from each coulomb of charge in a component.
- Internal resistance equation, and the graph used in Core Practical 8
- E = I(R + r), so V = E − Ir. A graph of terminal p.d. V against current I has gradient −r and intercept E.
- A cell (e.m.f. 6.0 V, r = 0.50 Ω) drives a 5.5 Ω resistor. Current and terminal p.d.?
- I = 6.0 ÷ 6.0 = 1.0 A V = 6.0 − 1.0 × 0.50 = 5.5 V
- Potential divider output
- V_out = V_in × R₂ / (R₁ + R₂). Using a thermistor or LDR as one resistor makes the output depend on temperature or light.
- Core Practical 7
- Determine the resistivity of a material: measure length and diameter (micrometer) and use the gradient of a graph of R against length (or against l/A).