30 11 Solar panels consisting of combinations of photovoltaic cells use energy in the radiation received from the Sun to generate electricity. (a) An advertisement for solar panels claims that the intensity of radiation from the Sun incident at the top of the Earth’s atmosphere is more than 2 kW m−2. Assess the validity of this claim. radius of Sun = 6.96 × 108 m surface temperature of Sun = 5790 K distance from Sun to Earth = 1.50 × 1011 m (4) .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. (b) The average intensity of radiation from the Sun incident at the Earth’s surface over a 24-hour period has been determined to be 164 W m−2. Radiation from the Sun 31 Turn over (i) The average intensity of radiation from the Sun at the Earth’s surface is much less than the intensity incident at the top of the Earth’s atmosphere. Explain why. 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(ii) It is claimed that the area of solar panels needed to generate 100 GW of power is about 0.5% of the surface area of the Earth. Assess the validity of this claim. radius of Earth = 6.4 × 106 m typical efficiency of solar panels = 25% (4) .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. 32 (c) Scientists are developing a space station equipped with large solar panels. The space station would be located in a geostationary orbit. The space station would transfer energy to Earth as microwaves. (i) A space station in a geostationary orbit is above the equator and has a period of 24 hours. Explain one advantage of locating the space station in a geostationary orbit. 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(ii) Calculate the height h of the space station above the equator when it is in a geostationary orbit. mass of Earth = 6.00 × 1024 kg 24 hours = 8.64 × 104 s (4) .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. h = ....................................................... (Total for Question 11 = 18 marks) TOTAL FOR PAPER = 120 MARKS 33 Turn over List of data, formulae and relationships Acceleration of free fall g = 9.81 m s−2 (close to Earth’s surface) Boltzmann constant k = 1.38 × 10−23 J K−1 Coulomb law constant k = 1 4πε0 = 8.99 × 109 N m2 C−2 Electron charge e = −1.60 × 10−19 C Electron mass me = 9.11 × 10−31 kg Electronvolt 1 eV = 1.60 × 10−19 J Gravitational constant G = 6.67 × 10−11 N m2 kg−2 Gravitational field strength g = 9.81 N kg−1 (close to Earth’s surface) Permittivity of free space ε0 = 8.85 × 10−12 F m−1 Planck constant h = 6.63 × 10−34 J s Proton mass mp = 1.67 × 10−27 kg Speed of light in a vacuum c = 3.00 × 108 m s−1 Stefan-Boltzmann constant σ = 5.67 × 10−8 W m−2 K−4 Unified atomic mass unit u = 1.66 × 10−27 kg Mechanics Kinematic equations of motion s =  (u + v)t 2 v = u + at s = ut + 1 2 at2 v2 = u2 + 2as Forces ∑F = ma g =  F m W = mg moment of force = Fx Momentum p = mv Work, energy and power ΔW = FΔs Ek = 1 2 mv2 ΔEgrav = mgΔh P = E t P =  W t efficiency = useful energy output total energy input efficiency =  useful power output total power input 34 Electric circuits Potential difference V =  W Q Resistance R = V I Electrical power and energy P = VI P = I 2R P = V 2 R W = VIt Resistivity R =  ρl A Current I = ΔQ Δt I = nqvA Materials Density ρ =  m V Stokes’ law F = 6πηrv Hooke’s law ΔF = kΔ x Young modulus Stress σ =  F A Strain ε = Δ x x E =  σ ε Elastic strain energy ΔEel = 1 2 FΔ x Waves and particle nature of light Wave speed v = f λ Speed of a transverse wave on a string v = T μ Intensity of radiation I =  P A Power of a lens P =  1 f P = P1 + P2 + P3 + … Thin lens equation 1 u  +  1 v  =  1 f Magnification for a lens m =  image height object height =  v u Diffraction grating nλ = d sin θ Refractive index n1 sin θ1 = n2 sin θ2 n = c v Critical angle sin C = 1 n Photon model E = h f Einstein’s photoelectric equation hf = ϕ + 1 2 mv2 max de Broglie wavelength λ = h p 35 Turn over Further mechanics Impulse FΔt = Δp Kinetic energy of a non-relativistic particle Ek =  p2 2m Motion in a circle v = ωr T = 2π ω F = ma =  mv2 r a = v2 r a = rω2 F = mrω2 Fields Coulomb’s law F =  Q1Q2 4πε0r2 Electric field strength E =  F Q E = Q 4πε0r2 E =  V d Electric potential V = Q 4πε0r Capacitance C = Q V Energy stored in a capacitor W = 1 2 QV W = 1 2 CV 2 W = 1 2 Q 2 C Capacitor discharge Q = Q0e−t/RC I = I0e−t/RC V = V0e−t/RC ln Q = ln Q0 −  t RC ln I = ln I0 −  t RC ln V = ln V0 −  t RC In a magnetic field F = BIl sin θ F = Bqv sin θ Faraday’s and Lenz’s laws E =  −d(Nϕ) dt Root-mean-square values Vr ms =  V0 √2 Ir ms =  I0 √2 36 Nuclear and particle physics In a magnetic field r =  p BQ Thermodynamics Heating ΔE = mcΔθ ΔE = LΔm Molecular kinetic theory 1 2 mác2ñ = 3 2 kT pV = 1 3 Nmác2ñ Ideal gas equation pV = NkT Stefan-Boltzmann law L = σAT 4 L = 4πr2σT 4 Wien’s law λmaxT = 2.898 × 10−3 m K Space Intensity I =  L 4πd 2 Redshift of electromagnetic radiation z = Δλ λ  ≈ Δf f  ≈ v c Cosmological expansion v = H0d Nuclear radiation Mass-energy ΔE = c2Δm Radioactive decay A = λN dN dt  = −λN λ =  ln 2 t½ N = N0 e−λt A = A0 e−λt Gravitational fields Gravitational force F = Gm1m2 r 2 Gravitational field strength g = Gm r 2 Gravitational potential Vgrav = −Gm r Oscillations Simple harmonic motion F = −k x a = −ω2x x = A cos ωt v = −Aω sin ωt a = ‒Aω2 cos ωt T = 1 f  =  2π ω ω = 2π f Simple harmonic oscillator T = 2π m k T = 2π l g