Modern Physics and Fields
Modern Physics and Fields covers photons, the photoelectric effect, wave–particle duality, nuclear decay and the field concepts of gravitation and electrostatics.
Photons and the photoelectric effect
A photon has energy E = hf = hc/λ. Emission occurs only if hf ≥ φ, the work function. Then hf = φ + KEmax, and the stopping potential satisfies eVs = KEmax. Threshold frequency f₀ = φ/h. Increasing intensity increases the number of photons per second, so the saturation current rises, but KEmax depends only on frequency.
Matter waves and nuclei
de Broglie wavelength λ = h/p = h/(mv). Activity A = λN; after n half-lives the remaining fraction is (1/2)ⁿ. In alpha decay A falls by 4 and Z by 2; in beta-minus decay Z rises by 1.
Fields
Gravitational field strength g = GM/r²; electric field strength E = F/q. Field lines point in the direction of the force on a positive test charge, or towards a mass. Potential and potential energy are scalar quantities.
Worked example
Light of wavelength 400 nm strikes a metal of work function 2.0 eV. E = hc/λ = (6.63 × 10⁻³⁴ × 3.00 × 10⁸)/(4.00 × 10⁻⁷) = 4.97 × 10⁻¹⁹ J. In eV: 4.97 × 10⁻¹⁹ / 1.60 × 10⁻¹⁹ = 3.11 eV. KEmax = 3.11 − 2.0 = 1.11 eV = 1.78 × 10⁻¹⁹ J.
Traps
Using wavelength in nm without converting; mixing joules and electronvolts; claiming brighter light gives faster electrons; treating half-life as a linear decrease; confusing field strength with potential.