A photon and an electron, each of 20 eV energy, move in free space. The ratio of the linear momentum of the electron pₑ to that of the photon p_Ph, that is (pₑ)/(p_Ph), is: (Take the speed of light =3×10⁸ m s⁻¹, the charge of the electron =-1.6×10⁻¹⁹ C and the mass of the electron =9×10⁻³¹ kg)

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(4) 225. Given: Eₑ=E_Ph=20 eV=20×1.6×10⁻¹⁹=3.2×10⁻¹⁸ J. Idea: at this energy the electron is non-relativistic, so pₑ=√2mₑ E, while a photon always satisfies p_Ph=E/c. pₑ=√2×9×10⁻³¹×3.2×10⁻¹⁸=√5.76×10⁻⁴⁸=2.4×10⁻²⁴ kg m s⁻¹. p_Ph=(3.2×10⁻¹⁸)/(3×10⁸)=1.067×10⁻²⁶ kg m s⁻¹. (pₑ)/(p_Ph)=(2.4×10⁻²⁴)/(1.067×10⁻²⁶)=225. At equal energy the massive particle carries far more momentum than the photon, which is why the ratio is a large number rather than a small one.

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