Dimensional Analysis for Hydrogen-Like Atoms

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Dimensional Analysis for Hydrogen-Like Atoms In this example we are considering atoms with a heavy nuclear charge of +Ze (Z is the number of protons) with total mass Mnucl, and a single electron with charge e and mass me Mnucl. − We'll apply the procedure for doing systematic dimensional analysis that was presented in an earlier handout to ﬁnd out about the characteristic size of the radius and energy. General Procedure 1. Determine the relevant quantities from physics considerations, often based on the equation(s) that determine how the system behaves. 2. Determine the fundamental units (e.g., [M], [L], [T ]) of each quantity. 3. Postulate an equation relating the quantities, with unknown exponents (a, b, . ). 4. Substitute units. 5. Equate exponents of [M], [L], [T ] and solve the resulting equations simultaneously. 6. Check your results by plugging back into the equations. Check whether the physics makes sense. Ok, now we apply the steps . 1. Physics determining the typical radius and energy. 2 The kinetic energy is p =2µ, where µ is the reduced mass: µ = meMnucl=(me + Mnucl). • Since me Mnucl, µ me, so the relevant quantity is me. ≈ The Coulomb potential is • k(Ze)e 1 V (r) = = (Zke2) ; − r − r so the relevant quantity is Zke2. Relativity? If so, the speed of light c would be relevant. Plan: assume not, then check • 2 speed at end of compare the energy to mec . Quantization = h¯ is relevant. (Whyh ¯ and not h = 2πh¯? In general,h ¯ is the • ) combination that appears in the Schr¨odingerequation.) 2. Units of each quantity. 1 2 2 2 E mv [M][L] [T ]− r [L] me [M] ∼ 2 ∼ ∼ ∼ 2 3 2 2 1 Zke Er [M][L] [T ]− h¯ ∆x∆p [M][L] [T ]− ∼ ∼ ∼ ∼ 1 3. Equations for each quantity of interest a 2 b c α 2 β γ r = Crh¯ (Zke ) m E = CEh¯ (Zke ) me ; where Cr and CE are dimensionless constants that we expect to be of order unity (e.g., between 1/3 and 3 or so). 4. Substitute units a 2a a b 3b 2b c radius: [L] = ([M] [L] [T ]− )([M] [L] [T ]− )([M] ) 2 2 α 2α α β 3β 2β γ energy: ([M][L] [T ]− ) = ([M] [L] [T ]− )([M] [L] [T ]− )([M] ) 5. Equate exponents of units and solve the simultaneous equations. radius: [M] : 0 = a+b+c [L] : 1 = 2a+3b [T ] : 0 = a 2b = a = 2; b = 1; c = 1 − − ) − − energy: [M] : 1 = α+β+γ [L] : 2 = 2α+3β [T ] : 2 = α 2β = α = 2; β = 2; γ = 1 − − ) − − 6. Do the results make sense? Our results for the radius and energy are: h¯2 1 r = Cr 2 = Cr a0 ; (Zke )me Z where a0 = 0:0529 nm is the Bohr radius and 2 2 2 (Zke ) me 2 ke E = CE 2 = CEZ : h¯ a0 We can verify that the trends make sense: If me (e.g. muonic atom), then r p • " # If Z (stronger Coulomb force), then r p • " # 2 Note that unless Z is large, E mec , justifying our assumption that the atom is nonrelativistic. We can check our estimates against the exact expectation values for the ground state of the hydrogen atom: Cr = 1 and CE = 1=2 = not bad! ) 2.

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