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A simplified model of a hydrogen atom is that the electron cloud is a sphere of radius Rwith uniform charge density and total chargee. (The actual charge density in the ground state is nonuniform.) See Figure 15.74.

a) For the uniform-density model, calculate the polarizability of atomic hydrogen in terms of R. Consider the case where the magnitude Eof the applied electric field is much smaller than the electric field required to ionize the atom. Suggestions for your analysis: Imagine that the hydrogen atom is inside a capacitor whose uniform field polarizes but does not accelerate the atom. Consider forces on the proton in the equilibrium situation, where the proton is displaced a distance from the center of the electron cloud ( s<<Rin the diagram). (b) For a hydrogen atom, Rcan be taken as roughly 110-10m(the Bohr model of the H atom gives R=0.510-10m). Calculate a numerical value for the polarizability of atomic hydrogen. For comparison, the measured polarizability of a hydrogen atom is =7.410-41Cm(N/m); see the note below. (c) If the magnitude Eof the applied electric field is 1106N/m,use the measured value of to calculate the shift shown in Figure 15.74. (d) For some purposes it is useful to model an atom as though the nucleus and electron cloud were connected by a spring. Use the measured value of to calculate the effective spring stiffness ksfor atomic hydrogen. For comparison, measurements of Young鈥檚 modulus show that the effective spring stiffness of the interatomic force in solid aluminum is about 16N/m. (e) If 伪 were twice as large, what would ksbe?

Note: Quantum-mechanical calculations agree with the experimental measurement of 伪 reported in T. M. Miller and B. Bederson, 鈥淎tomic and molecular polarizabilities: a review of recent advances,鈥 Advances in Atomic and Molecular Physics 13, 1鈥55, 1977. They use cgs units, so their value is 1/(4蟺蔚0 ) greater than the value given here.

Short Answer

Expert verified

(a) The expression for the polarization of atomic hydrogen in terms ofR is 40R3.

(b) The value of the polarization for the atomic hydrogen is11.12110-41Cm/N/C and it is very close to measured polarization.

(c) The shift of the proton is 4.62510-16m.

(d) The value of the value of the effective string stiffness is 346N/m.

(e) The value of the effective string stiffness for twice measured value of polarization is 173 N/m.

Step by step solution

01

Write the given data from the question.

The radius of the electron cloud from the hydrogen atom is R.

The total charge is e.

The magnitude of the applied electrical field is E.

The polarization of the atomic hydrogen is .

The effective stiffness string for atomic hydrogen is ks.

02

Determine the formulas to calculate the expression and value of polarization of atomic hydrogen, shift of the proton and stiffness for atomic hydrogen.

The expression to calculate the applied electric field is given as follows.

E=14蟺蔚0esR3.........i

Here, s is the displacement distance of the proton from the centre of the electron cloud.

The expression to calculate the shift of the proton is given as follows.

s=measuredEe......(ii)

Here, measuredis the measured polarization.

The expression to calculate the force acting electron cloud due to nucleus is given as follows.

F=eE......(iii)

The expression to calculate the force due to the stiffness of the string is given as follows.

F=kss.....(iv)
03

Calculate the expression for the polarization of atomic hydrogen.

(a)

Calculate the expression for the polarization of atomic hydrogen.

Substitute Efor into equation (i).

E=14蟺蔚0ER31=14蟺蔚0R3=4蟺蔚0R3.......(v)

Hence the expression for the polarization of atomic hydrogen in terms ofR is 4蟺蔚0R3.

04

Calculate the numerical value of the polarization of atomic hydrogen.

(b)

The radius of the electron cloud from the hydrogen atom, R=110-10m

The measured value of the polarization for hydrogen atom, measured=7.410-41Cm/N/c

Calculate the polarization for the atomic hydrogen.

Substitute 110-10mfor Rand 8.8510-12F/mfor 0into equation (v).

=48.8510-12110-103=111.2110-1210-30=111.2110-42=111.2110-41Cm/N/C

Hence the value of the polarization for the atomic hydrogen is111.2110-41Cm/N/C and it is very close to measured polarization.

05

Calculate the shift of the proton.

(c)

The applied electric field,E=1106N/C

The measured value of the polarization for hydrogen atom, measured=7.410-41Cm/N/c

Calculate the shift of the proton.

Substitute 1106N/Cfor E, 1.610-19Cfor eand 7.410-41Cm/N/cfor measuredinto equation (ii).

s=7.410-4111061.610-19s=7.410-351.610-19s=4.62510-16m

Hence the shift of the proton is 4.62510-16m.

06

Calculate the value of the effective string stiffness for atomic hydrogen.

(d)

Derive the expression for the effective stiffness by equate the equation (iii) and (iv).

eE=ksseE=kseseks=e2Ees

Substitute measuredEfor esinto above equation.

ks=e2EmeasuredEks=e2measured.....(vi)

Substitute 1.610-19Cfor eand 7.410-41Cm/N/c for measuredinto equation (vi).

ks=1.610-1927.410-41ks=2.5610-387.410-41ks=0.346103ks=346N/m

Hence the value of the effective string stiffness is 346N/m.

07

Calculate the value of the effective string stiffness for atomic hydrogen when α is twice as large.

(e)

Since the value of the polarization is twice as large therefore measured value of the polarization for hydrogen atom,

measured=27.410-41Cm/N/cmeasured=14.810-41Cm/N/c

Calculate the value of the effective string stiffness.

Substitute 1.610-19Cfor eand 7.410-41Cm/N/cfor measuredinto equation (vi).

ks=1.610-19214.810-41ks=2.5610-3814.810-41ks=0.173103ks=173N/m

Hence the value of the effective string stiffness for twice measured value of polarization is 173 N/m.

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