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CaptainEvil
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Homework Statement
Consider an atom of mass m bonded to the surface of a much larger immobile body by
electromagnetic forces. The force binding the atom to the surface has the expression
F = eacosz + bsinz + dtanz
where a, b and d are constants and z is positive upwards. The equilibrium point is defined
to be the origin, so z=0 there. Ignore any motion except in the vertical direction. The
whole assembly is subject to normal Earth gravity.
a) For small oscillations, give an approximate expression for the binding force on the
atom.
b) What restrictions are there on the values of a, b and d so that the force on the atom
actually is a restoring force and the atom can reach a stationary equilibrium?
c) What is the angular frequency ω0 and frequency ν0 of oscillation of this undamped
system?
d) Would the oscillation frequency change if there were no gravity? Why?
e) Subject to the considerations above, if the atom has a mass of 1 atomic mass unit,
30 b = −1.658×10 and 4 d = −3.2361×10 N, what is the frequency ν0 of the oscillation?
f) Suppose that a photon with this frequency that is incident on this atom would be
absorbed. What wavelength does this correspond to? What part of the spectrum does it
fall in?
Homework Equations
The Attempt at a Solution
I have completed the problem, and think I'm correct. Any confirmation, thoughts would be greatly appreciated!
a) in the z component, we can simplify the expression as a taylor series, disregarding the z terms with higher power terms than 2.
F(z) = F0 + (dF/dz)0z
F0 = 0 at equilibrium so F(z) is simply the first derivative of F evaluated at 0
=(bea + d)z
b) restoring force means that k (spring constant) must be negative.
so bea + d < 0
re-arranging yields that d < 0, b > 0 and a < ln(-d/b)
c) w0/SUB] = [tex]\sqrt{k/m}[/tex] where k = bea + d
and v0 = w0/2[tex]\pi[/tex]
d) No, because with microscopic distances and particles, electromagnetic force dominates over gravity, so gravity has a negligible effect.
e) Need to solve for a, so under equilibrium, force must balance gravity so F0 = mg. Fo from original eqn is ea so we can solve for a.
a = ln(mg) = 64.96 N
plugging in then yields vo as 6.42 x 1014 Hz, which is 467.3 nm, in the visible.
Thoughts?