Phy 122 – Assignment 13
A. 1. Center (n) layer: donors. Donors donate electrons; with all those free
electrons, conduction is dominated by negative particles. n is for negative.
Outer (p) layers: acceptors. Acceptors take on electrons. Holes are left behind, so
conduction is dominated by positive particles. p is for positive.
2. For everyday objects like baseballs, the wavelength is extremely small. ( =
h/(mv), and m is huge compared to an electron.) Wave properties, like diffraction,
depend on , and so are far too small to measure.
3.
B.
C. 1. (a) No. The energy levels in hydrogen are –(13.6 eV)/n2 . That makes the
first and second levels -13.6 eV and -3.4 eV, with the higher levels getting closer to
zero after that. So, the electron would be starting with –13.6 eV, and there is no
level at –3.6 eV for it to end up in.
(b) Yes. This would get the electron up to +.4 eV. An energy of zero or more
knocks the electron right out of the atom. The energy of a free electron is not
quantized, so +.4 eV works just fine.
2.
D. 1. The energy gap equals the energy of the emitted photon: E = hf where f = c/ .
2. Charge may or may not build up in the center
layer, depending on whether it is drained off by
the small current Ib. (The top picture shows
holes getting stuck in the n layer of a pnp.) This
layer of charge repels additional charge trying to
flow toward it, shutting off the current. If it’s
not there, the current flows.
3. Note that it asks for the wavelength of the electron, not the wavelength of the light it
gives off if it jumps to another level.(A common mistake.)
For constructive interference, the circumference of the electron’s orbit must be a whole
number of wavelengths: 2πr = nλ.
(B should actually be exactly double A, but number rounding made them a little off.)
E. . 1.
Connecting the battery one way sucks free charges out of the junction, making it
like an insulator. (No current due to lack of anything free to flow.) The other way
dumps lots of free charges in, for the opposite effect.
2. To be absorbed by a semiconductor, a photon must make
electron jump across the gap from the valence band. That
the photon’s energy must be at least as much as the size of
gap, or there won’t be an allowed energy for it to land in.
an
means
the
So, when the electron in the H atom falls from 4 to 2, or 5 to 2, or …, CdS can absorb
the resulting photon. But, the 3 to 2 line has too little energy to get CdS’s electrons
across the gap, so those photons pass through the crystal instead. (It’s transparent to
that color, but opaque to the others.)
F. 1. a. - 90° through +90°. (-90° if the circuit is purely capacitive, +90° if purely
inductive. Various mixtures of C, L and R can give you any angle in between.)
b. 0° If XL = XC, the opposite side of this triangle has a
length of zero, making the phase angle zero.
2.