Unreasonable Results A student in a physics laboratory observes a hydrogen spectrum with a diffraction grating for the purpose of measuring the wavelengths of the emitted radiation . In the spectrum, she observes a yellow line and finds its wavelength to be 589 nm. (a) Assuming this is part of the Balmer series, determine n i , the principal quantum number of the initial state. (b) What is unreasonable about this result? (c) Which assumptions are unreasonable or inconsistent?
Unreasonable Results A student in a physics laboratory observes a hydrogen spectrum with a diffraction grating for the purpose of measuring the wavelengths of the emitted radiation . In the spectrum, she observes a yellow line and finds its wavelength to be 589 nm. (a) Assuming this is part of the Balmer series, determine n i , the principal quantum number of the initial state. (b) What is unreasonable about this result? (c) Which assumptions are unreasonable or inconsistent?
A student in a physics laboratory observes a hydrogen spectrum with a diffraction grating for the purpose of measuring the wavelengths of the emitted radiation. In the spectrum, she observes a yellow line and finds its wavelength to be 589 nm. (a) Assuming this is part of the Balmer series, determine
n
i
, the principal quantum number of the initial state. (b) What is unreasonable about this result? (c) Which assumptions are unreasonable or inconsistent?
3:09 O O O 63° A X •
N N O 5G „ll
Quizzes
a
(absorption)
Brackett
series
Paschen
series
Lyman
series
(emission)
Balmer series
Paschen
series
(emission)
n= 2
n=3
n=4
....
Lyman series
n-5
(a)
(b)
e These pictures refer to the energy levels
of a hydrogen atom.
You can find the error in both parts, (a) and
(b). The arrows labeled "emission" in (a), and
all the arrows in (b), indicate a transition in
which an electron jumps from a higher-
energy state to a lower-energy state. The
different "series" of emission lines are
characterized by the index n of the low-
energy state in which the electron ends up.
In particular, the Lyman series consists of all
transitions that end up in the n=1 energy
level, with an initial energy level that
corresponds to the label n = 2, 3, 4, 5, etc.
One of these values of n is not shown as an
arrow in the Lyman emission series in figures
(a) or (b). This is a significant error because
that particular spectral line is very important
in astronomy.
Pick the value…
In an experiment of photoelectric effect, no current flows through the circuit when the
voltage across the anode and cathode is -1.70 V. Calculate
a)
b)
If it is illuminated by ultraviolet radiation of frequency 1.70 x 1015 Hz.
5.
the work function
the threshold wavelength of the metal (cathode)
[Ans: 8.55×1019 J, 233 nm]
a) [2018] State Kirchhoff’s two Laws, and give the fundamental physical principles which each law expresses.
b) De Broglie explained the Bohr model of the hydrogen atom by attributing a wavelength λ toa particle, with the value of λ coming from the momentum p = mv as λ = h/p.i) Calculate the de Broglie wavelength of a bird (a racing pigeon) that weighs 0.350 kgand flies at 100 km per hour.ii) Equating the centripetal force on an electron orbiting around a proton with the Coulomb force gives the expression v2 = e2/(4πε0mer). Calculate the speed of an electron or- biting at the Bohr radius, rB = 0.053 nm.iii) Calculate the momentum and the de Broglie wavelength of an electron moving at the speed you obtain in (ii).iv) Compare the wavelength you obtain in (iii) with the circumference of the orbit. Com- ment on this comparison. Explain briefly what it implies about the possible orbits of the Bohr model and how the higher orbits might be predicted.
c) Thorium-234 is…
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