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Unformatted text preview: Midterm II
Physics 7C
Fall 1999
R. Packard
Name: SID Discussion Section # Work all problems. Do not perform any numerical calculation until you
have a ﬁnal boxed algebraic answer. In order to obtain maximum partial
credit show all your work clearly. Cross out work that you do not wanted
graded. Work the easiest problem ﬁrst and then move on to harder ones. If you don’t understand what is being asked in a problem, ask the proctor
for clariﬁcation. All problems are weighted equally. Good luck. 1.
2.
3.
4. Total dx x Integral table: I Constants: c=3x108m/s, h=6.6x10'34js, e=l.6x10"9C, me=9x103‘kg,
G=6.67x10'“Nm2/kg2 l. A photon of energy 2x105eV collides with a free electron, initially at
rest. The photon emerges at an angle of 60°. A) At what angle will the
electron emerge? B) What is the emerging electron’s kinetic energy? 2. The Stanford Linear Accelerator (SLAC) accelerates electrons in a
beam tube. Assume that the electric ﬁeld along the tube is
E=7x106V/m. Calculate how much time it takes to reach 90% of the
speed of light. Hint: F=dp/dt works in special relativity. 3. In principle, Bohr’s theory can be used to describe the motion of the
Earth around the Sun. The Earth plays the role of the electron, the Sun
that of the nucleus and the gravitational force that of the electric force.
A) Derive the formula for the radii, rn, of the circular Bohr orbits for
the EarthSun system. (Derive does not mean take a formula and
change the constants. It means start from F=ma, etc. and get the result)
B) The radius of the Earth’s orbit is 1.5x1011m. What value of the
quantum number n does this correspond to. The mass of the Earth is
6x1024kg. Mass of the Sun is 2x103°kg. 4. A star whose surface temperature is 6000K is known to be 13 light
years away. Looking at the star with the unaided eye permits 29
photons per second to enter the eye. Assume the eye’s diameter is
5mm and that all of the star’s radiation is at the peak of the thermal
spectral distribution. Estimate the radius of the star. ...
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