 
Summary: Physics 208 Exam 2 Name_______________________________________________
You are graded on your work, with partial credit. See the last pages of the exam for formula sheets.
Please be clear and wellorganized, so that we can easily follow each step of your work.
1. A mass spectrometer is used to determine the atomic mass number of singly charged selenium ions.
(a) (7) The velocity of each ion is first determined by a velocity selector, in which the magnetic field is
1.08 T and the electric field is 2.24 !105
V/m. What is the magnitude v of the velocity of each ion
emerging from the velocity selector?
(b) (7) The ions then enter a region in which the electric field is zero and the magnetic field B is still 1.08
T. By setting the centripetal force equal to the magnetic force, derive the expression for R , the radius of the
orbit of an ion, in terms of B , the mass m of an ion, the fundamental charge e , and the speed v of an ion.
(c) (7) R for these ions is measured and found to be 15.5 cm. Calculate the numerical value of the mass m .
(d) (7) Given that 1 u = 1 atomic mass unit = 1.66 !10"27
kg, determine the atomic mass number for this
isotope of selenium.
2. Electronic flash units for cameras contain a capacitor for storing the energy used to produce the flash.
(a) (9) Suppose that the charge stored on the capacitor is 0.5 C when the applied potential difference (across
the capacitor) is 125 volts. What is the capacitance?
(b) (9) How much energy is stored in the capacitor when it is fully charged (with 0.5 C)?
(c) (9) If the flash lasts for 1/250 of a second, what is the power of the flash during that period (in watts)?
