Physics 222 Final 4/29/98 Name _____________
1eV = 1.6x10-19 J o = 8.85x10-12 m2/C2 o = 4¼x10-7 Tm/A
mp = 1.67x10-27 kg e = 1.6x10-19 C
1) An electronic smoke precipitator uses a high potential difference to maintain a uniform electric field of 10 kV/m inside an industrial smoke stack. The particles which are to be removed have a charge of 3.2x10-19 C each.
a) What is the force on each particle?







b) What potential difference must be maintained between the two sides of the stack to produce this electric field if the sides are 2 m apart? (Assume the precipitator is built like a very large capacitor, one side is positive the other negative.)







c) Suppose 1x1020 particles are trapped per second by the precipitator (each with a charge of 3.2x10-19 C). What current must flow from the precipitator to carry off the charge of these trapped particles?






d) How much work is done in moving one particle from one side of the precipitator to the other?







e) Explain the difference between electric potential and electric field?






2) A proton coming from the sun and entering Jupiter's magnetic field at the equator will travel in a circle. If the radius of the circle is found to be 2.0 m and the proton is traveling at 3x105 m/s, what must be the magnetic field at that point?















3) The changing magnetic field of a cassette tape as it passes over the head of a cassette player causes an induced electric current in the head (which is then amplified and turned into sound). Suppose the player head has 1000 turns of wire with a cross sectional area of 0.002m2.
a) Suppose the field in the tape varies from +0.001T to -0.001T in 0.1s as the tape passes over the player head. What maximum emf is induced in the coil in the head?






b) Explain why faster tape speeds make for more accurate recording of soft music.









4) Inside a TV tube electrons travel in a beam of radius 0.03m at a speed of 2000m/s. 3000 electrons pass a point in the tube per second, each with a charge of e = 1.6x10-19 C.
a) What is the current due to the beam?







b) Find the current density (current per area) for the beam.






c) Use Ampere's law to find the magnetic field at points outside the beam as a function of distance from the beam center, r.












d) Use Ampere's law to find the magnetic field at points inside the beam as a function of distance from the beam center, r.








5) In a Helium Neon laser electrons fall from an energy level of -20.66eV to a level of 22.62eV.
a) What frequency of photon is given off?




b) What is the wavelength of this photon?





c) The energy levels for electrons in an atom can only hold a certain number of electrons because of the Pauli exclusion principle. Why doesn't this principle prohibit the phonons in a laser from all being in the same energy state?


6)







a) Given the radius a and the current I, use the Biot Savart law find and expression for the magnetic field at point p.











b) Explain why the leads of the wire don't contribute.










Bonus:
What is the wavelength of your favorite color?



In the falling magnet experiment (to prove Faraday's law) what happened to the graph on the calculator when the magnet was turned over and allowed to fall? Why?






Describe Noether's contribution to the structure of physical theories.