Phys 312 - Assignment 3 - Due 11 Feb 99

1.

1

In a desktop electrostatic air cleaner, air is driven through the "cell" by a fan that draws 55 W (when set at "high"). The cross sectional dimensions of the cell are 25 cm by 5 cm. Neglecting losses, you can compute the speed of air flow, v, from these data. Compare this computed value of v with the value "measured" in class. (At the calculated v) how many minutes does it take to process a volume of air equal to that of a typical room? (Take the room to be 4 by 4 by 2 cubic meters).

2

Grains of pollen of mass M are entrained (carried along) by air moving at speed v, as calculated in problem 1, between plates held at a potential difference V = 5500 Volts. The distance between plates is 0.6 cm and the depth of the cell is 8 cm. Will the pollen be collected for sure on the positive plate when a grain picks up the elementary charge e or does it have to be multiply charged? I am particularly concerned with oak pollen that has very small grains, only 10 micrometers in diameter (you can estimate M from this or you can find out what M really is).

3

Adapted from Tipler, page 1168). True or false (explain briefly why, or comment briefly):

1. The peak intensity of blackbody radiation varies linearly with the temperature of the body.
2. In the photoelectric effect, the current is proportional to the intensity of the incident light.
3. The work function of a metal depends on the intensity of the incident light.
4. An electron microscope is used to look at electrons.
5. The shortest wavelength of the spectrum emitted by an x-ray tube is proportional to the voltage of the tube.
6. If an atom or molecule emits photons of a certain frequency, it will also absorb photons of that frequency.
7. In the ground state of the hydrogen atom, the kinetic energy is 27.2 eV
8. The De Broglie wavelength of a proton varies inversely with its momentum.
9. The energy of a photon is proportional to its wavelength.
10. Electrons can be diffracted, neutrons can be diffracted, Helium atoms can be diffracted, benzene molecules can be diffracted, tennis balls can be diffracted, tennis shoes can be diffracted.