# 1-The concept of thermionic emission is the basis

1-The concept of thermionic emission is the basis of modern X-ray tubes, which are the workhorse of a medical X-ray image. In this process a filament (a piece of metal in a vacuum) is heated by the application of electrical current. The heat (energy) is transferred to the \'free\' electrons of the metal which allow some electrons to escape from the surface. After a short period of operation the filament has lost enough electrons to give it an overall charge of 9.50 × 10-7 C. How many electrons, ne, has the filament lost? If the filament is 1.40 g of tungsten, what fraction of electrons, ef, does this represent? (HINT: The atomic number of tungsten is 74.0, the atomic weight is 183.8 g/mol and Avogadro\'s constant = 6.02 × 1023 1/mol.)

2-A 0.00275-kilogram air inflated balloon is given an excess negative charge, q1 = -4.00 × 10-8 C, by rubbing it with a blanket. It is found that a charged rod can be held above the balloon at a distance of d = 0.0680 m to make the balloon float. In order for this to occur, what polarity of charge must the rod possess? How much charge, q2, does the rod have? Assume the balloon and rod to be point charges. The Coulomb force constant is 1/(4π ε0) = 8.99 × 109 N·m2/C2 and the acceleration due to gravity g = 9.81 m/s2.

3-Two particles, one with charge -4.61 × 10-6 C and one with charge 4.39 × 10-6 C, are 0.0659 meters apart. What is the magnitude of the force that one particle exerts on the other?

Two new particles, which have identical positive charge q3, are placed the same 0.0659 meters apart, and the force between them is measured to be the same as that between the original particles. What is q3?

• Marco Lee
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