The concept of thermionic emission is the basis of modern X-ray tubes, which are

Question

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 8.00 × 10-7 C. How many electrons, ne, has the filament lost? If the filament is 1.25 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.)

Explanation / Answer

a)
Q = ne*e
8*10^-7 = ne*(1.6*10^-19)
ne = 5.00*10^12
Answer: 5.00*10^12 electron has the filament lost

b)
ne = 5.00*10^12
mass of 1 electron, me = 9.109*10^-31 Kg = 9.109*10^-28 g
total mass if electron, m = ne*me = 5.00*10^12 * 9.109*10^-28 = 4.55*10^-15 g
ef = mass of electron/mass of tungsteen
     = (4.55*10^-15) / (1.25)
      = 3.64*10^-15
Answer: 3.64*10^-15

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