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 1.100 × 10-6 C. How many electrons, ne, has the filament lost? If the filament is 2.45 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

ne = 1.1E-6/1.6E-19=6.88E12

ef = ne/total = 6.88E12/(2.45/183.8*6.02E23*74)=1.16E-11

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