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 Times 10^-7 C. How many electrons, n_e, has the filament lost? If the filament is 2.00 g of tungsten, what fraction of electrons, does this represent?

Explanation / Answer

Charge of 1 electron = 1.6 * 10^-19

Total Charge = 8.0 * 10^-7

No of Electrons, = (8.0 * 10^-7 )/ (1.6 * 10^-19 )
No of Electrons lost , = 5* 10^12 electrons

Atomic mass of Tungsten = 183.8
183.8 gm = 6.022 *10^23
2.0 g = (6.022 *10^23)/ 183.8 * 2.0
N = 6.55 * 10^21

Total Electrons, = Atomic Number * N
Total Electrons, = 74.0 * 6.55 * 10^21
Total Electrons, = 4.847 * 10^23


Fraction of electrons = (5.0*10^12)/(4.847 * 10^23) = 1.03 * 10^-11

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