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Question

Points will only be awarded to full answers with complete solutions worked.

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 x 10-7 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 x 1023 1/mol.) ne= ef=

Explanation / Answer

electron charge=1.6*10^(-19) C

so no of electron lost=(8*10^(-7))/(1.6*10^(-19))=5*10^12

b)tungsten atomic number=74

so each atom has 74 electrons.

atomic weight =183.8 g/mol

==>2.45 g=0.0133 mol=8.03*10^21 atoms=5.94*10^23 electrons

so %=(5*10^12)/(5.94*10^23 )=8.4175*10^(-10) %

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