Question

The electron beam in a television picture tube travels a distance l in evacuated space of the tube. If the speed of the electrons is v and the beam current is i, then the number of electrons in the beam at any instant will be

• A. $\frac{il}{ev}$
• B. $\frac{i}{lev}$
• C. $\frac{v}{ile}$
• D. $\frac{e}{ilv}$

Answer 1

Answer: (A)

$\frac{il}{ev}$

By definition the current is the flow of charge in time t,

Let, n be the number of electrons each having charge e then

$i=\frac{ne}{t}$

$n=\frac{it}{e}$ .......................(1)

The speed of electrons in the electron beam traveling a distance l in

evacuated space of the tube is

$v=\frac{l}{t}$

$\therefore t=\frac{l}{v}$

Using this in equation (1) we get

$n=\frac{il}{ev}$