AS Physics question.. please help!!?

Question

The electron beam in a TV tube travels 0.25m from the gun to the screen. The beam current is 4.0mA. For electrons traveling at 3.0x10^7 m s ̄ ¹ (0.1 of the speed of light), calculate the number of electrons in the beam at any instant. The charge on one electron is 1.6x10^-19C.

(^ = power of, couldn't get some numbers in superscript!)

Please could someone try and explain how to do this calculation step by step because I'm really confused!

Thanks! =)

Answer 1

The key is that current is charge per unit time. since the current is given as .004A (4.0milliamps) the charge traveling through a point is 0.004coulombs/ sec. so 0.004 / 1.6x10^-19 is the number of electrons crossing an imaginary plane per second.

so now how long are the electrons in the beam? given velocity of 3.0x10^7 m/s and distance  0.25 m, the time is 0.25/ 3.0x10^7

and finally the number of electrons in the beam is the first answer (electrons / sec) times the second answers (secs).

Answer 2

Take this with a grain of salt, it looks too easy.

1 A = 1 C/s

4.0 mA = 4.0e-3 C/s

4.0e-3 C/s  * 1 /1.6e-19 C = 2.5e16 electrons/s