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## Homework Statement

The accelerating voltage in a cathode ray tube is ##1000 V##, the electron current density is ##150 \mu A##.

a) How many electrons arrive at the screen in a second?

b) What's the magnitude of the force exerted on the screen by the electrons if they stop on collision?

(Sorry for my bad English)

## Homework Equations

## The Attempt at a Solution

a) The definition of current is ##I=\frac{Q}{t}## so if we divide the electron current density by the charge of a single electron and multiply it by 1 second that should give the answer:

##N = \frac{1,5 \cdot 10^{-4} A}{1,6 \cdot 10^{-19} C} = 9,3 \cdot 10^{14} \frac{1}{s}##

b) This is where my confusion starts. The force should be given by ##F = N \frac{\Delta p}{\Delta t}## where ##p = mv## is the momentum of one electron but I don't know what's the velocity of the electrons when they arrive at the screen. I can calculate their acceleration from the acceleration voltage like this ##a = \frac{UQ}{sm}## where s is the distance the electron travelled but that's unkown too.