Problem 18.25: Cathode-ray tube. A cathode-ray tube (CRT) is an evacuated glass
ID: 2288385 • Letter: P
Question
Problem 18.25: Cathode-ray tube.
A cathode-ray tube (CRT) is an evacuated glass tube. Electrons are produced at one end, usually by the heating of a metal. After being focused electromagnetically into a beam, they are accelerated through a potential difference, called the accelerating potential. The electrons then strike a coated screen, where they transfer their energy to the coating through collisions, causing it to glow. CRTs are found in oscilloscopes and computer monitors, as well as in earlier versions of television screens.
Part A
If the accelerating potential is 93.0V , how fast will the electrons be moving when they hit the screen?
v = m/sExplanation / Answer
At the beginning, we can assume that they have no energy.
As they move through the electric field, they gain kinetic energy.
The energy they gain is the product of the voltage (potential difference) and their charge.
i.e. 93V x e, where e=1.6x10^-19 Coulombs.
This is also known as 93eV,
where eV is a unit of energy equivalent to 1.6x10^-19 Joules. Using the usual formula for velocity ( E=(mv^2)/2 or v = sqrt(2E/m) ),
we find that the velocity is 5,720km/s.
5.72 x 106 m/s Ans.