TV Cathode Ray Tube Consider a potential

Tufts
Messages
5
Reaction score
0

Homework Statement


In an old TV cathode ray tube, electrons are accelerated by a potential of 15 kV between the gun and screen. Find:

a) Kinetic energy of the electrons when they reach the screen
b) Its respective wavelength
c) Wavelength of the photons with maximum energy that are emitted from the screen.

Homework Equations



E = hf (Energy of a photon with frequency f)
f = c/\lambda (\lambda is the wavelength of respective photons)

\lambda = h/p (wavelength of electron depending on p, its momentum)

The Attempt at a Solution


a) E = e*V = 15000 eV = 15000 * 1.6 x 10-19 = 2.4 x 10-15 J

b) \lambda = h/p
But p = mv (where m = 9.1 x 10-31 kg)
We also know that 1/2 * mv2 = E
So, v = (2/m * E)1/2 => v = (2/(9.1 x 10-31)*2.4 x 10-15)1/2 = 0.73 x 108 m/s (not close enough to c, so we can use our electromagnetism approximation)

Therefore, \lambda = h/p = 6.626 * 10-34 / (9.1 x 10-31 * 0.73 x 108) = 9.97 x 10-12 m

c) Let us suppose that there is 100% efficient exchange of energy between electron and photon. E = hf => f = E/h => f = (15000 * 1.6 x 10-19)/(6.626 x 10-34) = 3.62 x 1018 Hz

But \lambda = c/f => \lambda = 8.28 x 10-11 m



To me, I am using the formulas correctly... but the wavelength of light on part C is not in the visual spectrem. Since we are talking about a TV here... that certainly does not make sense! What am I doing wrong?? Thanks before hand for all the help.

Tufts..
 
Physics news on Phys.org
Tufts said:
But \lambda = c/f => \lambda = 8.28 x 10-11 m



To me, I am using the formulas correctly... but the wavelength of light on part C is not in the visual spectrem. Since we are talking about a TV here... that certainly does not make sense! What am I doing wrong??
Nothing. CRT screens can and do emit a small amount of xrays. Phosphorescence is a complicated phenomenon by which energy of the electrons stimulates the target molecules to produce visible light.

AM
 
Thanks alot, makes sense... I do remember the professor talking about x-rays emitted from CRT screens.

Tufts..
 

Thread 'Please tell me where I am going wrong in this integral'

##|\Psi|^2=\frac{1}{\sqrt{\pi b^2}}\exp(\frac{-(x-x_0)^2}{b^2}).## ##\braket{x}=\frac{1}{\sqrt{\pi b^2}}\int_{-\infty}^{\infty}dx\,x\exp(-\frac{(x-x_0)^2}{b^2}).## ##y=x-x_0 \quad x=y+x_0 \quad dy=dx.## The boundaries remain infinite, I believe. ##\frac{1}{\sqrt{\pi b^2}}\int_{-\infty}^{\infty}dy(y+x_0)\exp(\frac{-y^2}{b^2}).## ##\frac{2}{\sqrt{\pi b^2}}\int_0^{\infty}dy\,y\exp(\frac{-y^2}{b^2})+\frac{2x_0}{\sqrt{\pi b^2}}\int_0^{\infty}dy\,\exp(-\frac{y^2}{b^2}).## I then resolved the two...
View full post »

Thread 'Direction of the magnetic field of an oscillating charge'

Hello everyone, I’m considering a point charge q that oscillates harmonically about the origin along the z-axis, e.g. $$z_{q}(t)= A\sin(wt)$$ In a strongly simplified / quasi-instantaneous approximation I ignore retardation and take the electric field at the position ##r=(x,y,z)## simply to be the “Coulomb field at the charge’s instantaneous position”: $$E(r,t)=\frac{q}{4\pi\varepsilon_{0}}\frac{r-r_{q}(t)}{||r-r_{q}(t)||^{3}}$$ with $$r_{q}(t)=(0,0,z_{q}(t))$$ (I’m aware this isn’t...
View full post »

Thread 'Initial conditions for orbits around a wormhole'

Hi, I had an exam and I completely messed up a problem. Especially one part which was necessary for the rest of the problem. Basically, I have a wormhole metric: $$(ds)^2 = -(dt)^2 + (dr)^2 + (r^2 + b^2)( (d\theta)^2 + sin^2 \theta (d\phi)^2 )$$ Where ##b=1## with an orbit only in the equatorial plane. We also know from the question that the orbit must satisfy this relationship: $$\varepsilon = \frac{1}{2} (\frac{dr}{d\tau})^2 + V_{eff}(r)$$ Ultimately, I was tasked to find the initial...
View full post »

Similar threads

I Postive rays in cathode ray tube experiments?
Replies
2
Views
1K
Obtain Cathode Ray Tubes Without X-rays
Replies
9
Views
2K
I Why Bremsstrahlung <20keV for Tungsten?
Replies
17
Views
4K
Electrons in a cathode ray tube
Replies
4
Views
2K
Can a cathode ray tube be made from a light bulb?
Replies
15
Views
5K
How do we know the mass of an electron?
Replies
28
Views
4K
The GZK Bound and Cosmic Rays: Calculating Photonic Energy in Outer Space
Replies
6
Views
4K
Cathode ray tube, Voltage needed to move the beam impact spot
Replies
3
Views
2K
What are the limitations of CRT display technology? resolution, nits, ....
2
Replies
66
Views
8K
Cathode ray tube- thermionic emission
Replies
4
Views
2K

Hot Threads

Recent Insights

Back
Top