What Is the Problem with the Electron's Travel Distance in the Field?

[ksle82]

Anybody body know how to do this problem or have an idea what the problem is about.

See attached for diagram and question. I would like to understand and possible solve the problem if anyone can help.


https://www.physicsforums.com/attachment.php?attachmentid=7406&stc=1&d=1154053940
electron.JPG

 

[HallsofIvy]

You have an electron moving through a magnetic field. The problem asks two things: to determine what properties affect the motion and how far up or down the electron will move during its flight.

 

[andrevdh]

The electron is moving at some speed Vo to the right towards the screen of a TV. On its way it passes below a conducting plate (indicated by the characters TV). This plate can be charged electrically. The rest of the TV structure is electrically neutral (earthed) and will not cause any deflection in its motion.

 

[Chi Meson]

Opinion number three:

the electron has a known charge and a known mass. It passes through an ELECTRIC field of a capacitor (air filled) that is at a certain voltage, and a certain distance between plates. V=Ed . [even though the bottom plate is grounded, there still is a potential difference between the two plates and there will be an electric field between them that can be assumed to be uniform]

As the electron moves through the field it will experience a force. F=qE . this will cause an acceleration. (you should know that equation). resulting motion will be like projectile motion.

BTW, the question says "L" while the diagram indicates "lambda." Eityher way, this is the horizontal distance traveled by the electron. Also, what the heck is with "TV" as the voltage? Maybe that's why Halls thought it was a magnetic field? What's a "Tesla-Volt" anyhow? Ah well, just state the variable voltage as "V," and no one can say it's wrong.

Edit: I just looked at the drawing again, and I am certain that it is horrifically poorly drawn. I would assume the following: d is the distance between the plates (even though it is unclear) and "L/lambda" is the distance the electron travels through the electric field, even though the plates appear to extend only halfway across "L." You could decide to assum that the filed exists only for L/2, but make it clear that it is an assumption according to the drawing; there is no precise indicator of this actual length.

 

[Office_Shredder]

Edit: I just looked at the drawing again, and I am certain that it is horrifically poorly drawn. I would assume the following: d is the distance between the plates (even though it is unclear) and "L/lambda" is the distance the electron travels through the electric field, even though the plates appear to extend only halfway across "L." You could decide to assum that the filed exists only for L/2, but make it clear that it is an assumption according to the drawing; there is no precise indicator of this actual length.

It's not drawn poorly, it's just not standard is all.

d is the distancebetween the plates, d/2 is indicating that the electron is halfway between the plates. L/lamda is the distance it travels, you can see there is a line drawn out to either side of lamda... it's labelled in the middle of the distance it measures.

And on the TV: It says TV because the electron is moving through a TV capacitor

 

[andrevdh]

The charge on the upper TV plate will be mirrored by a charge of opposite magnitude on the lower plate (bad drawing - the intention was probably to have a separate earthed plate at the bottom and also Earth the front of the screen separately to get rid of the arriving electrons, so there should be a break between the screen and bottom earthed plate, or the screen is assumed to be nonconductive). This will then in effect produce a uniform electric field between the top and bottom earthed plate.

 

[Chi Meson]

It's not drawn poorly, it's just not standard is all.

d is the distancebetween the plates, d/2 is indicating that the electron is halfway between the plates. L/lamda is the distance it travels, you can see there is a line drawn out to either side of lamda... it's labelled in the middle of the distance it measures.

And on the TV: It says TV because the electron is moving through a TV capacitor

I see that, but there is still a line that is clearly the upper plate. THis line extends some distance that is approximately half of the distanc "L."

And if the "TV" stands for "television," all I can say is that it is an odd-as-heck place to put it (on the upper plate, where one is expecting a "V" to refer to voltage). I'm NOT saying you are wrong on this point, I'm just giving my reasons for saying that this problem is poorly drawn/presented. If there was an intent to state that this was a television, it should have said so in the text. Even still, this problem is a generic "charge moving through electric field" problem, and there is no additional information that comes from knowing it is a television and as presented the "TV" is confusing and distracting.

 

[andrevdh]

Continuing with this badly designed problem the next error is that the indicated distance, lambda, is such that for a portion of the distance the electron will travel outside of the field. There is no indication of what horizontal distance the electron travel while in the electric field. One will therefore have to introduce one's own symbol for this distance.

 

[Chi Meson]

Continuing with this badly designed problem the next error is that the indicated distance, lambda, is such that for a portion of the distance the electron will travel outside of the field. There is no indication of what horizontal distance the electron travel while in the electric field. One will therefore have to introduce one's own symbol for this distance.

Well yeah, that's what I said. Twice!