TBR electrostatics Q

[capn jazz]

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This involves a cathode ray tube - It's the Electrostatics chapter, passage X, Q 65 (P 149).

On occasion, the electrons are made to pass through the plates undeflected. If the electrons enter the region between the plates with the same direction, but with excessive velocity:

A) they will be deflected upward
B) they will be deflected downward
C) they will remain undeflected
D) they will be turned back the way they came.

The answer is B. I understand the explanation about an increase in v only affecting the magnetic force (qvB) but I do not understand how they determined they would deflect downward, especially since the - plate is down, and you would think electrons wouldn't like that.

Is this an instance of the right hand rule? Can anyone clear this up for me?

 

[Seraph 84]

There are a couple variations to the right hand rule. I'll explain the one I learned.

For a charged particle moving in a magnetic field B at some velocity will experience a force

F = qv x B (the x is called the "cross product', you may know this already, I don't know)

To determine the direction of the force on the charge take your fingers and point them in the direction of v. Next you have to "cross" that direction with the B field, so now rotate your fingers ninety degrees in the direction of the B field. You can keep your hand steady if you want, but the important part is that your thumb remains stationary. After you've crossed your fingers with the direction of the B field, point your thumb outward, and that will be the direction of the force on the charge, but only if it's positive; if the charge is negative then the force will be in the exact opposite direction what you would normally expect for a positive charge.

Example:

Say I have a stream of positively charged particles moving across my monitor from left to right and there is a magnetic field B pointing out of the screen. To find the force, we point our fingers in the direction of v and cross it with B (this example may be kinda awkward when you're doing it because you need to start with your right hand upside down, but you'll get the idea). After using the right hand rule, you should get a force pointing downward.

Now say I have a stream electrons moving in the same direction with the same magnetic field. Using the right hand rule, our thumb points downward, but since q is negative, the force is actually the opposite direction that we would expect; so the force is upward.

Hopefully that explanation helps you get the answer. But it looks like the force due to the magnetic field exceeds the force of an electric field; it just sounds like a capacitor passage, but I'm just speculating.

Hope that helps.

 

[capn jazz]

That helps a lot, thanks, but the passage, as far as I can tell, never specifies a direction for the magnetic field...

 

[Seraph 84]

That helps a lot, thanks, but the passage, as far as I can tell, never specifies a direction for the magnetic field...

Hmm, wierd. Maybe you have to deduce it using the right hand rule. I'd help you more, but I don't have my physics books yet.

Good luck.

 

[DamienPro]

The initial condition already signifies that the upward electric force is equal to the downward magnetic force. With an increase of v, the net force pointing downward, qvB - qE, causes the electrons to start deflecting downward. (Note: The direction of the magnetic field, B, in this case is always perpendicular to both v and E; it can point in and out of the page depending on where the direction of v is.)

 

[609718]

This thread is a couple of years old but I have the same exact question.

How do we determine the direction of the magnetic field? Anyone who has TBR books can you please explain this?

Thanks

 

[simmac]

bump

 

[simmac]

yes, it doesn't specify magnetic field direction so how know that there's a downward magnetic force??

 

[simmac]

anyone?..

 

[simmac]

nm, this thread helped http://forums.studentdoctor.net/threads/tbr-passage-question-physics.1083360/