Electric field force & deflection

[SaintJude]

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Consider the picture showing a cathode ray above. So Kaplan explains that there is no deflection cause by the electric field by the potential difference V (left side of picture). The electrons are accelerated by antiparralel to the electric field by the potential difference V. But why?

Edit: Is this because the electric field is parallel to the electron's motion? Just as in magnetic field, is it required that the electric field is perpendicular to the electron's motion to create a deflection in its path?

 

[MedPR]

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Consider the picture showing a cathode ray above. So Kaplan explains that there is no deflection cause by the electric field by the potential difference V (left side of picture). The electrons are accelerated by antiparralel to the electric field by the potential difference V. But why?

Edit: Is this because the electric field is parallel to the electron's motion? Just as in magnetic field, is it required that the electric field is perpendicular to the electron's motion to create a deflection in its path?

Yea, before the ray hits the deflecting plates it is moving in a direction parallel to the electric field. Since its a cathode ray, it is actually moving antiparallel to the direction of the E-field.

Also, yes your edit is right. It's the same as any sort of deflection though. What happens if you kick a ball horizontally off a cliff and immediately turn off gravity? The ball will continue in the same direction as you kicked it because gravity is no longer there to deflect it downward. Parallel forces cannot deflect objects because they can't do any work in the perpendicular direction.

 

[SaintJude]

Hmm, I see. Does the magnetic or electric field have to be completely perpendicular for deflection to occur?

 

[MedPR]

Hmm, I see. Does the magnetic or electric field have to be completely perpendicular for deflection to occur?

Well I'm pretty sure the magnetic field is always perpendicular to the direction of motion -- this is why the right hand rule always works.

I don't think the electric field needs to be completely perpendicular. As long as there is some perpendicular component it will exert a force on the ray. If you draw two capacitor plates at a 30 degree angle to the horizontal and shoot a cathode ray through it, the electrons in the ray will still deflect in the direction antiparallel to the electric field because they will be attracted to the proton plate and repelled by the electron plate. At least that's what I think would happen...?

 

[SaintJude]

The magnetic field is not always perpendicular to the direction of the motion. I've seen scenarios/questions where the particle was moving in direction of the magnetic field. The charge just does not experience a force due to the magnetic field. The right hand rule doesn't work precisely b/c there is no force the charge feels.

I think you're right about the electric field. That's why a dipole, even slightly misaligned with the Electric field, will "realign" and thus change its direction according to the field lines.

 

[MedPR]

The magnetic field is not always perpendicular to the direction of the motion. I've seen scenarios/questions where the particle was moving in direction of the magnetic field. The charge just does not experience a force due to the magnetic field. The right hand rule doesn't work precisely b/c there is no force the charge feels.

I think you're right about the electric field. That's why a dipole, even slightly misaligned with the Electric field, will "realign" and thus change its direction according to the field lines.

Oh that's true. So it's just a coincidence that all the problems we do have a perpendicular mag field?

Also, does the magnetic field need to be compeltely perpendicular or is it the same as any other force with a perpendicular component?

 

[chiddler]

Oh that's true. So it's just a coincidence that all the problems we do have a perpendicular mag field?

Also, does the magnetic field need to be compeltely perpendicular or is it the same as any other force with a perpendicular component?

you do components with it. A more accurate formula to use for Bfield is q(vB)sin(theta)=F, for example.

I just read this last post. Please excuse me if this is irrelevant or not what you are asking.

/lazy

 

[SaintJude]

It might be a coincidence in TBR, but it ain't in Kaplan.

I also think that my question about electric fields didn't consider the definition of electric field lines. Electric field lines show the direction a proton would move. So, the direction of the electric field lines should guide me to seeing how a charge will adjust its path to realign itself with the direction of the elecric field lines.

For example, for the path due to the deflection plates: if the electric field lines point down, then I should have realized that the electron's path will deflect upwards.

the force due to a magnetic field is perpendicular both to its velocity and to the direction of the magnetic field.

 

[MedPR]

you do components with it. A more accurate formula to use for Bfield is q(vB)sin(theta)=F, for example.

I just read this last post. Please excuse me if this is irrelevant or not what you are asking.

/lazy

This is exactly what I was getting at. Thanks

It might be a coincidence in TBR, but it ain't in Kaplan.

I also think that my question about electric fields didn't consider the definition of electric field lines. Electric field lines show the direction a proton would move. So, the direction of the electric field lines should guide me to seeing how a charge will adjust its path to realign itself with the direction of the elecric field lines.

For example, for the path due to the deflection plates: if the electric field lines point down, then I should have realized that the electron's path will deflect upwards.

the force due to a magnetic field is perpendicular both to its velocity and to the direction of the magnetic field.

Yea you're right on all counts. I was saying the same thing you just did about magnetic fields, but I think I wasn't clear/I got confused. A particle will create a mag field perpendicular to its velocity, but it is possible to introduce a charge into an existing mag field and have it move in a non-perpendicular direction to that mag field. However, that particle will still create its own perpendicular mag field.

 

[SaintJude]

Yea you're right on all counts. I was saying the same thing you just did about magnetic fields, but I think I wasn't clear/I got confused. A particle will create a mag field perpendicular to its velocity, but it is possible to introduce a charge into an existing mag field and have it move in a non-perpendicular direction to that mag field. However, that particle will still create its own perpendicular mag field.

Actually, thank you for stating this. I think I ignored the distinction between the existing and a charge's own magnetic field created by its motion.

 

[milski]

The force from a uniform magnetic field on a particle moving in that field with no other forces acting on it is perpendicular to its velocity. Changing the field or applying other forces on the particle will make it non-perpendicular. The former is still an interesting and easy to create situation, so you get plenty of problems about it.


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[MedPR]

The force from a uniform magnetic field on a particle moving in that field with no other forces acting on it is perpendicular to its velocity. Changing the field or applying other forces on the particle will make it non-perpendicular. The former is still an interesting and easy to create situation, so you get plenty of problems about it.


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This is what I originally thought as well, until Saint Jude mentioned something a few posts up. Say you have an existing magnetic field pointing into the page. If you wanted to, you could introduce a charge moving at a constantly velocity out of the page and parallel to the existing magnetic field. This new charge (the one moving out of the page), however, would create a new magnetic field perpendicular to itself. So, you can move a charge through a magnetic field in a non-perpendicular direction, but the magnetic field created by a charge is always perpendicular to that particular charge.

Right?