Electron deflection TBR Physics

[ashtonjam]

(Discrete problem, no passage info)
15. If an electron is observed to travel through a certain region of space undeflected, what can be concluded?
A. There is definitely no electric field in that region, but perhaps there is a perpendicular magnetic field.
B. There is definitely no magnetic field in that region, but perhaps there is an electric field.
C. No field of any kind is present.
D. Both an electric and magnetic field may be present parallel to one another.

Answer: C

Why couldn't D be correct? I'm imagining a situation in which the electron is moving through space parallel to both uniform magnetic and electric field lines.

The electron would move in the opposite direction of the electric field, so it would accelerate in a straight line and not be deflected. The magnetic field lines would be parallel to the electron's velocity and this would mean that there is 0 magnetic force on the electron; F = qv x B = qvBsinθ = qvBsin0 = 0 N.

Maybe by 'deflection,' they mean net force? I'm not even sure.



Answer rationale:

15. Choice C is the best answer. The electron is undeflected, which can be explained by the absence of a net force acting on the electron. This is the case if (a) the electric and magnetic forces are of equal magnitude but opposite direction or (b) there are no electric and magnetic forces acting on the electron. Because there could be both an electric field and magnetic field, as opposed to just one or the other, choices A and B are eliminated. In order to produce a force on a moving electron, the magnetic field must be aligned perpendicular to the velocity of the particle, resulting in a force that is mutually perpendicular to both the magnetic field and the velocity. If the electron moves in the same direction as the magnetic field, it will feel no magnetic force. This means that the force exerted by the magnetic field is perpendicular to the magnetic field itself; so if the magnetic and electric fields are parallel, then the magnetic force is perpendicular to the electric force. This would not allow the forces to cancel, so the electron would be deflected as it traveled. Choice D is not possible. The best answer is that there is no field of any kind present. The best answer is choice C.

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[sps27]

If an electron travels in a uniform electric and magnetic field and parallel to those fields,

1) the direction of force on the electron due to magnetic field will be perpendicular to the magnetic field and its velocity. the magnitude of the force on the electron due to magnetic field will be 0 as v and B are parallel so theta = 0 and sin 0 = 0
2) there will be a net force on the electron due to electric field which will cause it to deflect no matter what (I think). I don't think it is possible for charged particles to travel in electrical fields undeflected.

The only thing I can think of (and this is overthinking the question for sure) is if the electron is in an orbit around a nucleus, it is essentially moving in an equipotential surface with fixed angular momentum, but even then it still has angular displacement.

 

[dechristine]

Electrons can travel in a straight line between parallel E and B fields if the forces of both fields are the same and both fields are uniform. This is what occurs in CRT screens: electrons are accelerated (and deflected) in a uniform E field, they travel straight through parallel E and B fields, and are again deflected onto a display by a pure B field. I think the problem with answer choice D is that it is not specific enough because if both fields aren't uniform and have the same force, then the electron will travel in a corkscrew-like motion.

For an electron to travel straight through parallel E and B fields:
qE = qvB
E = vB
B = E/v

 

[ashtonjam]

Does this mean electrons are considered to be deflected if they're accelerated by the electric force in a straight line? I guess I assumed that deflection meant changing the angle of motion.

 

[dechristine]

No, deflected usually means "knocked off course from moving in straight line". My point was that the question asks what you can conclude from observing an electron traveling without deflection. While you can conclude that there aren't any fields present (because the electron moving in a straight line means the net force on the electron is zero), you can't conclude that there are an E and B field parallel to each other without more information.

 

[ashtonjam]

The thing is, choice D has the softening word 'may' that makes it seem like it could include both possibilities. There could be no field, or there could be two parallel fields. On the other hand, choice C seems like a very strong conclusion that excludes that possibility, and we know how those answer choices work on Verbal reasoning. You said there's not enough information to know if there are two parallel fields or not but that's exactly it, isn't the problem asking for a conclusion to be drawn from the limited information? And the parallel fields case is one in which there's no deflection as well, so it's just weird that you can conclude only one out of the two possibilities 😕

 

[sps27]

Electrons can travel in a straight line between parallel E and B fields if the forces of both fields are the same and both fields are uniform. This is what occurs in CRT screens: electrons are accelerated (and deflected) in a uniform E field, they travel straight through parallel E and B fields, and are again deflected onto a display by a pure B field. I think the problem with answer choice D is that it is not specific enough because if both fields aren't uniform and have the same force, then the electron will travel in a corkscrew-like motion.

For an electron to travel straight through parallel E and B fields:
qE = qvB
E = vB
B = E/v

Not sure if I understand you correctly here. Are you saying in a CRT an electron travels straight un-deflected in a region with parallel electric and magnetic fields? How is that possible? What does parallel mean here? Like E and B fields pointing in the same direction or opposite direction?

 

[ashtonjam]

I don't know, but this is what I had in mind. The electric field lines and the magnetic field lines are uniformly straight and parallel to each other. The electron is accelerated to the right by the electric field lines. The magnetic field, since it's exactly opposite of the electron's velocity, does nothing because F = qvB sin180 = 0. The magnetic field lines could be pointing the opposite direction as well and still cause 0 magnetic force since sin0 = 0. Is there anything wrong here?

 

[sps27]

I don't know, but this is what I had in mind. The electric field lines and the magnetic field lines are uniformly straight and parallel to each other. The electron is accelerated to the right by the electric field lines. The magnetic field, since it's exactly opposite of the electron's velocity, does nothing because F = qvB sin180 = 0. The magnetic field lines could be pointing the opposite direction as well and still cause 0 magnetic force since sin0 = 0. Is there anything wrong here?

Thanks. I see it now. This is like a straight anode to cathode flow. So yes, both C and D seem to be correct for this ans. I understand your dilemma and agree with you. It is a little ambiguous.

 

[vpanopoulos]

Just did this passage and question. I stared at the answer explanation for about 10 minutes. I understand everything up until "this means the force exerted by the magnetic field is perpendicular to the magnetic field itself". Huh?