TBR cathode ray problem

[greenseeking]

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Assume that an apparatus similar to the one shown in figure 1 could accelerate protons rtaher than electrons and at the same velocity. If we wanted the undefelected protons to follow the same path as the undefelected electrons in Figure 1, then:

a. direction of the eletric field would have to be reverse
b. direction of magnetic field would have to be reversed
c. directions of both fields would have to be reversed
d. netiehr field would have to be changed in any way.

Answer is D.

Why isn't it C???
Since you are now using Protons, rather than Electrons, the directions of the fields should change. Back of the book is saying that since Eq=qVB, E and B only depends on v.

I'm so confused why the directions of the fields wouldn't change. For example, in the milikin oil drop experiement, if you use a proton, then the drop would drop because of the sign. That's why you would have to change the orientation of the anodes/cathodes if you want to suspend the protons.

 

[BB8730]

The oil drop experiment uses an electric field to cancel out the force of gravity. If I'm understanding the problem correctly, this problem is using an electric field and a magnetic field that results in the forces cancelling each other out, resulting in an undeflected electron. If you use a proton instead, the forces would be reversed, but they will still cancel each other out. You could change the directions of both fields, but you don't have to.

 

[Dreamer29]

From my understanding of how the device works, charges flowing through the tube are exposed to both an electric force and a magnetic force. However, the two forces are in opposite direction, but of equal magnitude (and hence F=Eq and F=qvB can be set equal to each other). Thus, one directs the charges to one plate, and the other directs the charges to the opposite plate. However, the combination of these two forces will allow the particles to flow through the plates without hitting either of the plates.

Take a loot at this diagram: http://upload.wikimedia.org/wikiped..._selector.svg/300px-Velocity_selector.svg.png

A positive charge would be "pushed" towards the upper plate by the electric force. At the same time, the magnetic force is in the downward direction. Thus, because they are in opposite direction, they will cancel out, and the particle will continue forward. Now let's assume that the particle was an electron. The same thing essentially happens, however this time each of the forces act in the opposite direction. The electric force is now acting in the downward direction because it's a negative charge; and the magnetic force is now acting in the upward direction. Yet again, the forces are equal in magnitude and opposite in direction. Thus, the negative particle will be on the same path as the positive particle.

We can see this in the equations as well

Eq=qvB
The "q" cancels out on both sides (and thus whether or not it's a positive or negative charge will have no effect).

Hope this helps!

 

[greenseeking]

The oil drop experiment uses an electric field to cancel out the force of gravity. If I'm understanding the problem correctly, this problem is using an electric field and a magnetic field that results in the forces cancelling each other out, resulting in an undeflected electron. If you use a proton instead, the forces would be reversed, but they will still cancel each other out. You could change the directions of both fields, but you don't have to.

Thanks! Perfectly clear now.

 

[greenseeking]

From my understanding of how the device works, charges flowing through the tube are exposed to both an electric force and a magnetic force. However, the two forces are in opposite direction, but of equal magnitude (and hence F=Eq and F=qvB can be set equal to each other). Thus, one directs the charges to one plate, and the other directs the charges to the opposite plate. However, the combination of these two forces will allow the particles to flow through the plates without hitting either of the plates.

Take a loot at this diagram: http://upload.wikimedia.org/wikiped..._selector.svg/300px-Velocity_selector.svg.png

A positive charge would be "pushed" towards the upper plate by the electric force. At the same time, the magnetic force is in the downward direction. Thus, because they are in opposite direction, they will cancel out, and the particle will continue forward. Now let's assume that the particle was an electron. The same thing essentially happens, however this time each of the forces act in the opposite direction. The electric force is now acting in the downward direction because it's a negative charge; and the magnetic force is now acting in the upward direction. Yet again, the forces are equal in magnitude and opposite in direction. Thus, the negative particle will be on the same path as the positive particle.

We can see this in the equations as well

Eq=qvB
The "q" cancels out on both sides (and thus whether or not it's a positive or negative charge will have no effect).

Hope this helps!

Thanks! Helps a lot!! 😀