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theMan2012

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I had a question about the following question:

In a CRT with an electromagnetic deflector, how does the energy supplied to the electron by the anode compare with the energy of the photon that is emitted after the electron strikes the screen?
*Cathode ray tubes (CRT) shown below



A. The energies are equal
B. The energy from the anode is greater than the photon's energy, because multiple photons are ejected for each electron striking the screen.
C. The photon's energy is greater than the energy from the anode.
D. The energy from the anode is greater than the photon's energy, because some energy is dissipated as heat from the screen.

I thought the answer was C because the electric field does work on the electron and this increases its energy. Therefore the energy of the electron striking the screen must be greater then the energy when it was released from the anode. However the answer stated is D. Can anyone explain this to me and why we don't account for the extra energy added by the Electric Field disregarding the magnetic field portion since it does no work. Thank you!
 
Jan 4, 2012
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Ugh, this cathode ray tube is so damn annoying.

Why is the energy not equal, since the tube is evacuated?! See quote from textbook. Is it because they didn't indicate it was evacuated?

The electrons at the cathode are sitting in an electric field, which means that they must
experience a force given by Equation 1. Since the cathode ray tube is evacuated, there are
essentially no molecules inside the glass case. As such, the electrons will not collide with
any particles and energy is conserved.
 
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milski

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The electric field is created by the anode - there are no other source of energy for the electron, so the energy of the photon cannot be larger than that.

SaintJude, the energy of the electron is preserved while it travels from the anode to the screen. Not all of this energy is converted to the photon - some of it is dissipated as heat from the screen.
 

MedPR

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D seems obvious to me. You shoot out something with some amount of energy. When it hits a wall or some other object, it must transfer some of its energy, therefore leaving it with less energy than it started.
 

milski

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D seems obvious to me. You shoot out something with some amount of energy. When it hits a wall or some other object, it must transfer some of its energy, therefore leaving it with less energy than it started.
It's not that obvious, if you hit the wall with the exact amount of energy, you should be able to shoot out a photon with the same energy as the photon that hit. But you need a coherent source like a laser for that. In the CRT you just heat up a filament, so you get all sorts of energies.
 

MedPR

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It's not that obvious, if you hit the wall with the exact amount of energy, you should be able to shoot out a photon with the same energy as the photon that hit. But you need a coherent source like a laser for that. In the CRT you just heat up a filament, so you get all sorts of energies.
Well it's obvious when you are simple minded - like me - and forget about special cases like lasers.
 

milski

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Well it's obvious when you are simple minded - like me - and forget about special cases like lasers.
It was not a dig at you - just a reminder that at particle-sized scale behavior tends to be ideal. There could be questions that could trip you up if you miss that.
 
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Glad MedPr made that comment! And glad Milski replied! I wouldn't have realized the importance of the coherent laser. :)
 

MedPR

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It was not a dig at you - just a reminder that at particle-sized scale behavior tends to be ideal. There could be questions that could trip you up if you miss that.

So laser = particle-sized scale? Are lasers just a beam single particles thick?
 

BerkReviewTeach

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I thought the answer was C because the electric field does work on the electron and this increases its energy.
This is an area where you need to consider the orientation of the fields and forces. Only paralell forces can do work on a particle. A perpendicular force can deflect a particle, but it can't speed it up or slow it down, so it can't give it kinetic energy. The particle is deflected by a perpendicular magnetic force and perpendicular electric force as passes through each deflection region. Because the electron leaves the two regions without gaining kinetic energy, it retains its original energy.

This is a tough question, because you have to look at the answer choices and figure out what the author is trying to emphasize here. The multiple photon answer looks like filler, so it's out. Deflection doesn't add KE, so C is out. The question somes down to whether or not energy is lost as heat when the electron strikes the screen. I have no way of deciding between A and D based on what is given, but from all the extra information the writer included with choice D, it's like they're telling me to take notice of the heat dissipation.

I get frustrated by ambiguous questions where you have to read into the answer choices, but I recall doing that quite often on test day.
 

milski

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So laser = particle-sized scale? Are lasers just a beam single particles thick?
Two aspects here:

- laser are coherent source, in other words they emit at one exact frequency. Another example of a coherent source would be a sodium lamp. Using one of these allows you to emit electrons with a very well known and precise energy. If you had the right wavelength laser, you would be able to shoot electrons at the screen which are going to covert all of their energy to a photon and there would not be any heat coming from the screen.

- the comment I made about particle size scale was more about the fact events with particles are much closer to the ideal case than experiments with objects that you can hold and see. For example, collisions between atoms are real elastic collisions, unlikely say collisions between balls which always lose a bit of energy. I guess what I'm trying to say is that for macro objects you always have some sort of inefficiencies resulting in some heat. That's not the case when we talk about particles. I might be splitting hairs here, the first paragraph is the more important one to keep in mind.
 

MedPR

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Two aspects here:

- laser are coherent source, in other words they emit at one exact frequency. Another example of a coherent source would be a sodium lamp. Using one of these allows you to emit electrons with a very well known and precise energy. If you had the right wavelength laser, you would be able to shoot electrons at the screen which are going to covert all of their energy to a photon and there would not be any heat coming from the screen.

- the comment I made about particle size scale was more about the fact events with particles are much closer to the ideal case than experiments with objects that you can hold and see. For example, collisions between atoms are real elastic collisions, unlikely say collisions between balls which always lose a bit of energy. I guess what I'm trying to say is that for macro objects you always have some sort of inefficiencies resulting in some heat. That's not the case when we talk about particles. I might be splitting hairs here, the first paragraph is the more important one to keep in mind.

I understand. Thanks again!
 

cfive22

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Two aspects here:

- laser are coherent source, in other words they emit at one exact frequency. Another example of a coherent source would be a sodium lamp. Using one of these allows you to emit electrons with a very well known and precise energy. If you had the right wavelength laser, you would be able to shoot electrons at the screen which are going to covert all of their energy to a photon and there would not be any heat coming from the screen.

- the comment I made about particle size scale was more about the fact events with particles are much closer to the ideal case than experiments with objects that you can hold and see. For example, collisions between atoms are real elastic collisions, unlikely say collisions between balls which always lose a bit of energy. I guess what I'm trying to say is that for macro objects you always have some sort of inefficiencies resulting in some heat. That's not the case when we talk about particles. I might be splitting hairs here, the first paragraph is the more important one to keep in mind.
I was reviewing TBR CBT 5 and came upon this question and thread as well. Thanks for all the info milski. Were you a physics major? Your physics skills are off the charts...
 

milski

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I was reviewing TBR CBT 5 and came upon this question and thread as well. Thanks for all the info milski. Were you a physics major? Your physics skills are off the charts...
I am a physics major but so far I've taken only the 1 year intro sequence plus some math classes.
 
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