TBR Physics Mass Spec Question

[vpanopoulos]

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TBR Chapter 8, Passage VI Question 41 states:

To accelerate anion particles from left to right in Region 1, and deflect those particles out of the page in Region 2, how must the E and B fields be aligned in Figure 1?

A) Anode plate left, cathode plate right, B field up
B) Anode plate left, cathode plate right, B field down
C) Anode plate right, cathode plate left, B field up
D) Anode plate right, cathode plate left, B field down

The answer is: D, I chose B

I understand that the B-field has to be down because of the RHR: thumb points L to R, B field points down, which gives you a force into the page (for a proton) and out of the page (for an electron).

What I don't understand is the cathode/anode answer. I figured, electrons move opposite the electric field; so if the field points R to L (anode plate left, cathode plate right), then the electron will move L to R. If the electron is moving towards the anode plate, wouldn't it be repelled by it?

 

[ashtonjam]

Here's a picture I got from TBR chemistry. Remember that electrons always flow from anode to cathode. For the battery, that does mean that the - terminal has all the electrons (so it is the anode) and the + terminal doesn't (so it is the cathode). However, if you connect a capacitor to the battery, the anode/cathode definition is not quite the same.

Since the + terminal of the battery is starved for electrons, it will suck up electrons from the plate and oxidize it (an-ox). That means the plate connected to the battery cathode is deemed the anode; the plate has electrons to send over, and it actually builds up a positive charge that attract anions.

The opposite is also true. The - terminal of the battery (anode) has an excess of electrons and sends them to its side of the capacitor plate, reducing it (red-cat). The capacitor plate attached to the anode is considered a cathode.

TL;DR: Notice that in both cases, electrons flow from anode to cathode. For capacitors and gel electrophoresis, anodes attract anions and cathodes attract cations (work out the electric charges based off that saying). For your question, it makes sense to have the anode on the right because anodes attract anions.

 

[vpanopoulos]

You said that for capacitors and gel electrophoresis, anodes attract anions and cathodes attract cations. But in the question, it's a mass spectrometer. Does that make any difference in the answer?

 

[ashtonjam]

You said that for capacitors and gel electrophoresis, anodes attract anions and cathodes attract cations. But in the question, it's a mass spectrometer. Does that make any difference in the answer?

No difference because it's actually a capacitor in the problem. Check out the diagram and you'll see a battery connected to a parallel plate capacitor.

 

[vpanopoulos]

What if there were no capacitor present? Then which way would the electrons move?

 

[ashtonjam]

If there were no capacitor present, then the problem would be different and it would be hard to say because all the answer choices refer to anode/cathode plates. Electrons would definitely move in the opposite direction of electric field lines, just as they do with charged capacitor plates (cathode -> anode). In real world applications like mass spectrometers, something needs to be generating the field to begin with.