tbr test 3 physical sciences q 27

[2010premed]

---

Members don't see this ad.

Which of the following changes would have to be made to the mass spectroscopy apparatus as it is described in the passage, if the cation source in the experiment were replaced with an anion source?

I. The polarity of the battery would have to be reversed.
II. The orientation of the magnetic field would have to be reversed.
III. The location of the double filter would have to be changed.

Answer: I and II

Any help on this 1 is appreciated....

 

[2010premed]

the explanation says anions are accelerated to regions of higher voltage (greater positive charge), while cations are accelerated to regions of lower voltage (greater negative charge).

... good to know?

 

[PiBond]

the explanation says anions are accelerated to regions of higher voltage (greater positive charge), while cations are accelerated to regions of lower voltage (greater negative charge).

... good to know?

anions are attracted to the positively charged plate (by convention we call it the plate of higher voltage). thus, if we were to swap an anion for a cation we would have to switch the the terminals of the battery in order to get the particle to accelerate through the potential difference toward the plate that is opposite its charge.

in contrast, the positively charged ions are accelerated to the negatively charged plate (by convention called the plate of lower voltage)

we need to reverse the direction of the magnetic field because our anions would slam into the bottom and wouldn't be able to hit the detector. doing so would allow them travel in the path represented by your diagram

 

[UIUCstudent]

I'm a bit confused by this circuit (it's not connected into a loop?). If we don't switch the battery plates, would the anions accelerate to the left or would it be toward the bottom toward the battery? I'm just not sure what's causing the particle to go a certain path before reaching the magnetic field.

 

[ilovemcat]

Which of the following changes would have to be made to the mass spectroscopy apparatus as it is described in the passage, if the cation source in the experiment were replaced with an anion source?

I. The polarity of the battery would have to be reversed.
II. The orientation of the magnetic field would have to be reversed.
III. The location of the double filter would have to be changed.

Answer: I and II

Any help on this 1 is appreciated....

Protons flow away from the region of Higher Potential (+) to a region of Lower Potential (-). Electrons flow from a region of Lower Potential (-) to a region of Higher Potential (+). For the voltage source in this diagram, the longer line represents the region of Higher Potential (+), and the Shorter Line the region of Lower Potential (-).

If we're considering the flow of Protons, then the set-up is just fine as is. The protons would flow away from the Positive Potential to the region of Lower Potential. This is the direction we want because we need the protons to get ejected from the ion chamber. However, the question is asking you what would happen in the presents of an anion source (ie. a negatively charged electron). An electron would flow from a region of lower potential to a region of higher potential. But that's just opposite of what you want. If you placed a bunch of electrons in the ion chamber, they would just flow to the voltage source. So we need to reverse the polarity of the battery so that the electrons could flow away from the region of lower potential towards the region of higher potential.

I'm assuming for Part II the particle detector cannot be moved (it isn't a choice). So, the only way for us to measure the mass of the particle, it needs to hit the particle detector. But there's a problem. If the magnetic field remained the way it is in the diagram - an electron would rotate clockwise. We want it to rotate counter-clockwise so that it hits the detector. The only way this could happen was if we made the magnetic field point out of the page. This way, upon entering the magnetic field, the magnetic force would point upwards. Your magnetic force is your centripetal force - this would cause the particle to curve (like you see in the diagram). Therefore as the particle moves, the magnetic force would also change.

For III, technically you could change the location of the double filter and place it somewhere up above the particle detector - but then you have the change the entire set-up. You'd have to move the Ion chamber up as well and extend the wire connected towards the voltage source, but that isn't included in the answer choice so this can't be true.