BR GenChem Ch 2 Milkman Oil Drop question

[stephster416]

In Ch. 2 Passage 1 of BR GenChem in my edition, there's a question about a Milkman oil drop experiment where the bottom plate is positively charged and the top plate is negatively charged (which you can tell from the passage and they explicitly state that as well in an explanation for one of the other questions), so the oil drop ionized by the electron beam bends towards the bottom plate. In Q8 they ask "If an oil drop were slowly descending once it was in the electric field between the two charged plates, then which of the following changes would NOT stop it from continuing to fall?"

The answer they give as correct is B, "decrease the number of electrons on the drop." I thought in this case since the positively charged plate here is on the bottom drawing the drop downward, decreasing electrons WOULD slow its fall (and the answer I had chosen, A, was to increase the number of electrons since that should not stop it from continuing to fall, and in fact make it fall faster towards the positively charged plate).

In the explanation they state "if an electron is descending, a net upward force must be applied to accelerate the particle in the opposite direction" and that you'd' need to either increase the upward force or decrease the downward force. Then they state "increasing the number of electrons in the drop increases the charge, which should stop (or at least slow) the descent" - I don't see why this would be true at all if the bottom plate is positively charged. They also say in the last sentence of the explanation "The only choice that would definitely not stop the descent, but would in fact increase the descent, is to increase the mass of the oil droplet" - which isn't an answer choice anyway...

Am I missing something really obvious or is there a mistake somewhere in the explanation?

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[gongshow89]

I could be wrong about this, but when I read the passage it reads as if the bottom plate is actually negative. The oil drop is bombarded with an "ionizing electron beam" and gains a charge. So I'm guessing the oil drop becomes negative. The passage says that the bottom plate is a repellent (therefore also negative).

If you decrease the number of electron it will give the ion a "more positive charge" and thus accelerate toward the bottom plate.

Again, I could be wrong.. but it seems to line up.

 

[ilovemedi]

I could be wrong about this, but when I read the passage it reads as if the bottom plate is actually negative. The oil drop is bombarded with an "ionizing electron beam" and gains a charge. So I'm guessing the oil drop becomes negative. The passage says that the bottom plate is a repellent (therefore also negative).

If you decrease the number of electron it will give the ion a "more positive charge" and thus accelerate toward the bottom plate.

Again, I could be wrong.. but it seems to line up.

Okay I'm confused. In this passage, it says an ionizing electron beam was used to give the oil droplets a charge ("it gained a charge"). Shouldn't the particle be POSITIVE then if it was ionized? Why did TBR assume it to be negative when they said it was ionized..........

 

[lexw]

particles dont have to be positive to be an ion (think anions or any of the halogens) when they bombard with electrons, theyre creating droplets with a negative charge by giving them extra electrons. This makes sense because the millikan experiment was supposed to show the charge of an electron. by bombarding with protons it would change the mass of the droplets. Also from the picture the top plate is the positive and the bottom is the negative. you can tell by looking at the circuit.

 

[Paean1]

I don't have the BR genchem book with me, but I can try to help here.

The ionizing electron beam works by ionizing the gas molecules (not the oil droplets) in the chamber between the 2 charged plates. This ionization process releases electrons which attach to the oil droplets. Therefore the only charge the oil droplets should have is a negative charge from those attached electrons.

The whole point of the experiments is to make qE = mg, so we know that if the oil droplets are negatively charged the bottom plate must be negatively charged and the top plate must be positively charged (in order for the electrostatic force to oppose the gravitational force).

If the oil droplets are falling, then the only way to affect their rate of descent is by changing the charge of the oil droplet or in other words the # of electrons on the oil droplet, the strength of the electric field or the mass of the droplets. Increasing # of electrons on the oil droplet would increase the electrostatic force (upward force), thus decreasing rate of descent. Conversely, decreasing # of electrons would decrease electrostatic force (upward force), thus increasing rate of descent. Since the question asks which would not stop it from continuing to fall we can safely choose to decrease the # of electrons since that should not only keep the oil droplet falling but even increase its rate of descent.

 

[ilovemedi]

particles dont have to be positive to be an ion (think anions or any of the halogens) when they bombard with electrons, theyre creating droplets with a negative charge by giving them extra electrons. This makes sense because the millikan experiment was supposed to show the charge of an electron. by bombarding with protons it would change the mass of the droplets. Also from the picture the top plate is the positive and the bottom is the negative. you can tell by looking at the circuit.

Thank you for your response! Yes, by looking at the diagram I did make the assumption that the top must be positive (it's a battery, and the big | = +). But if that wasn't here, should we mainly assume the charge will be negative if ionized by an electron beam? THANK YOU again

 

[ilovemedi]

@Paean1 Wow thanks for the response. I get it. I guess I was confused by the term "ionizing electron beam". Makes sense. THanks.

 

[benjaminl1nus]

lol, "Milkman" oil drop. almost as good as "Taco Bell's Canon"