Homework Help: Polarity of two oil droplets

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1. Mar 23, 2017

randomgamernerd

1. The problem statement, all variables and given/known data: .
in a milikan type experiment there are two oil droplets P and Q between the charged horizontal plates, as shown in the figure. Droplet P is in rest while Q is moving upwards. The polarity of charges on P and Q is:
P. Q
a). +. +
b)neutral. -
c). - -
d). +. -
select the correct option.
Please check the image I've attached

2. Relevant equations: Its a concept based question I guess

3. The attempt at a solution:
Q is moving towards +ve plate, it must be negatively charged
P is not accelerating. It must be neutral.
So option B should be the correct option.
But the text says its C

2. Mar 23, 2017

Staff: Mentor

What forces are acting on each droplet?

3. Mar 23, 2017

mjc123

Does B indicate a neutral particle?
Must P be neutral? What forces are acting on it besides electrical?

4. Mar 23, 2017

randomgamernerd

I'm sorry, I didn't get you guys..I think only electrical forces are acting...
option B implies P is neutral and Q is negatively charged.

5. Mar 23, 2017

Staff: Mentor

Oil drops are not massless...

Have you read an overview of the Millikan Oil Drop Experiment?

6. Mar 23, 2017

randomgamernerd

I' e not read milikan's experiment.
I am only aware of the concept of electric field and electric field lines(the beginnig of electrostatics)..the question has been asked in the text wihout me tion pf Mi Oil exp.

7. Mar 23, 2017

Staff: Mentor

A quick google search for "millikan oil drop" would be well worth the effort.

In a nutshell, you can't disregard gravity here.

8. Mar 24, 2017

randomgamernerd

ok i will check it out.
But are you sure this question cant be answered without knowledge of the exp?

9. Mar 24, 2017

Staff: Mentor

It can be answered so long as you get all the assumptions right for the given problem. You assumed that gravity was not a factor, which turned out to be incorrect given the "Millikan context".