Charge on plates in Millikan's Experiment

[Zahid Iftikhar]

Homework Statement

Why no charge left on a plate if disconnected from a battery?

Relevant Equations

No supporting equations

When a capacitor is connected to a battery, it gets charged according to the voltage of the battery. If battery is disconnected, the charge stays on the capacitor. In Millikan's experiment used to find charge on an electron, when two plates are connected to a voltage source, they acquire opposite charge as they are behaving just like the aforementioned capacitor. If we put a switch on positive terminal of the battery and switch it off, we say the electric field is turned off, allowing the oil droplet to fall under gravity. My question is why both the plates don't work as capacitor when switch is turned off? Further problem is that one of the plate (negative) ,still connected to battery ,should have negative charge and repel the negatively charged oil drop. So the droplet can't fall freely as we read in the books?
I 'd be thankful to learned PF mentors to spare some time for my question.
Regards

 

[TSny]

I think the electric field between the two plates is switched off as follows. The switch removes one of the plates from the voltage source while, simultaneously, connecting both of the plates together (electrically). Thus, the two plates very quickly come to the same potential. This eliminates the electric field between the plates.

 

[Zahid Iftikhar]

I think the electric field between the two plates is switched off as follows. The switch removes one of the plates from the voltage source while, simultaneously, connecting both of the plates together (electrically). Thus, the two plates very quickly come to the same potential. This eliminates the electric field between the plates.

Thanks for the reply.
You meant that general Physics books at secondary level generally miss this explanation for the sake of simplicity. If so, fine. It means plates do have charge left on them if suddenly separated from the battery.

 

[TSny]

Thanks for the reply.
You meant that general Physics books at secondary level generally miss this explanation for the sake of simplicity.

Yes. I do happen to have a high school text that has the following figure:

You can see that when the switch S at the lower right is thrown to the upward position, the upper plate is removed from the battery and both plates are grounded. But, I think you are right that few general physics textbooks take the trouble to show the grounding of the plates.

If so, fine. It means plates do have charge left on them if suddenly separated from the battery.

Yes

 

[Zahid Iftikhar]

Thanks dear Sir. The figure you have added has helped me a lot. I ll surely take it down for my class notes. Bundle of thanks for the help.

 

[TSny]

I enjoyed thinking about your question. If you are interested in seeing Millikan's own diagrams of his apparatus, see

http://gsganden.blogspot.com/2011/01/oil-drop-experiment.html
https://en.wikipedia.org/wiki/Oil_drop_experiment
I have some difficulty interpreting the switch symbols in his drawings, but I believe they do show that the upper plate is connected to ground (as is the lower plate) when the switch is in a certain position.