# Charge on plates in Millikan's Experiment

• Zahid Iftikhar
In summary, when a capacitor is connected to a battery, it gets charged according to the voltage of the battery and the charge stays on the capacitor even when the battery is disconnected. In Millikan's experiment, the two plates behave like a capacitor and acquire opposite charges when connected to a voltage source. When the switch on the positive terminal of the battery is turned off, the electric field between the plates is turned off, allowing the oil droplet to fall freely under gravity. However, this raises the question of why both plates do not act as capacitors when the switch is turned off. The answer is that the switch removes one plate from the voltage source and connects both plates together, eliminating the electric field between them. This explanation is often omitted
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

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.

TSny said:
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.
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.

Zahid Iftikhar said:
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

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.

Zahid Iftikhar

## What is Millikan's Experiment and why is it important?

Millikan's Experiment, also known as the oil drop experiment, was conducted by Robert Millikan in 1909 to determine the fundamental unit of electric charge. This experiment was important because it provided the first accurate measurement of the charge of an electron, which led to a better understanding of the structure of atoms.

## How does Millikan's Experiment work?

In this experiment, Millikan sprayed oil droplets into a chamber with an electric field between two charged plates. By measuring the rate at which the droplets fell or rose in the field, he was able to calculate the charge of the droplets and determine that the charge was always a multiple of a single, fundamental unit of charge.

## What is the significance of the charge on plates in Millikan's Experiment?

The charge on the plates in Millikan's Experiment is crucial because it creates an electric field that affects the motion of the oil droplets. By varying the charge on the plates, Millikan was able to calculate the charge of a single electron and determine its magnitude and sign.

## What are the limitations of Millikan's Experiment?

Although Millikan's Experiment provided an accurate measurement of the charge of an electron, it had some limitations. For example, it assumed that the oil droplets had a constant mass, but in reality, they may have evaporated or acquired a charge from the air in the chamber. Also, this experiment only measured the charge of the electron and did not provide any information about the structure or behavior of the atom.

## How has Millikan's Experiment impacted our understanding of electricity and atoms?

Millikan's Experiment has had a significant impact on our understanding of electricity and atoms. It provided evidence for the existence of individual, discrete units of electric charge and helped to confirm the existence of subatomic particles such as electrons. This experiment also paved the way for further research and advancements in the field of atomic and quantum physics.

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