not just described, Explained


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Millikan's oil drop experiment simulation

This experiment is a little simplified

We’ve simplified the experiment a little from the one Millikan actually performed.  But it should give you some of the flavour of how meticulous Millikan (and/or his assistant Harvey Fletcher) had to be.

Each drop must have a whole number of electrons

Remember the general technique.  Find lots of charges due to unknown numbers of electrons then find the common factor.

Try Why Do Astronauts Float by Julian Hamm

It's all about timing drops on the way up and on the way down

The equation the simulation uses to calculate the charge on the oil drop depends on the rising and falling velocities of the oil drop.

So we’ll be timing oil drops and the simulation will calculate the velocities for us.  The simulation keeps the mass of all the oil drops the same for simplicity.

You may remember that we (or at least the simulation will) find the mass of the drop from the speed it falls through the air.

You only need to time the drop falling once

When the drop's falling there's no electric field so the number of electrons on it doesn't matter.  When it's rising because of the electric field, the more electrons means the quicker it rises.

So we change the number of electrons on the drop lots of times and time it rising on each occasion.

Finding the charge on the electron

The results for charge come in some random order.

There's a cunning little tool to help find how many electrons each charge is caused by.  If you know the charge and the number of electrons then you can easily work out the charge on a single electron, which is what the experiment is all about.

back to Lesson 8: Atoms 2: Electrons