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Right now Sure, you can ask your phone to say the charge of a single electron – the basic charge unit. (It has a magnitude of 1.6 x 10–19 coulombs, a common unit of electric charge.) But in 1909 things were not so simple. Then physicists Robert Millikan and Harvey Fletcher invented it using oil. Their “drop of oil” experiment wasn’t the first method to find that value, but it’s perhaps the most famous, and it took them to Millikan In 1923 he received the Nobel Prize.
This historical experiment shows some important concepts in physics, and it is not very complicated, so let us examine them!
Four Forces
This experiment deals with oil droplets, which is right there in the name. But it really depends on understanding four different forces: the gravitational force, the electric force, the buoyancy, and the air resistance force. The intention is to use these four to measure the value of the electric charge of a single drop of oil.
Sure, you already know the force of gravitation. If I had to guess, I would say you are somewhere on the surface of the Earth. This means that you are probably experiencing the force of gravity as an interaction between your mass and the mass of the Earth. We can model this interaction as a vector pointing downwards like the Earth creating a gravitational field, with a magnitude of 9.8 newtons per kilogram. The mass of this gravitational field will be subjected to a force equal to the mass of the object and the product of the gravitational field. (Of course, it’s just a model. If you move too much above the ground, you’ll need another model.)
Next up is the electric force. It is the interaction between two objects with an electric charge. As with gravitational force, we can find the electric force by placing a single charge in a region with an electric field (E) in newton units per coulomb. The electric force will be the product of the charge of the object (q) and electric field.
The previous two forces seem to complement each other. But the next two are slightly different. They are related to the interaction between oil and falling air. You already understand the trace of air if you ever get your hand out of the window of a moving car. As the speed of the car increases, so does the drag force of the air in your hand.
In hand-sized objects, the drag force of the air is proportional to the square of the speed of the hand. However, if you have a very small spherical object (like a drop of oil) that moves through the air, we can model this force with this equation:
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