At the moment, you Maybe just let your smartphone tell you the electric charge of a single electron-the basic unit of electric charge. (Its size is 1.6 x 10^{–19} Coulomb, a common unit of charge. ) But in 1909, things were not that simple. At the time, physicists Robert Millikan and Harvey Fletcher discovered this with oil.Their “oil drop” experiment was not the first method to find this value, but it may be the most famous method, which led to Millikan Won the Nobel Prize in 1923.

This historic experiment illustrates some important physical concepts, it is not too complicated, let’s review it!

Four powers

This experiment involves oil drops-I mean, its name is there. However, in reality, it depends on the understanding of four different forces: gravity, electricity, buoyancy and air resistance. The idea is to use these four to measure the charge value on a drop of oil.

Of course, you already know the force of gravity. If I were to guess, I would say that you are somewhere on the surface of the earth. This means that you may be experiencing gravity as an interaction between your mass and the mass of the earth. We can simulate this interaction by thinking of the Earth as a gravitational field—a downward-pointing vector on the order of 9.8 Newtons/kg. The mass in this gravitational field will receive a force equal to the product of the mass of the object and the gravitational field. (Of course, this is just a model. If you move too high above the earth, you will need a different model.)

The next one is electricity. This is the interaction between any two charged objects. Just like gravity, we can place a single charge in an area with an electric field (*Second*) In units of Newtons per coulomb. Electricity will be the product of object charges (*q*) And electric field.

The first two forces seem to complement each other. But the next two are a little different. They are related to the interaction between the oil and the air it falls. If you have ever extended your hand out of the window of a moving car, you already know air resistance. When you increase the speed of the car, this air resistance on your hands will also increase.

For an object the size of a hand, the air resistance is proportional to the square of the speed of the hand. However, if you have a very small spherical object (such as a drop of oil) moving in the air, we can model this force using the following equation: