Hi. It's Mr. Andersen and this is AP Physics essentials video 8. It is on elementary charge,
which is the smallest unit of charge that an object can have. And it was determined
by Robert Millikan in the Millikan oil drop experiment. And what he was doing was dropping
oil through a little hole. And then using a microscope to see how he could change the
voltage to make the oil drops hover. And as he did that he found elementary charge. And
what he found is that that charge was either no charge or a multiple of a positive x or
an elementary charge or a negative x. But it was never in the middle. In other words
charge is quantized. What that means is that charge smallest unit is going to be 1.6 times
10 to the negative 19th Coulombs. Sometimes positive, sometimes negative, but never in
the middle. And so electric charge is quantized. What that means is it is indiscrete units.
And this was calculated to be 1.6 times 10 to the negative 19th Coulombs. We now refer
to this as elementary charge. And so if you are looking at protons and electrons they
are both going to have charge. In electrons it is going to be a negative elementary charge.
And a proton is going to be a positive elementary charge, even though protons have a much larger
mass, the charge is going to be the same. And so if we look at Millikan's experiment
what he had set up was an atomizer up here. It is kind of like squirting perfume out.
What was being squirted out were these little drops of oil. He then had a microscope down
here and then two charged plates. And so he could look at the oil as it falls right down
this hole right here. And he could change the voltage applied to those plates. And then
x-rays as well. And we will get to those in a second. So let's look at one of these oils
as it drops. And so what happens? Well it is being acted on by gravity. And so if we
look at that one drop of oil there is a gravitational pull that is pulling it down. But as it falls
there is going to be air resistance as it moves through air. It is based on the viscosity
of the air. And so he was able to figure out that all of these drops should fall. So if
we take another drop right here it will just fall down to the bottom. But what he used
was a x-ray tube. And what that x-ray tube would do is fire x-rays across that hole.
What it would do is it would knock electrons off or sometimes certain atoms would gain
electrons. We know this now that it is built on the charge. But watch what happens if we
take one of these and now drop it. What happens to it is it moves down but then it is in electromagnetic
field. And so since it is charged it is being attracted to this positive charge up here.
And so what Millikan could do is he could regulate the amount of that voltage and the
oil would go up. And the oil would go down. And he could get it so it would balance out.
And then he would just measure that voltage. And so once he did the physics of, okay we
have a gravitational force down. We have got viscosity going up. But now we have got this
electromagnetic charge. He was able to calculate, based on the size of that droplet how much
is this elementary charge. And so he determined it to be 1.6 times 10 to the negative 19th
Coulombs. And so now we refer to that as the elementary charge. And he was like one percent
off the value. So it is an incredible experiment. And so we can have charges of 0, no charge
or we could have elementary charges in the positive or elementary charges in the negative.
In other words there is charge quantization. What that means is you can not have a charge
in the middle. You can not have like a half of a charge. Now if you are really smart and
you have watched all of the videos so far, that might not make sense. Because when we
were talking about a proton, a proton you know is made up quarks. And those quarks have
partial charges. Remember each of the up quarks are going to have a two-thirds positive charge.
And that down quark is going to have a negative one-third e or negative one-third elementary
charge. But those quarks do not exist by themselves. They always are together as one object. And
you can see that they all add up, if I add up all of those charges, to 1 or a whole number
of the elementary charge. And so again, if we are looking at an atom, what is it made
up of? Protons and neutrons and electrons. So those protons are in the nucleus with the
neutrons and then electrons outside. Protons have a positive e or a positive elementary
charge. Neutrons have no charge. And electrons have a negative e. Even though the mass of
all of these objects is going to be different. And so did you learn to challenge the claim
that an electric charge smaller than the elementary charge has been isolated? Again it is quantized.
And I hope that was helpful.