What's a capacitor? Well this is a capacitor. OK, but what's inside of this? Inside of this capacitor
is the same thing that's inside basically
all capacitors. Two pieces of
conducting material like metal, that are
separated from each other. These pieces of
paper are put in here to make sure that the two
metal pieces don't touch. But what would
this be useful for? Well, if you connect two
pieces of metal to a battery, those pieces of metal
can store charge. And that's what
capacitors are useful for. Capacitors store charge. Once the battery is
connected, negative charges on the right side get attracted
towards the positive terminal of the battery. And on the left side,
negative charges get repelled away from
the negative terminal of the battery. As negative charges leave the
piece of metal on the right, it causes that piece of metal
to become positively charged, because now that
piece of metal has less negatives than
it does positives. And the piece of
metal on the left becomes negatively
charged, because now it has more negatives
than it does positives. It's important to note
that both pieces of metal are going to have the
same magnitude of charge. In other words, if the charge
on the right piece of metal is 6 coulombs, then the charge
on the left piece of metal has to be negative 6 coulombs. Because for every
1 negative that was removed from the right
side, exactly 1 negative was deposited on the left side. Even if the two pieces of
metal were different sizes and shapes, they'd
still have to store equal and opposite
amounts of charge. Now I've only show
negative charges moving, because in reality it's the
negatively charged electrons that get to move freely
throughout a metal, or a piece of wire. The positively charged protons
are pretty much stuck in place, and have to stay where they are. This process of
charge switching sides won't continue to
happen forever, though. Negative charges
on the right side that are attracted toward
the positive terminal of the battery
will start to also get attracted toward the
positively charged piece of metal. Eventually the
negative charges will get attracted to the positive
piece of metal, just as much as they're attracted toward
the positive terminal of the battery. Once this happens,
the process stops, and the accumulated
charge just sits there on the pieces of metal. You can even remove the
battery, and the charges will still just
continue to sit there. The negatives want to go
back to the positives, because opposites attract. But there's no path for
them to take to get there. This also explains why
the pieces of metal have to be separated. If the pieces of metal were
touching during the charging process, then no charges
would ever get separated. The negatives would just
flow around in a loop because you've
completed the circuit. That's why you want
the paper in there, to keep the two pieces
of metal from touching. So capacitors are devices
used to store charge. But not all
capacitors will store the same amount of charge. One capacitor hooked
up to a battery might store a lot of charge. But another capacitor hooked
up to the same battery might only store a
little bit of charge. The capacitance of a
capacitor is the number that tells you how good that
capacitor is at storing charge. A capacitor with a
large capacitance will store a lot of
charge, and a capacitor with a small capacitance will
only store a little charge. The actual definition
of capacitance is summarized by this formula. Capacitance equals the charge
stored on a capacitor, divided by the voltage across
that capacitor. Even though technically the
net charge on a capacitor is 0, because it stores
just as much positive charge as it does
negative charge. The Q in this
formula is referring to the magnitude of charge
on one side of the capacitor. What the voltage is
referring to in this formula is the fact that when a
capacitor stores charge, it will create a
voltage, or a difference in electric potential, between
the two pieces of metal. Electric potential is high
near positive charges, and electric potential is
low near negative charges. So if you ever have positive
charges sitting next to, but not on top
of, negative charges, there's going to be a
difference in electric potential in that region, which
we call a voltage. It's useful to know if you
let a battery fully charge up a capacitor, then the
voltage across that capacitor will be the same as the
voltage of the battery. Looking at the formula
for capacitance, we can see that the units are
going to be coulombs per volt. A coulomb per volt
is called a farad, in honor of the English
physicist Michael Faraday. So if you allow a 9 volt
battery to fully charge up a 3 farad capacitor,
the charge stored is going to be 27 coulombs. For another example, say
that a 2 farad capacitor stores a charge of 6 coulombs. We could use this formula
to solve for the voltage across this capacitor, which
in this case is 3 volts. You might think that as
more charge gets stored on a capacitor, the
capacitance must go up. But the value of the
capacitance stays the same. Because as the charge
increases, the voltage across that capacitor
increases, which causes the ratio
to stay the same. The only way to change the
capacitance of a capacitor is to alter the
physical characteristics of that capacitor. Like making the pieces
of metal bigger, or placing the pieces
of metal further apart. Just changing the
charge or the voltage is not going to
change the ratio that represents the capacitance. [MUSIC PLAYING]
