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What's a capacitor?

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Well this is a capacitor.

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OK, but what's inside of this?

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Inside of this capacitor
is the same thing

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that's inside basically
all capacitors.

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Two pieces of
conducting material

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like metal, that are
separated from each other.

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These pieces of
paper are put in here

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to make sure that the two
metal pieces don't touch.

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But what would
this be useful for?

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Well, if you connect two
pieces of metal to a battery,

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those pieces of metal
can store charge.

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And that's what
capacitors are useful for.

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Capacitors store charge.

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Once the battery is
connected, negative charges

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on the right side get attracted
towards the positive terminal

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of the battery.

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And on the left side,
negative charges

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get repelled away from
the negative terminal

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of the battery.

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As negative charges leave the
piece of metal on the right,

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it causes that piece of metal
to become positively charged,

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because now that
piece of metal has

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less negatives than
it does positives.

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And the piece of
metal on the left

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becomes negatively
charged, because now it

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has more negatives
than it does positives.

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It's important to note
that both pieces of metal

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are going to have the
same magnitude of charge.

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In other words, if the charge
on the right piece of metal

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is 6 coulombs, then the charge
on the left piece of metal

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has to be negative 6 coulombs.

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Because for every
1 negative that

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was removed from the right
side, exactly 1 negative

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was deposited on the left side.

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Even if the two pieces of
metal were different sizes

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and shapes, they'd
still have to store

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equal and opposite
amounts of charge.

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Now I've only show
negative charges moving,

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because in reality it's the
negatively charged electrons

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that get to move freely
throughout a metal,

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or a piece of wire.

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The positively charged protons
are pretty much stuck in place,

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and have to stay where they are.

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This process of
charge switching sides

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won't continue to
happen forever, though.

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Negative charges
on the right side

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that are attracted toward
the positive terminal

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of the battery
will start to also

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get attracted toward the
positively charged piece

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of metal.

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Eventually the
negative charges will

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get attracted to the positive
piece of metal, just as much

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as they're attracted toward
the positive terminal

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of the battery.

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Once this happens,
the process stops,

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and the accumulated
charge just sits there

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on the pieces of metal.

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You can even remove the
battery, and the charges

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will still just
continue to sit there.

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The negatives want to go
back to the positives,

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because opposites attract.

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But there's no path for
them to take to get there.

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This also explains why
the pieces of metal

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have to be separated.

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If the pieces of metal were
touching during the charging

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process, then no charges
would ever get separated.

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The negatives would just
flow around in a loop

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because you've
completed the circuit.

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That's why you want
the paper in there,

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to keep the two pieces
of metal from touching.

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So capacitors are devices
used to store charge.

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But not all
capacitors will store

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the same amount of charge.

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One capacitor hooked
up to a battery

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might store a lot of charge.

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But another capacitor hooked
up to the same battery

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might only store a
little bit of charge.

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The capacitance of a
capacitor is the number

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that tells you how good that
capacitor is at storing charge.

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A capacitor with a
large capacitance

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will store a lot of
charge, and a capacitor

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with a small capacitance will
only store a little charge.

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The actual definition
of capacitance

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is summarized by this formula.

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Capacitance equals the charge
stored on a capacitor, divided

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by the voltage across
that capacitor.

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Even though technically the
net charge on a capacitor

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is 0, because it stores
just as much positive

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charge as it does
negative charge.

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The Q in this
formula is referring

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to the magnitude of charge
on one side of the capacitor.

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What the voltage is
referring to in this formula

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is the fact that when a
capacitor stores charge,

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it will create a
voltage, or a difference

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in electric potential, between
the two pieces of metal.

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Electric potential is high
near positive charges,

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and electric potential is
low near negative charges.

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So if you ever have positive
charges sitting next

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to, but not on top
of, negative charges,

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there's going to be a
difference in electric potential

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in that region, which
we call a voltage.

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It's useful to know if you
let a battery fully charge up

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a capacitor, then the
voltage across that capacitor

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will be the same as the
voltage of the battery.

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Looking at the formula
for capacitance,

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we can see that the units are
going to be coulombs per volt.

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A coulomb per volt
is called a farad,

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in honor of the English
physicist Michael Faraday.

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So if you allow a 9 volt
battery to fully charge up

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a 3 farad capacitor,
the charge stored

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is going to be 27 coulombs.

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For another example, say
that a 2 farad capacitor

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stores a charge of 6 coulombs.

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We could use this formula
to solve for the voltage

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across this capacitor, which
in this case is 3 volts.

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You might think that as
more charge gets stored

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on a capacitor, the
capacitance must go up.

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But the value of the
capacitance stays the same.

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Because as the charge
increases, the voltage

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across that capacitor
increases, which

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causes the ratio
to stay the same.

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The only way to change the
capacitance of a capacitor

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is to alter the
physical characteristics

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of that capacitor.

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Like making the pieces
of metal bigger,

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or placing the pieces
of metal further apart.

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Just changing the
charge or the voltage

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is not going to
change the ratio that

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represents the capacitance.

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