Capacitors and Capacitance - Electronics & Communication Engineering Video Lecture - Electronics and Communication Engineering (ECE)

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.
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