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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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電容器和電容| 電路| 物理| (Capacitors and capacitance | Circuits | Physics | Khan Academy)

37 分類 收藏
Xiang-hao Lin 發佈於 2019 年 11 月 12 日
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