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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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