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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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What's a capacitor?

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

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    Xiang-hao Lin 發佈於 2021 年 01 月 14 日
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