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  • In a metal conductor, the positively charged particles are fixed in place,

  • whereas the negatively charged particles, called electrons, are free to move around.

  • A metal typically has an equal amount of

  • positively and negatively charged particles,

  • making the metal as a whole electrically neutral.

  • If we apply a force trying to push extra electrons into this metal plate,

  • we will not be successful, because particles with the same charge repel one another.

  • If extra electrons accumulate inside the metal plate,

  • they will repel each other and repel any of the new electrons trying to enter.

  • Therefore, even if we apply a force,

  • the metal will remain electrically neutral.

  • Now, let us consider a new scenario where there

  • is another metal plate close to the one shown.

  • As electrons accumulate in the first metal plate,

  • they will repel the electrons in the second metal plate.

  • The positively charged particles left behind in the second plate

  • will exert an attractive force on the

  • negatively charged particles in the first metal plate.

  • This makes it possible for the first plate to have

  • more negative particles than positive particles.

  • As we add electrons to the first metal plate, an equal

  • number of electrons leave the second metal plate.

  • After a certain number of extra electrons accumulate on the first plate,

  • they will repel any new electrons trying to enter.

  • The first metal plate has developed a net negative charge,

  • and the second metal plate has developed

  • an equal and opposite net positive charge.

  • We can cause this equal and opposite charge on each plate to be larger

  • by applying a larger external force.

  • If we remove the external force we are applying,

  • the extra electrons in the first metal plate

  • will continue to repel one another.

  • We have now returned to our original condition

  • where the net charge on each plate is zero.

  • The two metal plates are what we call a capacitor.

  • Note: Voltage is shownupside downin this video

  • because the blue arrows will show electron flow.

  • By briefly connecting the capacitor to a battery as shown,

  • the two plates have developed equal and opposite charges.

  • If we then connect the capacitor across the light bulb,

  • all the extra electrons in the negatively charged plate will move

  • to the positively charged plate through the light bulb.

  • If we use multiple batteries in series, then

  • we can force the two plates of the capacitor to develop larger amounts of charge.

  • When the capacitor is charged,

  • its voltage will be the sum of the voltages of all the batteries.

  • Charging the capacitor to a higher voltage will cause the

  • light bulb to stay on longer when the capacitor discharges.

  • Now suppose that we increase the areas of the two metal plates.

  • This increased area allows us to push more extra electrons

  • into the first metal plate, and remove more electrons from the second metal plate,

  • without applying a larger external force.

  • When a larger capacitor is connected to our battery,

  • it will take longer for the capacitor to charge to the voltage of the battery.

  • Although the final voltage is still equal to the voltage of the battery,

  • for a larger capacitor, this voltage represents

  • a larger amount of charge on the two metal plates.

  • The larger capacitor will take longer to discharge through the light bulb,

  • thereby allowing the light bulb to stay on longer.

  • Suppose that we add a special material in between the two plates.

  • This material does not allow any particles to flow through it,

  • but it contains molecules that change their orientation

  • based on the charges on the two plates.

  • These molecules exert forces that attract more electrons to the negative plate,

  • and that repel more electrons from the positive plate.

  • The presence of this material has the exact same effect

  • as does increasing the areas of the two metal plates.

  • This allows the two plates to develop larger net charges,

  • for the same amount of applied voltage.

  • When we increase the area of the two plates or add this special material,

  • we say that we have increased thecapacitanceof the capacitor.

  • There is also a third way to increase the capacitance of a capacitor.

  • This is by moving the two metal plates closer together.

  • Because the two plates are now closer together,

  • they exert greater forces on each other's electrons.

  • This attracts more electrons to the negative plate

  • and repels more electrons from the positive plate,

  • allowing the two plates to develop larger net charges,

  • for the same amount of applied voltage.

  • This has the same effect as inserting the special material,

  • and as increasing the areas of the metal plates.

  • To create the largest capacitance possible,

  • we want to make the areas of the plates as large as possible,

  • insert the special material in between the plates,

  • and move the two plates as close together as possible.

  • The larger the capacitance, the more energy

  • will be stored for the same amount of voltage.

  • Much more information about electric circuits

  • is available in the other videos on this channel.

  • Please subscribe for notifications when new videos are ready.

In a metal conductor, the positively charged particles are fixed in place,

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    Amy.Lin 發佈於 2021 年 01 月 06 日
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