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Capacitors and capacitance

A basic overview of capacitors and capacitance. By David Santo Pietro. . Created by David SantoPietro.

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  • winston baby style avatar for user Ali Arshad
    why the electric potential is high near positive charges ?
    (46 votes)
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    • blobby green style avatar for user iqacmes
      Electric potential at a point near a charger is measured in terms of the work done(by an external) agent in bringing an unit positive charge from infinity to that point. Clearly to bring an unit +'ve charge from infinity to a point near a positive charge would be more than the work done to bring the unit positive charge from infinity to an identical point near a negative charge since in the former case the agent would have to overcome a repulsive force which opposes the bringing of the charge..while in the latter case the attractive force helps in brining the unit +'ve charge
      (71 votes)
  • leafers seedling style avatar for user Sampath Rachumallu
    sir u said that two metal pieces store same amount of charge independent of sizes. but capacitance is dependent on area. then how can it be possible.suppose if two pieces of different areas are taken then what will happen?
    (17 votes)
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  • leaf green style avatar for user woozooroot
    i have really simple question...
    why need capacitor??
    why do we need to store charge?
    what could possibly go wrong if there is no capacitor??
    plz tell me i want to explain to my future student
    (11 votes)
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  • spunky sam blue style avatar for user GDDF4D
    If you removed the salt bridge in a galvanic cell, would it hold the charge like a capacitor?
    Or would the charges just equilibrate?
    (13 votes)
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    • blobby green style avatar for user rich.p.fields
      I think this is a very interesting question! If you break the salt bridge in a cell (either of the galvanic or electrostatic variety) while it is charged then there is no way for the electrolyte (salt + solvent) to regain electroneutrality (neutral and homogeneous charge distribution). Take lithium ion batteries as an example, during the charging process lithium ions are released from the positive electrode (typically a metal oxide structure i.e. LiFePO4) into the electrolyte while electrons move from the same positive electrode over to the negative electrode via an electric circuit. Once the electrons arrive at the negative electrode they attract the positive lithium ions out of the electrolyte, subsequently becoming stored (intercalated) in a material such as carbon. If you break the path (salt bridge) through which the lithium ions move then it would be impossible to discharge the cell i.e. put lithium ions back into the positive electrode. If you tried to discharge it there would be an excess of lithium ions in the electrolyte and they would be forced back into storage, subsequently drawing back the electrons you wanted to discharge around a circuit. - Prospective Ph.D student.
      (3 votes)
  • starky tree style avatar for user pentazu
    Why exactly is electric potential lower at where the negative charges are than at where the positive charges are? If the electric potential is the amount of work which will be used or gained per charge for or from moving an amount of charge, why would it matter if the charge creating the field is positive or negative?
    (5 votes)
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    • leaf green style avatar for user Mark Zwald
      It takes positive work to move a positive test charge closer to a positive charge, so that means the electric potential energy is positive. Likewise it takes negative work to move a positive test change closer to a negative charge, so the electric potential energy around negative charge is negative. Electric potential (voltage) is a measure of the electric potential energy per unit charge, so if the electric potential energy is negative, the voltage will also be negative.
      (13 votes)
  • blobby green style avatar for user Ahensen2014
    Does the charge just travel through the air then? How does the current continue to flow through the entire circuit when a capacitor is in it?
    (9 votes)
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  • leafers tree style avatar for user 'Sangamesh Raghoji'
    how potential difference is developed when positively charged pate and negatively charged plates are separated?(exact reasoning )
    (4 votes)
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    • blobby green style avatar for user Daniel Lengyel
      The potential difference is simply the work that has to be done to bring a unit charge from one terminal to the other. This is also the case for a battery, there is a potential difference across the two terminals, because near the negative terminal there is a lower electric potential than at the positive terminal. Think of it as the free electrons really want to go to the positive side, but can't, as in the case of a battery, but as soon as there is a closed circuit the electrons will move to the positive terminal. So the potential difference, or the voltage, is simply a measure of how much work has to be done, to get from one point to the other, or how much of this work is stored in the separate electrons, which want to go back to the positive side.
      If this doesn't make much sense, think of when you are lifting a book. You had to do work to overcome the force of gravity. As soon as you release that book, it will fall towards the ground. The work that you had to put in to lift the book, and which is now stored in the book, is analogous to the potential difference.
      (11 votes)
  • leaf orange style avatar for user ktlelito
    @ David states that you can change the capacitance of a capacitors by making the plates smaller or larger. Does that mean that capacitors have limits or ranges on what voltages they can maintain? For example, smaller plates could only store smaller voltages because they can't separate as much charge? And does that mean that for example that a small capacitor attached to a 9 volt battery maybe could not get up to 9 volts?
    (4 votes)
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  • orange juice squid orange style avatar for user Abraham George
    Is capacitance kind of like inductance?
    (3 votes)
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  • starky seedling style avatar for user deepprakash8a
    whats the use of a capacitor if it has to be charged by a battery , why cant we just use a battery instead of a capacitor.....
    (1 vote)
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    • leaf green style avatar for user Mark Zwald
      Capacitors are capable of supplying much higher currents than batteries, albeit for a much shorter period of time. Besides being used to store energy, capacitors are also used for signal filtering, impedance matching, signal coupling, touch sensing, oscillators, etc...
      (5 votes)

Video transcript

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. [MUSIC PLAYING]