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What Is the difference between capacitance and voltage ?

Discussion in 'General Electronics Discussion' started by Reprovo, Jan 10, 2014.

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  1. Reprovo

    Reprovo

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    Jan 5, 2014
    If a capacitor stores an electrical charge ,why Is a value for capacitance necessary? Why can't we measure capacitance only In terms of the maximum voltage It can store without using the Farad ? I understand the capacitor doesn't generate a potential difference but only stores It.

    I think I understand It now.The capacitor releases charge whereas a battery does not so It's a combination of charge and voltage.
     
    Last edited: Jan 10, 2014
  2. BobK

    BobK

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    Jan 5, 2010
    C = q / V

    Capacitance is not voltage, it is charge divided by voltage. Capacitors with different capacitance will have the same voltage when storing different amounts of charge. And there is no max voltage for an ideal capacitor, there is for real capacitors when they physically break down.

    Bob
     
  3. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    A simplified explanation:
    The difference between a capacitor and a battery is the way the charge is stored.
    A capacitor works on the physical principle of opposite charges being stored on conductive plates separated by an isolator.
    A battery uses a chemical reaction to generate electric power.
    A rechargeable battery uses a kind of chemistry wehre the chemical reaction can be reversed by charging the battery with a current.

    Wikipedia gives some useful information on capacitors and batteries.
     
  4. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    A battery contains chemical energy which is released when a load is placed across the battery, a capacitor contains charge not energy. My concern is you may think charge is energy at this earley stage. This is incorrect. Tell me if you need any help and I can help you.
    Adam
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    How about a car analogy?

    Voltage is the momentum the car has, charge is the distance traveled, and current is the speed. Resistance would be anything that slows the car down (let's say air resistance -- because that's kinda proportional to the square of the speed.).

    A capacitor is like a car without an engine it won't move without some external influence. If you give it some momentum (say by pushing it), it will begin to move, but its momentum and speed will reduce the further it goes. Eventually it will have zero momentum and zero speed.

    This is like charging a capacitor, by charging it up to a certain voltage (momentum). As it discharges (moves some distance) it does so at a particular current (speed). The voltage (momentum) and current (speed) reduce as it discharges (as it travels).

    A battery is like a car with an engine. The engine acts to maintain the momentum.

    So it can retain the voltage as it move charge at a certain rate (the current). Both the voltage and current can be maintained.

    I'm not sure if that helps...
     
  6. Laplace

    Laplace

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    Apr 4, 2010
    Even though we say that a capacitor stores charge, how often do you stop to consider that a charged capacitor and a discharged capacitor both contain the exact same amount of electrical charge? A surplus of charge on one capacitor plate is balanced by an equal deficit of charge on the other plate.
     
  7. davenn

    davenn Moderator

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    Sep 5, 2009
    Reprovo,
    the above that Laplace wrote is what you need to get your head around.
    Its the one that has most people fooled. The net charge value in a capacitor doesnt change when going from its "discharged" to its "charged" state. its just the distribution of charge on the plates that has changed. The net charge is still zero

    And as touched on above, a battery, contrary to popular belief, doesnt hold a charge, a huge mass of electrons, ready to flow out when a circuit is connected to it. The battery through a chemical reaction ( only when the circuit is made) generates a flow of electrons out of the negative terminal whilst positive ions move towards the positive terminal. For every electron that leaves the negative terminal, another electron comes into the positive terminal from the circuit.
    The chemical reaction generates a potential difference across the terminals and that PD generates an EMF that "starts the electron flow.

    NOTE: the number of electrons in the battery DOESNT change and the net charge in the battery doesnt change

    When a battery goes "flat" all that is happening is that the chemical reaction is slowing down and can no longer cause a movement of electrons and ions in the battery, so the potential difference across the terminals drops.

    In rechargable batteries, when we apply a charging voltage, we are NOT filling the battery up with electrons to make it a storehouse of charge.
    All we are doing is reforming the chemical bonds within the battery back to their (close to their) original state so the battery can once again generate a PD across the terminals by the movement of electrons and ions.

    NOTE: where ever I have said battery, you can read that also as a cell in the non electronics minded world, people have got so used to calling AA penlight cells etc a battery. They are cells. A battery is a collection of cells, be it a bunch of AA's or D cells in series or parallel or the collection of plates in a car battery etc

    Dave
     
    Last edited: Jan 11, 2014
  8. Reprovo

    Reprovo

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    Jan 5, 2014
    Thanks for all the Information everyone.
    It's pretty clear to me now.
     
  9. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    You beat me to it lol, I was going to say the exact same thing this morning. It is nice to see people being advised with the correct information.
    Thanks
    Adam
     
  10. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    As briefly mentioned above a battery contains chemical energy. The electrons for the current are already in the wires as a sort of electron sea. This is why metals are shiny.

    The battery does not produce any electrons but sort of pumps them around the circuit.

    The energy from the battery flows in one direction to the load and in a dc circuit flow in the opposite way to the electrons, the electrons are attracted to + terminal of the battery.

    The electron does not have enough any energy of its own to jump across the battery so it uses the ions from the chemical reaction to do this, sort of like a raft going between two river banks.
    The electron and it's associated charge is unchanged by the load but it is the energy that is used up. The actual energy flows as an EM wave in the space around both wires and travels to the load in the same direction.
    I think that's how it works in basic form.
    Thanks
    Adam
     
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