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Resistance and Reactance

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  • Resistance and Reactance

    I'm having some difficulty with the AC electrics topic in attempting to understand the difference between reactance and resistance. When outlining the use of a capacitor, the notes indicate that capacitive reactance is the same as resistance, but a little later on in the chapter, the vector sum indicates that they are not the same, and resistance is good, but reactance wastes power.

    Can someone furnish me with a definition of both? Also, if someone has a link to electric theory for simpletons, then I would appreciate it.


  • #2
    Its common to equate electrics with the flow of water in tubes, at least it works up to a point. Voltage can be compared to water pressure, current compared to the flow rate.

    An electrical resistance is like a water wheel, it restricts the flow of the electricity but it does work, usually producing heat. In a resistive circuit current and voltage are in phase, that is to say more water pressure leads to an increased flow rate leads to the water wheel going round faster and more work is done. Of course, in an AC circuit the water rushes backwards and forwards and the wheel changes direction with the water flow but the analogy is still sound.

    A capacitive circuit is comparable to a bath, with bathwater and you in it. As you slosh backwards and forwards the water goes with you. At one point, where the movement of the water has stopped for a second, it's all hanging over the tap end of the bath (electrical equivalent, high voltage or potential, no current) then it whooshes back past your thighs (electrical equivalent, low voltage and high current) then it hanges over the head end of the bath (high potential, no current) then it whooshes back etc. What we have here is voltage and current out of phase, when one is big the other is small.

    If we put the two circuits together we have two opposing effects. In the resistive circuit more volts means more current but, if we put a capacitor in the circuit it creates a current that can oppose and reduce the original flow leading to a less efficient water wheel. This opposing force is called capacitive reactance.

    The effect, then, of putting a capacitor in a circuit is to make the resistive devices less efficient. That's why capacitive reactance is seen as 'bad' and that's why we try to cancel out the effect with the opposite effect, inductive reactance.

    There's a link to some electrics animations on the 'Web Resources' forum. I don't think the bath analogy is there, though!


    • #3

      I have dug up this post as i have the same problem of understanding(no back ground in this area) capacitors/inductors.After reading the water analogy i have slightly more understanding (i think).Basically the "whoosh-ing" is reduced with the capacitor fitted in the circuit,storing charge with current reducing as charge bulids and zero voltage at max charge?But i just don`t get the inductor/inductance Please help spent last two days on it and going no where fast!
      P.S i did a search on the data bank for capacitors,inductors,inductance ect not many direct questions?
      Thanks in advance
      Last edited by pipertommy; 20-08-2006, 11:04.


      • #4
        No takers?Its just mainly the combination of capaciters and inductors working in opposite senses to balance things out the actual concept?


        • #5

          Hi Pipertommy

          Read the section in your notes on the effects of electricity flowing through a conductor and generators. If a conductor (wire) in a circuit is passed through a magnetic field a voltage will be induced in the conductor. (generation)

          If you pass electricity through a conductor it produces a magnetic field. (effects of electricity) The strengh and direction of the magnetic field is dpendant on the size and direction of the current flow and can be increased by coiling the conductor.

          In an AC circuit the value of current goes up and down and changes direction. This changes the size and direction of the magnetic field. If that electrical circuit has components in it that contain coils of wire ie transformers, motors ! As the AC goes up and down, the magnetic field goes up and down and it passes through the coils of the transformers/motors ect in the circuit. From generation you saw that this will induce a voltage in the coil ! The supply AC voltage and current is going down but the collapsing magnetic field is buiding up another voltage and current flow in oppostion to it. This is known as the back EMF (electro motive force) it will affect the supply current. This is inductive reactance!

          In a AC electrical circuit you will have resistance that affects current flow, you will have capacitive reactance that affects current flow and your will have inductive reactance that will affect current flow. All of these must be considered when looking at the current flow and therfore power of an AC circuit. Read up on page 9.7 of your notes.

          Hope this has helped.

          John H


          • #6
            Thanks John!I`ve been going over it all the last few days,bits have been starting to click.It was the fact i have no background in this subject,and all seemed a bit confusing