basic electronics

Discussion in 'General Electronics Chat' started by siva p, Mar 14, 2005.

  1. siva p

    Thread Starter New Member

    Mar 14, 2005
    2
    0
    :p

    hai

    i have a basic doubt. there is a concept that current leads voltage lags ans current lags voltage leads. what is actually happening current is going front voltage is comming back. can any one give me explanation to this with pratical example.
     
  2. Erin G.

    Senior Member

    Mar 3, 2005
    167
    1
    Electrimotive force, or voltage, is the electrical "pressure" that make electrons move. Current (amps) is a measurement of the volume of electrons actually in motion. The electrons were set in motion because a voltage was applied to the circuit.

    This is the relationship between an AC voltage & current in a circuit. In a resistive circuit (eg heater, toaster, etc.) the current is in phase with the voltage. However, some devices with magnetic (inductive) components in the circuit (eg ballasts, motors, etc.) alter the phase relationship, which causes extra current to be drawn for the same power output. Ballasts (being inductive components) cause the current to lag behind the voltage.
     
  3. siva p

    Thread Starter New Member

    Mar 14, 2005
    2
    0
    hai,

    can i get the answer still more pratical.not with more technical terms.
    thankyou for the response. i think in the voltage term should be replaced by potential.

    siva

     
  4. Erin G.

    Senior Member

    Mar 3, 2005
    167
    1
    Electrical potential is measured in volts, so voltage and potential are the same thing.

    Electricity is plotted on a sine wave. See the attachment for a picture of this. It starts at zero volts (zero potential) and goes up to a postive (+) peak voltage (1st 90 degrees). Then it goes back down through the zero volts (2nd 90 degrees, or 180 degrees total). It passes zero and goes on to a negative (-) peak voltage (3rd 90 degrees, or 270 degrees total), and back up to zero volts (4th 90 degrees, or 360 degrees total). This is one "cycle" of AC, or alternating current. It is called that because it is constantly "alternating" between postive (+) voltage and negative (-) voltage.

    A resistor "resists" the FLOW of current in a circuit.
    An inductor is a componant in that has magnetic characteristics, and opposes CHANGES in current within a circuit.

    Back to our examples:

    If you have a circuit that has only resistive loads, such as a toaster or light bulb, then your potential (volts) and current (amps) will be in phase, or together. In other words, when the volts are at peak +, the current is also at peak +.

    If you have a circuit that has only inductive (components that are magnetic) loads, such as a motor, the voltage and current will not be in phase. The magnetic components will cause more current (amps) to be drawn, and the current takes longer to get to peak + than the voltage. In other words, the current "lags" behind the voltage. The voltage is "leading" the current, usually by about 90 degrees.

    So in a motor, when the voltage is at peak + (90 degrees), the current (amps) is still at zero. As the voltage goes back down to zero (180 degrees), the current comes up to peak + (90 degrees). The voltage will always lead the current by 90 degrees, and the current will always lag the voltage by 90 degrees, in an inductive circuit.
     
  5. Kadmos

    Member

    Mar 15, 2005
    20
    0
    Yes but how and why electromotive force pushes the electrons to move??
     
  6. Erin G.

    Senior Member

    Mar 3, 2005
    167
    1
    The only real answer to your question, Kadmos, is "because". It's one of those funny things in physics that just is. When a voltage (electrimotive force) is applied to a circuit, the electrons within that circuit are forced to move. And they will continue moving until that voltage is removed from the circuit.
     
  7. n9xv

    Senior Member

    Jan 18, 2005
    329
    1
    A potential difference means an "unbalanced" electrical condition. The negative terminal of a voltage source has a surplus of electrons and the positive terminal of a voltage source has a deficiency of electrons. A potential difference (voltage) is necessary to set up the condition for current to flow. The free electrons are constently trying to fill in the "holes" of the depleted atoms (positive terminal) of the voltage source. There can be no current without a voltage because no voltage implies electrical balance. In reallity electrons flow from negative to positive. But you can also think of "holes" (hole current) as flowing from positive to negative. Hole current is commonly refered to conventional current. When you look at a any schematic, just follow the arrows - thats the direction of hole (conventional) current.
     
  8. Brandon

    Senior Member

    Dec 14, 2004
    306
    0
    Its like gravity.

    Gravity makes things more from a place of higher potential energy to a place of lower potential energy. In a way gravity is a mass-o-motive force (try ti be funny). But, we still don't know why even gravity exhist or where is comes from. We just know a relationship between it and mass and its observed effect.

    Same with voltage and current. Voltage is like gravity. When you allow voltage, your applying 'electron gravity' to the circuit. Ground is just like the real ground. The opposing terminal is the place of higher potential energy. The move 'electron gravity' you supply the faster and more electrons move, hense current. Just like if the gravity were stronger, things would fall faster.

    Now to the lagging and leading aspects, it has to do with the reaction of reactive components (caps, inductors)

    ICE, ELI I lead E in a C, E leads I in a L.

    Caps
    Caps are great for voltage buffering. That is, they can help hold a voltage constant and remove noise, ripples, sudden current rushes n what not in a circuit. Since they maintain voltage well, voltage on them can not change instantly. When look at a caps formula for current you have I=Cdv/dt. If voltage changes instantly, dv/dt = infinity, and we have a problem. But from this formula we also see that current CAN change instantly. If you stop charging a cap, dv/dt =0. Therefore I = 0. But as soon as you begin to change voltage, the current snaps to the appropriate level. As a result, the current is 'faster' than the voltage and end up leading it. Could get more techincal with the math as it come out when you use phasors or LaPlace that it leads by 90 degrees, but I don't think that was what you were looking for.

    Inductors
    The same applies to an inductor. THink of a pipe or water for this. If you pushing tons of water through a pipe, you can't stop the water instantly. Especially if there is something wanting it to keep flowing forward, which for the inductor is the magnetic field produce due to the flow of current. For the inductor we have V=Ldi/dt. Same idea, current cant change instantly, but now voltage can. Its ho flash bulbs work in cameras, and strobes. They use an inductor and quickly change the curent on it which creates very high voltages.

    The real why is still one of those things we don't have yet. We only know what we observed so far.
     
  9. Kadmos

    Member

    Mar 15, 2005
    20
    0
Loading...