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[noob] Resistance in paralel = increase in current?

Discussion in 'Electronic Basics' started by Simon, Apr 13, 2007.

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  1. Simon

    Simon Guest

    I'm following a book here and it was talking about resistance and
    how to calculate the total resistance of a circuit. I understand
    everything pretty well except the part that said something like: now
    having another resistance connected in paralel to the initial
    resistance, the current will double and go in both resistance... it
    also says something like the battery is producing more current because
    of this.

    I'm wondering, is this true? I don't have a multimeter (yet) to
    verify... would an ammeter show an increase of current in this
    circuit? And as I understand it, having a circuit in paralel is just
    a great way of depleting a battery's power faster! (Oh i'm sure there
    are good applications for this, i'm not there yet)

    But also, can you explain what happens with the current and why it
    increases when a paralel resistance is added?

    Thanks a lot!
  2. A resistance in parallel with another resistance is an additional path for
    current to flow. Given a fixed voltage, more paths (less resistance) means
    more current flow. You might want to search on "conductance," which is
    another way of understanding this phenomenon.
  3. Yes - this is true.
    The battery voltage will stay the same when you add a second resistor
    (or close enough for this discussion), so you can consider each
    resistor (each parallel branch of the circuit) independently.

    If you have, for example, a 10 volt battery, and two 10 ohm resistors
    connected in parallel across it, each resistor will draw 1 amp
    (10volts/10 ohms), so the total current delivered by the battery will
    be 2 amps.

    In house wiring, all the lights and outlets are connected in parallel
    across the 120V (in North America) or 240V (elsewhere) AC supply.
    Likewise, in a car, all the lights, radio, starter, etc. are connected
    in parallel across the 12 volt battery. In either case, each light or
    other load draws what it wants from the power source, and the power
    source therefore delivers the total of all currents demanded by the

    Peter Bennett, VE7CEI
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  4. That statement makes the assumption that the voltage across
    the two resistors in parallel is the same as the voltage
    across the original resistor and the second resistor is
    similar to the first one.
    You open the door of a car and the dome light comes on (is
    connected across the battery. The battery generates enough
    current to light that bulb. You leave the door open, and
    turn the headlights on. The battery now also generates
    enough current to light those bulbs (that are connected in
    parallel with the dome light). Those bulbs are resistors.
    Actually, you may notice the dome light dims, slightly, when
    you turn the headlights on, because the battery will not
    hold a perfectly steady voltage as it generates the higher
    current. It has a little effective internal resistance that
    is in series with those two sets of lights that are in
    parallel with each other. The real world is complicated.
    The second resistor forms a second circuit (path for
    current) that is sort of beside the first path. The current
    through the first resistor is little changed by this second
    path, if the battery has a low internal resistance compared
    to each of these two resistors.
    Exactly so, but sometimes you need a power source to do two
    things for you, at the same time.
    Have you ever seen a Y connector to put on a water faucet so
    you can connect two hoses? Running water through one hose
    lets less water out of the faucet than running water from
    two of them at, the same time.
  5. Puckdropper

    Puckdropper Guest

    Christmas lights. When one bulb burns out, the strand stays light.
    (Hm... 300 lightbulbs come on at once when the OP understands this. ;-))

  6. ehsjr

    ehsjr Guest

    Yes, as others have said.
    Sure you are, you just don't know it. You've seen it
    hundreds of times. For example a car's headlights are
    in parallel, the 120 volt electrical devices in your
    house are in parallel, the clock in a clock radio is
    in parallel with the radio, and so forth.
    That has already been explained, but I'll add another
    way of thinking about it. Say you have a light bulb
    lighting an area. You want to make it brighter, so you
    add another bulb, and now it's twice as bright. That
    means twice as much electrical energy needs to be consumed
    That's why the current increases - you are using twice
    as much energy.

    In mathematical terms, E = I * R where E is voltage, I
    is current and R is resistance. When E is a constant
    voltage, say a 12 volt battery, and you decrease R,
    I must rise. Say you have a 12 volt battery connected
    to a 12 ohm resistor. E=I*R, so I must equal 1 amp.
    Now, you add a second 12 ohm resistor in parallel
    with the first. The paralle resistance equals 6 ohms,
    so 12 = I * 6 which means I is 2. Each resistor will
    have 1 amp through it, making 2 amps total.

  7. Randy

    Randy Guest

    if you parallel two equal resistances the total resistance is half,
    current equals voltage devided by the resistance, so when the
    resistance is cut in half the current doubles. Also in a series
    circuit you have the same current everywhere but different voltage
    drops relative to the resistance of each component, while in a
    parallel circuit you have the same voltage in each branch but
    different amounts of current according to the resistance of the branch.
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