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Resistor Concept

  1. Mar 5, 2009 #1
    ~~Resistor Concept~~

    Hi. I am having trouble with understanding the resistor. What does a resistor do in SERIES or in PARALLEL circuit?

    My understanding: ( plz correct me if i am wrong!)

    In series circuit, Resistor is a device that reduce the current in the circuit. And according to ohm's law, V = IR, when the current is reduced, the voltage will also be reduced. So I think resistor will reduce BOTH voltage and current. And if the voltage doesn't change, then what is the point of connect a voltmeter across a resistor, Right? So I think BOTH voltage and current are reduced by a resistor.

    Here comes another problem. If a resistor in series would reduce current and/or voltage, will the current BEFORE passing thru the resistor be larger than the current AFTER passing thru the resistor ? Or the current will only change at the point where the resistor is put?

    Example:

    Compare

    How will the brightness of 2 lightbulb change if i put a resistor bewteen 2 identical lightbulbs?
    How will the brightness of 2 lightbulb change if i put a resistor NEXT TO 2 identical lightbulbs? ( NOT BEWTEEN )

    Thx for helping

    P.S. Lightbulb similar to this one below.

    Breaking%2520the%2520lightbulb%2520Movie.jpg
     
  2. jcsd
  3. Mar 5, 2009 #2

    S_Happens

    User Avatar
    Gold Member

    Re: ~~Resistor Concept~~

    A resistor is a device that opposes current by reducing the voltage across it. If you look at a series circuit (a bunch of resistors and battery in series) you will see a voltage drop across each resistor, but the same current through each resistor.

    No, current is equivalent.


    First, a lightbulb IS a resistor.

    For resistors in series, the current is the same, while voltage will be different (less for each proceeding resistor).

    For resistors in parallel, voltage is equivalent, while current MAY differ depending on the resistance of each "side."
     
  4. Mar 5, 2009 #3
    Re: ~~Resistor Concept~~

    Thank you


    What do you mean by opposes current? Push the current back?
    So resistor will reduce voltage, but not current?

    So will the voltage be lesser after it pass one or several resistors? Keep decreasing?
     
  5. Mar 5, 2009 #4

    S_Happens

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    Gold Member

    Re: ~~Resistor Concept~~

    Let's just stick to the concept and simply clarify your original ideas, as you weren't far off base to begin with. The equation V=IR can tell us many things, but it has to be applied correctly.

    You deduced, correctly, that due to V=IR a resistor that reduces voltage also reduces current. Only when you look at the entire circuit, does this really apply. An increased resistance in the circuit will mean less current through the ENTIRE circuit, at a constant value (using a single loop circuit). What I mean to say is that in a simple circuit the current will be the same throughout.

    If you want to apply the formula to a single resistor, then you must realize that the current is constant through the resistor. This means that the voltage must change indirectly with resistance (before the resistor there is none, and after the resistor it has seen the full resistance).

    Yes, there will be a voltage drop across every resistor. The sum of the voltage drops is equivalent to the voltage for the entire circuit.
     
  6. Mar 7, 2009 #5
    Re: ~~Resistor Concept~~

    Thank you for helping again!

    I don't quite understand this part. Do you mean voltage drop?
    So if I have a given voltage and resistance, i can calculate the current through the resistor and that current is the same throughout the resistor?

    "voltage must change indirectly with resistance"

    - so if I constant current and resistance are given, then voltage will be different across the resistor? "before the resistor there is none, what is none? voltage?

    thanks
     
  7. Mar 7, 2009 #6
    Re: ~~Resistor Concept~~

    Ok this what my book says about resistance:
    "The principal applications of resistors are to limit the flow of current and, in certain cases, to generate heat."

    The formula that you have V = IR I think you mean by conductance which has a symbol of G which is expressed my this formula:

    [tex]G = \frac{I}{R}[/tex]

    1 [tex] \Omega[/tex] is equivalent to 1A of current in a material when 1V is applied across the material. Voltage is the potential energy in electrical terms, but current is the movement of electrons in the wire where a resistor opposes this current.

    Most of this I derived this from one of my dad's old books of electricity.
     
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