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What will be the total resistance of ideal voltage source?

  1. Apr 2, 2015 #1
    If we connect a single resistor in the circuit of ideal voltage source then what'll be its total resistance? ? Isn't it zero? As shown in image .so is it possible that the total resistance of a circuit is 0 Although it has 5ohm resistor!? IMG_20150403_091252.jpg
  2. jcsd
  3. Apr 2, 2015 #2


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    Staff: Mentor

    An ideal voltage source can power any load you throw at it while continuing to maintain a constant voltage. So its resistance remains zero.
  4. Apr 2, 2015 #3
    yaa..Sir, I know that its internal resistance must be zero. But what's about total resistance of the circuit? Is it also be zero?
  5. Apr 2, 2015 #4


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    Staff: Mentor

    If that wasn't the answer to your question, then you weren't asking the right question. The voltmeter is across the source, so regardless of what else is going on, its voltage is held fixed by that source.

    The effective resistance of a circuit comprising an ideal voltage source in parallel with anything is zero ohms.
  6. Apr 4, 2015 #5
    I think maybe you are confused about the calculating the equivalent impedance (resistance in your case) of a circuit.
    There are 2 circuits to draw.
    The first circuit is the Thevenin equivalent of the circuit. In your case, it would be an ideal voltage source with a 0 Ohm series resistance. This circuit is used to calculate DC values mostly. So in this case if the voltage is 5 V and the load resistor is 5 Ohms, the DC current is 1 Amp.
    The second circuit is the equivalent impedance (or resistance in your case) of the circuit. This circuit is calculated by replacing ideal voltage sources with a wire (I.e., shorted). In your case, this circuit becomes just a resistor of 0 Ohms (Note that there is no voltage source in this circuit). The 5 Ohm load is still across the terminals. Normally this circuit is used for AC analysis. Suppose you connect a 110 VAC supply with a 110 Ohm resistor in series across the 5 Ohm resistor. What is the AC voltage across the 5 Ohm resistor? By superposition, the voltage across the terminals is 5 Volts DC (calculated from the first circuit) plus 0 VAC (calculated from the second circuit). Normally the equivalent impedance is not zero and there is an AC component across the terminals. By using the equivalent circuit impedance, the task of calculating signal levels is made easier.
    Hope this helps.
  7. Apr 4, 2015 #6

    jim hardy

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    A question well stated is half answered.
    Above question is not well stated. You introduced a term that you did not first define.

    Just what do you mean by "total resistance" ?
    The "total" of all the resistances in your circuit is 5 ohms + Rvoltmeter.

    Are you considering your voltage source and resistor as being in series or being in parallel when you measure "total resistance" ??

    You must define between what two points you intend to measure said "total resistance" .
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