P=VI and V=IR??? Confusion!

Discussion in 'Homework Help' started by Engr Mick, Apr 6, 2014.

  1. Engr Mick

    Thread Starter New Member

    Jan 29, 2014
    According to Ohm's Law, Current increases, when Voltage increases,(I=V/R), But Current decreases, when Voltage increases according to (P=VI). Explain?
  2. ericgibbs

    Senior Member

    Jan 29, 2010
    hi Mick,
    Ohms Law states that the Current is directly proportional to the applied Voltage and inversely proportional to the Resistance.

    The Equation is: I = V/R

    In a circuit if you have a Fixed value of P, then the Current will decrease as the Voltage increases, otherwise the Fixed value of P would change

  3. ericgibbs

    Senior Member

    Jan 29, 2010
    hi Mick,
    As an example, say you wanted a resistor to be heated so that it dissipates 100Watts.

    You could have a voltage supply of 100V and the current would have to be 1Amp
    If you have a current source that has only a 0.5A output, you would have to increase the voltage to 200V in order to maintain the 100Watts.

  4. amilton542

    Active Member

    Nov 13, 2010
    Another way of looking at it is that in any circuit it will obey the law of energy conservation.

    Assume a voltage source V connected in series to a load resistance R:

    Then let  P_0=-V_0 I_0 be the power supplied by the source, the power absorbed by the load will be P_1=V_1I_1 assuming ideal conditions.

    The parameters will adjust to suit.
  5. #12


    Nov 30, 2010
    The point you're over looking is the phrase, "For a given wattage".

    For a fixed amount of watts, the current needed will be less if the applied voltage is increased.

    Of course, this demands that the resistance of the circuit be larger to support the increased voltage at the lower current, but this isn't (usually) how you design circuits. This is how you check your design to see if it will smoke. You learn Watt's Law and then set it aside until you need it to design a heater or double check a voltage design.
  6. Ramussons

    Active Member

    May 3, 2013

    You are trying to compare 2 different parameters.

    Ohms law, as you have put it is I = V/R.

    The other equation is Power P = Voltage V x Current I

    Just where is the contradiction?