ohm's law states that voltage and current are proportional to each other. but my doubt is that "to keep power as constant either voltage or current changes" here the voltage and current are inversely proportional to each other.when does the power satisfies the ohm's law?


closed as unclear what you're asking by pjc50, pipe, R Drast, Voltage Spike, Dmitry Grigoryev Mar 21 '17 at 17:39

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  • \$\begingroup\$ Explain why Power (voltage * current) would need to satisfy Ohm's law. Also you forgot about the Resistor which is what Ohm's law is all about, that's why we honor Mr Ohm by mentioning the value of resistors in Ohms. \$\endgroup\$ – Bimpelrekkie Mar 21 '17 at 8:59
  • \$\begingroup\$ What are you quoting? And what exactly is your question? \$\endgroup\$ – user1890202 Mar 21 '17 at 9:05
  • \$\begingroup\$ when voltage increases current also will increase. for constant power voltage and current changes inversely. why it is happening? \$\endgroup\$ – muhammed esa Mar 21 '17 at 9:08
  • \$\begingroup\$ "when voltage increases current also will increase" - Your initial thinking is already fundamentally flawed by not considering what part the resistor plays as FakeMoustache mentions \$\endgroup\$ – Doodle Mar 21 '17 at 9:14
  • \$\begingroup\$ I have to admit, the OP has a point. The statement "to keep power as constant either voltage or current changes" is flawed. It should read "To keep power constant, if voltage increases current must decrease and vica-versa." No idea where he is reading that from, since he quotes no source, but wherever it is, it is incorrect. Either that or he is quoting it out of context. \$\endgroup\$ – Trevor_G Mar 21 '17 at 14:25

Ohm's Law applies to resistors. Here voltage is proportional to current, if the resistance stays constant.

Power stays constant across an ideal transformer, or a DC to DC converter, which is a totally different beast, and has different laws. Here, power out is equal to power in (less losses), and a change of output voltage is met with an inverse change in output current to keep the power constant.

Trying to interpret the behaviour of one component with a law meant for another is dooooomed to failure.

  • \$\begingroup\$ mr.neil thamk you for your valuable answer. \$\endgroup\$ – muhammed esa Mar 21 '17 at 9:37

According to Ohm's law, since it's defined for resistors, if voltage increases then current will increase as well.

But if you're asking how can the current decrease if the voltage increases so that the power remains constant then the only example I can give is not a resistive system: Switchmode converter.

The following symbolizes a converter with transformer (Of course it can be without a transformer as well).


simulate this circuit – Schematic created using CircuitLab

Simply, to keep the power across the load constant, control circuitry maintains (via feedback) the input current if the input voltage varies.

  • 3
    \$\begingroup\$ They look like they're smooching eachother :) \$\endgroup\$ – Zimano Mar 21 '17 at 9:27

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