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