# Is VI=P strictly an ohmic relation

hover
Is VI=P strictly an ohmic relation or does it apply to all types of circuits?

Thanks

## Answers and Replies

Mentor

Is VI=P strictly an ohmic relation or does it apply to all types of circuits?

Thanks

What is the context of the question?

hover

What is the context of the question?

I'm just wondering if this equation predicts the power for any given circuit. Not just resistors but light bulbs and things that aren't Ohmic in nature.

hover

I want to know if a light bulb given a certain voltage and current will have VI=P power even thought the lighbulb isn't ohmic.

Homework Helper

power = volts x current is true at any point in time. Various components will affect how the power varies over time.

hover

So it doesn't matter whether the device is ohmic or not?

Mentor

So it doesn't matter whether the device is ohmic or not?

What do you mean by "ohmic"? Do you mean a power factor = 1?

hover

What do you mean by "ohmic"? Do you mean a power factor = 1?

I mean does the object have to follow ohms law V=IR?

Mentor

I mean does the object have to follow ohms law V=IR?

I'm not trying to be dense here, but I'm still not quite understanding. Do you mean where "R" is not a complex impedance? Could you please give an example of something that doesn't "follow Ohm's Law"? Thanks.

hover

I'm not trying to be dense here, but I'm still not quite understanding. Do you mean where "R" is not a complex impedance? Could you please give an example of something that doesn't "follow Ohm's Law"? Thanks.

Something like a light bulb. As the voltage increases, the resistance the lightbulb has isn't linear.

Mentor

Something like a light bulb. As the voltage increases, the resistance the lightbulb has isn't linear.

Ah. The resistance of the filimant varies with temperature (this is true of most conductors BTW). It doesn't vary with voltage, per se. A higher voltage causes a larger current to flow, which heats up the filament more, which changes its resistance. V=IR is true at any moment in time, even during the transients like at turn on or if you increase the voltage once the bulb is on.

Do you have another example of what you would consider non-Ohmic?

hover
Ah. The resistance of the filimant varies with temperature (this is true of most conductors BTW). It doesn't vary with voltage, per se. A higher voltage causes a larger current to flow, which heats up the filament more, which changes its resistance. V=IR is true at any moment in time, even during the transients like at turn on or if you increase the voltage once the bulb is on.

Do you have another example of what you would consider non-Ohmic?

That was basically it. So I'm guessing that VI=R does work for something like a light bulb right?

Mentor

That was basically it. So I'm guessing that VI=R does work for something like a light bulb right?

Yes, absolutely.

For circuits that have reactive devices like inductors and capacitors, the simple linear V=IR Ohm's Law equation is extended to include complex impedances. The equation still works, but "R" --> Z where Z is a complex number.

hover
Yes, absolutely.

For circuits that have reactive devices like inductors and capacitors, the simple linear V=IR Ohm's Law equation is extended to include complex impedances. The equation still works, but "R" --> Z where Z is a complex number.

Good! That clears everything up. Thanks! :)