# How can RMS current be phasor?

1. Sep 26, 2016

### Zheng_

If RMS current is equal to the value of the direct current that would produce the same power dissipation in a resistive load, how can it be phasor?

Last edited: Sep 26, 2016
2. Sep 26, 2016

### drvrm

In which type of circuits you have encountered complex voltages....
i think in AC circuits the elements may not dissipate energy but act as storage devices and are related by phase difference, there in the analysis the instant voltages may be complex.

3. Sep 26, 2016

### Zheng_

I
I'm sorry I mean RMS currect, i'd edited the title and yeah I mean AC circuit

4. Sep 26, 2016

### Zheng_

What I'm asking is RMS currect. I know that voltage and currect in AC circuit can be phasor, but why RMS current, which descibe a effective value, can be a phasor? It wouldn't change its value as the time is passing right?

5. Sep 26, 2016

### drvrm

the phase angle contains z the reactance which is complex

Suppose we have an inductive circuit, and here the voltage and current waves are not in-phase. Whenever a changing voltage is applied to an inductive coil, a “back” e.m.f. is produced by the coil due to its self-inductance. This self-inductance opposes and limits any changes to the current flowing in the coil.

Further the effect of this back e.m.f. is that the current wave form reaches its peak value some time after that of the voltage. The current always “lags” behind the voltage .

6. Sep 26, 2016

### Zheng_

Ok thanks

7. Sep 26, 2016

### nasu

But who said that the RMS value of the current is a phasor? You represent the variable quantities (current, voltage) by phasors.
The RMS value is just this, a value. Of course, you can take the magnitude of the phasor representing the current equal to the RMS value.

Same as the magnitude of the velocity vector is equal to the speed. This does not make speed a vector.

8. Sep 26, 2016

### Zheng_

But..one of the question in my homework let that I rms = 2∠10°?

9. Sep 26, 2016

### nasu

That may be but it's hard to say what they mean without context. It may be just a notation to say that the magnitude of the phasor is equal to the RMS value and has a phase angle of 10 degrees.
The conventions for notations depend on the specific textbook. Read the chapter about phasors and pay attention to the meanings of the notations.

10. Sep 26, 2016

### Zheng_

Ok thanks. I will go and read through my textbook.

11. Sep 26, 2016

### lychette

rms is a 'scaling factor'...all voltages in AC circuits (VR, VL and Vc can be quoted as rms, or average or maximum or whatever else.
This does not affect the angular relationship between these quantities...the phasor aspect of their representation.
It would be wise to be consistent

12. Sep 28, 2016

### Staff: Mentor

This tells you the current is i(t) = 2√2⋅sin(ωt+10°)