# Voltage from current across impedance

by perryben
Tags: current, impedance, voltage
 P: 8 So the power is S=VI*=ZI^2. the question is whether V=ZI* or just ZI. Using V=ZI when I calculate the voltage i get a negative answer. Thanks!
P: 1,011
 Quote by perryben So the power is S=VI*=ZI^2. the question is whether V=ZI* or just ZI. Using V=ZI when I calculate the voltage i get a negative answer. Thanks!
Are you using the rms value for I? You'll need that. RMS (root mean square) cannot be negative here. Also, rms current is either greater than or equal to the DC current (mathematics of rms vs. average).
Mentor
P: 41,303
 Quote by perryben So the power is S=VI*=ZI^2. the question is whether V=ZI* or just ZI. Using V=ZI when I calculate the voltage i get a negative answer. Thanks!
Thread moved from EE to homework help.

Why would V = ZI* ? Once you have the complex Z and I, the complex V should just be the product of those two. There are times that you use the complex conjugate of a quantity, but I wouldn't think you would use it here....

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