Electrical – Impedance phase angle relative to


In a simple PARALLEL RC circuit with two components of Resistor 150 Ohms and Capacitor 0.000000075F (75nF). AC supply 10V at 20,000Hz.

Impedance has a phase angle of -54.7 degrees if my calculations are correct.

What is it lagging -54.7 degrees behind? I suspect the answer is "Voltage supply" at 0 degrees but am unsure.

EDIT. Ill try to rephrase.

Can anybody complete this sentence:
"……" is lagging "……" by 54.7 degrees

This should enable me to understand what is at what angle

*EDIT 2:
For reference my understanding of phase angle has now improved and I will attempt to explain the answer I was looking for.

In the above described circuit the phase angle is 54.7 degrees. This angle represets the phase difference in degrees between:

Resistance and Impedance.

Conductance and Admittance.

True power and Apparent power.

And most importantly, Current (which is in phase with Resistor voltage) and Supply voltage.

Best Answer

When there are two dots (A and B) separated by an angle (say 60 degree) on the circumference of a rotating wheel, you may say... 1. A leads B by 60 degrees, or 2. B lags A by 60 degrees, or 3. A lags B by 300 degrees, or 4. B leads A by 300 degrees.

So the point is what have you choosed as your reference.

In parallel circuit what is common for both components is the voltage, thus voltage can be used as the reference. Current in the capacitor, by common convention, is said as leads the voltage by 90 degrees. But you can also put it as the current lag behind the voltage by 270 degrees. Mathematically it doesn't matter at all as long as you mark the vectors accordingly.

The current in the resistor is in phase with the voltage and the degree of separation between them is zero.

When you solve the resultant supply current vector of the capacitor and the resistor currents, the vector will fall between the two, thus the resultant current still leads the supply voltage by theta. Or you can also write this as the supply current lags behind the voltage by 360 minus theta.

Mathematically you will be right in either convention. But normally we practice to put phasors in small angle definition. So current leads the voltage is the common convention.

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