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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.

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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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  • \$\begingroup\$ Im marking this as correct. I believe the answer from Andy aka is also correct. I am going to add a second edit to my original post to clarify the answer I was specifically looking for as I was unable to articulate my question well initially. \$\endgroup\$ May 24 '16 at 18:06
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Edited to account for my random inability to read the question correctly.

An impedance is not normally described as lagging. The current is described as lagging/leading the voltage applied.

However, in the capacitance example, the current leads by 90 degrees and this forces the impedance to have a negative phase angle because if the angle in the denominator is positive it becomes equivalent to a negative angle in the numerator.

An easy way to remember is CIVIL - in a Capacitor, I leads V and V leads I in an inductor.

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  • \$\begingroup\$ I do not have a good understanding of everything relating to this topic. From your answer I look again at "Current Triangle" diagram. Is it true to say for above circuit: When "volt supply" and "current resistor" are 0 degrees, then "current capacitor" is 90 degrees and "current total" is 54.7 degrees? \$\endgroup\$ May 6 '16 at 16:33
  • \$\begingroup\$ Capacitor current is +90 degrees with respect to a sinewave voltage applied because I(cap) = C.dv/dt and the differential of a sinewave is a cosine wave which is 90 degrees in front. If your impedance is a capacitor then Z = V(cap)/I(cap) which implies a negative reactance because a +90 degree in the denominator becomes -90 degree in the numerator. The addition of a resistor forces the current from +90 degrees to closer to 0 degrees. \$\endgroup\$
    – Andy aka
    May 6 '16 at 17:19

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