# For an ideal capacitor, when is the current leading and when is the voltage leading by 90 degrees?

Say I have a purely resistive load as a reference in parallel with an ideal capacitive load. I know that the current through the capacitor is said to be leading by 90 degrees. Specifically, though, what is it leading?

With the two loads (resistive and capacitive) in parallel with the power source, can we say that the voltage will be the same both in magnitude and phase across both components (i.e. $$\V_{\text{source}} = V_{\text{resistor}} = V_{\text{capacitor}}\$$)?

I want to say: when in parallel, the currents of the two components are different and the voltage is the same. Likewise, in series, the voltages will be different and the currents will be different. Is this correct?

• 90 degrees only applies when the power supply voltage is a sine wave. Any other type of AC signal requires a different approach. May 30, 2018 at 8:39
• @Andyaka If any wave can be broken into sin/cos components with fourier, shouldn't it still hold true for other waves? Care to explain a bit please? Jun 1, 2018 at 12:20
• $I_C = C\cdot\dfrac{dv}{dt}$ is the best explanation. Jun 1, 2018 at 12:25

$$\text {C I V I L}$$

In a Capacitor the current (I) leads the Voltage which leads the current (I) in an inductor (L).

With the two loads (resistive and capacitive) in parallel with the power source, can we say that the voltage will be the same both in magnitude and phase across both components (i.e. $$\V_{\text{source}} = V_{\text{resistor}} = V_{\text{capacitor}}\$$)?

Yes. Both share common nodes so this must be true.

I want to say: when in parallel, the currents of the two components different and the voltage is the same.

Correct. Parallel components always have the same voltage.

Likewise, in series, the voltages will be different and the currents will be different.

Correct. Series components always have the same current.

• In the last quote, when I say voltages will be different, the voltages will differ in both magnitude and phase shift with respect to the current, correct? i.e. for the cap the voltage will be 90 degrees out of phase with the current. May 29, 2018 at 23:36
• You are correct. May 29, 2018 at 23:37

The current through a capacitor always leads the voltage across the capacitor by 90 degrees. The current through a resistor is always in phase with the voltage across the resistor. The voltage across elements in parallel must be the same. The current through elements in series must be the same. These are the rules...apply as necessary.

You are correct, when the two are in parallel, the voltage across them is the same (like any components in parallel). The current thru the capacitor in that case will be 90 degrees out of phase with the voltage they share.

In series, the current thru each component will be the same and the voltage across the capacitor will be 90 degrees out of phase with that current.

Hope this helps.