Voltage across the capacitor lags behind the current through capacitor as we know. But why the voltage across the capacitor lags behind the source voltage in this circuit ? Similarly it is found that voltage across the inductor leads the source voltage in RL circuit.

Why this happens when the resistor between source and capacitor in RC circuit & resistor between source and inductor in RL circuit has no effect on the phase of the voltage and current in these circuits? Shouldn't the source voltage apear across the Capacitor and Inductor although with less amplitude but with the same phase angle across capacitor/inductor in these cases ?

enter image description here


1 Answer 1


Imagine if the resistor is so large that the capacitor barely charges at all. Now consider what happens just after the source voltage reaches its positive peak. The capacitor voltage will stay below the source voltage for almost another 90 degrees. So the capacitor voltage will continue increasing through all that time.

It's easy to see it under these extreme conditions. Obviously, if the resistor or the capacitor is smaller the capacitor voltage will reach its peak sooner. But it will always be after the source voltage did since the source voltage will still be a bit higher than the capacitor's voltage and the capacitor voltage can't begin dropping until the source voltage is less than it is.

  • \$\begingroup\$ so if there is no resistor ? source and capacitor voltage will have exactly same phase angle ? right ? \$\endgroup\$
    – Alex
    Apr 21, 2021 at 8:05
  • \$\begingroup\$ If there's no resistor, then the source voltage and the capacitor voltage will be the same. But that's not possible in real life. \$\endgroup\$ Apr 21, 2021 at 8:05
  • 1
    \$\begingroup\$ David, I think you meant to say it's not possible to have no resistance in real life. It's very easy to have no resistor :-) But the wires will have resistance, the signal a source impedance and the capacitor an ESR. \$\endgroup\$
    – TonyM
    Apr 21, 2021 at 8:21
  • \$\begingroup\$ And why the inductor voltage will lead the source voltage in RL circuit? \$\endgroup\$
    – Alex
    Apr 21, 2021 at 9:58
  • \$\begingroup\$ @Alex Precisely the same argument, but with current instead of voltage. \$\endgroup\$ Apr 21, 2021 at 10:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.