# RLC circuit inductor/capacitor combined voltage?

1. Dec 11, 2003

### ike2010

RLC circuit inductor/capacitor combined voltage??

I have an RLC circuit. I understand that the combined sum of the voltage magnitudes across the three components will be greater than the signal generator. I don't understand, however, why the combined voltage across the inductor and the capacitor will always be greater than zero? Isn't the voltage sign of these two components opposite? And if so, as one is increasing and the other is decreasing, wouldn't there be a moment when the sum equaled zero? Thanks for any input on this. It's driving me crazy.

p.s. for clarity, i'm wondering why the sum of the voltage across the inductor and capacitor is always greater than zero?

2. Dec 11, 2003

### chroot

Staff Emeritus
I think you're going to have to give us a little more information before we're going to be able to make sense of your question. What kind of RLC circuit is this? Series? Parallel? What kind of signal is driving it? How is the signal source connected to the circuit?

- Warren

3. Dec 11, 2003

### ike2010

My bad. It's an RLC circuit in series. The signal generator is 6V. The circuit is set up like this sg ---> capacitor ---> resistor ---> inductor ---> sg

The only thing I can come up with is that maybe the sum will always be greater than zero because this isn't a perfect circuit and there is resistance in the wire and the inductor?

4. Dec 11, 2003

### chroot

Staff Emeritus

You mean it's just a constant 6V? DC?

- Warren

5. Dec 11, 2003

### ike2010

6V AC

6V AC

6. Dec 12, 2003

### gnome

Someone please correct me if I'm mistaken, but I think that statement is wrong. The sum of the maxima of the three voltages will be greater than the signal generator, but I believe that at any given moment, the sum of the time-values of voltage across the resistor, capacitor and inductor must be equal to that of the signal generator.
This too, I think is wrong. The voltage across the inductor leads the generator by 90o and the voltage across the capacitor lags by 90o, so they are exactly 180o out of phase with each other and therefore when the voltage across one is zero, the voltage across the other will always be zero. (At that moment, however, the voltage across the resistor will be equal to that of the generator, which will not be zero.)

Last edited: Dec 12, 2003
7. Dec 13, 2003

### MaxMoon

g

The inductor does have some resistance value. Ideally, there is no resistance associated with the inductor, but in reality there is. You can find this value by getting out an ohm-meter.

8. Dec 29, 2010

### bluewish

Re: RLC circuit inductor/capacitor combined voltage??

why does the sum of the voltage drops in RLC series circuit not equal to the source voltage?

9. Dec 29, 2010

### SammyS

Staff Emeritus
Re: 6V AC

I believe the question is referring to the RMS voltage - that is the voltage that is read by an AC voltmeter.

This is a series circuit, so the instantaneous current is the same throughout the circuit. The instantaneous voltage across an inductor (an ideal inductor) leads the current by 90°. The instantaneous voltage across a capacitor lags behind the current by 90°. The instantaneous voltage across these two, is always 180° out of phase with each other.

If the inductor and the capacitor have the same impedance, the sum of the instantaneous voltage across them is indeed zero. However, for that same case, the sum of the RMS voltages across each will not be zero.

The quick answer to the question, "Why (is) the sum of the voltage across the inductor and capacitor is always greater than zero?" is:

The sum of the RMS voltages across the individual elements of an RLC series AC circuit is greater than zero, because the instantaneous voltages are out of phase with each other.