# RLC Circuit qualitative explanation some1 ?

1. Nov 6, 2009

### sudar_dhoni

can some one explain me the qualitative aspects of RLC circuit
i am satisfied with the vector diagrams i want the physical meaning behind
say for instance in ac with inductor when current varies induced electric field is set up which opposes the current so there is a voltage drop
similarly in capacitor it stores charges and they repel the incoming current and also for resistor
like this i want a physical explanation of RLC series circuit what happens there and also about resonance and q factor because i study with proper reasoning
so PLZ PLZ PLZ any 1 to help me

2. Nov 6, 2009

### Bob S

In an RLC circuits, the voltage across the inductor is given by V = L dI/dt, and the current in the capacitor is given by I = C dV/dt. In an LRC circuit flows back and forth between the inductor and capacitor without loss, like the pendulum in classical mechanics. Energy loss in the resistor is like damping in the pendulum. the resonant frequency is given by 1/sqrt(LC) in radians per second.

Bob S

3. Nov 7, 2009

### sudar_dhoni

can u explain in terms of voltage drops in inductor capacitor and resistor
see the source supplies energy to current or charges,these charges then somehow or the other they have to return to lower energy state so they drop their energy (energy is same as voltage -enegy possesed by unit charge) so they drop their voltage.All i have said till now is in
http://www.physicsclassroom.com/Class/circuits/u9l1b.cfm
my doubt is that how does this happen in RLC circuit
plz dont explain in terms of vector diagrams as it is not convincing
i want the physical meaning behind

4. Nov 7, 2009

### Staff: Mentor

The energy stored in the capacitor is proportional to the square of the voltage, the energy stored in the inductor is proportional to the square of the current, the power dissipated by the resistor is proportional to both the voltage and the current. Energy oscillates between the capacitor and inductor and is damped by the resistor.

5. Nov 7, 2009

### Born2bwire

Do you understand how a capacitor and an inductor work? Bob S' explanation is pretty much complete, he already related the relationships between voltage and current for an ideal inductor and capacitor. Inductors and capacitors store energy. The energy is exchanged between the capacitor and inductor when they are in an isolated series circuit, in effect, they become a harmonic oscillator. The introduction of a resistor provides a means of dissipating the energy in the circuit, effectively damping the oscillator system.

6. Nov 7, 2009

### sudar_dhoni

do u mean that there is both gain as well as loss of voltage when it flows between capacitor and inductor?
could u explain much more deeper as i am not aware of oscillator
i think what u mean is that in one cycle capacitor may drop voltage and inductor might supply energy by means of induced electric field acting in same direction of current i.e while passing through capacitor it flows from higher potential to lower potential so it has to do work and voltage is dropped while in inductor it accelerates and supplies energy so that gain=loss and the only loss is in resistor. is this what happening ?
if not can u explain theoritically not mathematically

7. Nov 7, 2009

### Born2bwire

A capacitor works by storing charges on opposite plates when a voltage is applied across the plates. These charges build up an opposing voltage to the applied voltage and as more charges build up, eventually the built up voltage cancels out the applied voltage. If we were to remove the capacitor from this charging circuit, it can now act as a low-energy source. Equivalently, the energy is stored in the electric field that is built up by the charges. An inductor can be thought of as a coil of wire, like a solenoid. When current passes through the coils, it induces a magnetic field through the coils. Energy is stored in the magntic field as well.

In a time-varying circuit, the changing current in the inductor will cause a changing magnetic field in the coils over time. But, from Lenz's law we know that the changing magnetic field will impose an electromotive force that seeks to "oppose" the change in its field. This causes a voltage to be induced. In a sinusoidally applied signal, this causes the voltage and current to be 90 degrees out of phase with the current lagging the voltage.

Likewise, in a capacitor, a time-varying current will cause a time-varying voltage across the capacitor as the charges between the plates are constantly shifted. In a sinusoidal signal, the current leads the voltage by 90 degrees.

Thus, in a LC circuit, the phase of the energy in an inductor is 180 degrees out of phase with the capacitor. When one is fully energized, the other is fully dissipated of energy. This is what allows the energy to oscillate back and forth between the two elements at a specific resonant frequency. It is the same behavior as a harmonic oscillator, which is how an idealized pendulum behaves for small displacements. But an ideal capacitor and inductor do not dissipate energy, so the introduction of a resistor to make an RLC circuit causes power dissipation. However, since the phases of the current and voltages are not the same, the power dissipated is less than the power that is stored up in a capacitor or inductor over the same cycle. Only a part of the power (called reactive power) that is going back and forth between the inductor and capacitor is dissipated by the resistor over a cycle. So the resistor behaves as a dampener in our harmonic oscillator.

8. Nov 7, 2009

### sudar_dhoni

When one is fully energized, the other is fully dissipated of energy. This is what allows the energy to oscillate back and forth between the two elements at a specific resonant frequency

this means that energy of current is drained at one site and energy is gained at the other site such that at resonance both loss and gain are equal such that the energy of current is drained only by resistor.
if it is so what at happens at resonace is that inductor and capacitor actions are equal and opposite and they cancel off ,this is what resonace as power consumed is less.
my suggestion is that in order to achieve this state
no need to use inductor and capacitor.Use only resistor which is same as cancelling impedances of inductor and capacitor at resonace.Instead of struggling like this in order to achieve resonace dont use inductor and capacitor and use only resistor ????

9. Nov 7, 2009

### Staff: Mentor

Resistors do not store energy, they only dissipate energy (convert it from electrical energy to thermal energy).

10. Nov 7, 2009

### sudar_dhoni

what is the use of storing that energy by inductor and capacitor

11. Nov 7, 2009

### Staff: Mentor

Are you asking for practical energy-storage applications of capacitors and inductors or are you asking about something wrt the use of capacitors and inductors in RLC circuits. If the latter then I do not understand precisely what you are asking.

12. Nov 8, 2009

### sudar_dhoni

are you asking about something wrt the use of capacitors and inductors in RLC circuits. If the latter then I do not understand precisely what you are asking.[/QUOTE]

see
at resonance the impedance offered by capacitor and inductor are equal so the net effect on the voltage drop is cancelled. the voltage is only dropped by the resistor.which means only resistor is doing the dropping of voltage.
THEN why need to use capacitor and inductor which dont drop any voltage instead u use ony resistor

13. Nov 8, 2009

### Staff: Mentor

Usually because you want to suppress other frequencies as well as pass your resonant frequency. A resistor alone will not do that.

14. Nov 8, 2009

### sudar_dhoni

i cant understand what u mean could u plz explain elaborately
also in one cycle how much time the current changes its direction once or twice
i am clear about frequency of AC
PLZ DaleSpam i know u r a genius could u please explain the whole of RLC elaborately
also i have many doubts in electrical engineering could u plz shower upon me some of ur intellect by teaching me and answering my doubts

15. Nov 8, 2009

### Staff: Mentor

You are asking way too much. I cannot explain everything about a RLC circuit in a single post, there are literally whole textbooks on the subject.

However, it seems that your primary question is about impedance and since you mentioned some specifics I will assume that you understand impedance. Recall that the impedance of a capacitor is 1/(jwC) where j is the imaginary number, w is the frequency, and C is the capicatance. Similarly the impedance of an inductor is jwL where L is the inductance, and the impedance of a resistor is R where R is the resistance. If these are placed in series then the series impedance of the RLC circuit is jwL + 1/(jwC) + R = R + j(CLw²-1)/Cw which as you have mentioned is equal to R for the resonant frequency of w = sqrt(1/LC). However, consider other frequencies such as w = 0, in that case the impedance of the RLC circuit is infinite. So although the RLC circuit behaves just like the R by itself at the resonant frequency, the behavior of the RLC circuit at other frequencies is different. This can be used to create a "band pass filter", which will allow frequencies near the resonant frequency through, but suppress higher or lower frequencies.

16. Nov 8, 2009

### sudar_dhoni

thats what i asked
i asked at resonance the RLC circuit behaves just like the R by itself so at resonance without inductor and capacitor also is same is with both them connected

could u plz explain my doubt mentioned here no1 is responding to it