Physics - Electromagnetism - R-L-C circuits

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    Hello it's a me again Drifter Programming! Today we continue with Electromagnetism to get into Resistor-Inductor-Capacitor (R-L-C) circuits! I highly suggest you to go and read stuff about the components which are the Resistors, Capacitors, Inductors, and also about the previous circuits that we covered: R-C circuits, L-C Circuits and R-L circuits...The most important has to be the previous article about L-C circuits. So, without further do, let's get started!

R-L-C circuit

    Last time we talked about circuits that contain an inductor with inductance L and a capacitor with capacitance C. But, such a circuit is not feasible in real life, cause these components and even the wires that connect them together have some electrical resistivity. Considering that the total resistance R of a circuit is stored in one resistor we then define the so called Resistor-Inductor-Capacitor (R-L-C) circuits, which are closer to "real life". As with the previous circuits, we will again only get into the series connection of those three components. Connecting them in serial means that the current flowing through each element is equal to the total current I that is flowing through the circuit (serial -> same current, parallel -> same voltage).

Parallel circuits will be covered when we get into AC current.

     In L-C circuits we saw that the energy from the capacitor was "moved" to the inductor and vise versa in an continuous oscillation of frequency ω. At each period of this oscillation the capacitor started out fully charged, was discharged, charged again in inverse polarity and discharged again by getting back to the initial position. During that time the inductor started out fully de-energized, was energized, de-energized again and then energized in inverse polarity, de-energizing one final time to get back to the initial non-energized position. No energy is lost or created in such a circuit and so the total energy remained the same. The energy just changed from electric to magnetic, and vise versa. By including a resistor component, energy in form of heat will start to "leave" the circuit. This means that the total energy of the circuit will decrease at each oscillation, until it reaches zero. The capacitor will have a smaller charge Q at each oscillation giving the capacitor less electric energy, whilst the current in the circuit will be smaller and smaller, decreasing the magnetic energy stored in the inductor. I'm of course talking about the max-points, where each of those components has the total energy of the circuit, but the total sum of energy at each given point of the oscillation of course decreases as well! From all this we can understand that we now have a damping oscillation.


Let's analyze this oscillation a little bit better...

    Consider a series RLC circuit that contains a voltage source V. Applying KVL for this circuit we find that:

     A very similar equation to the one of last time, where we now also consider the voltage drop across a resistor of resistance R that can be found using Ohm's law (V = IR).

     By doing the Calculus needed (solving the differential equation etc.) we can find the final equation of the charge in the capacitor is:

      The coefficient of t is the so called oscillation frequency of a damping oscillation noted as ω':

     When R = 0 we of course get the oscillation frequency ω that we covered last time! As R is increasing the frequency becomes smaller and smaller. At the point where R^2 = 4L/C the subroot now gives zero and so the circuit isn't oscilatting at all (critical damping). If we increase the resistance R even more, the circuit will be over-damped and Q will now be calculated as the sum of two decreasing exponential functions!

A cool simulation can be found in the following link:

You can see how the individual voltages start dropping, until finally reaching zero.


     RLC circuits are being used in many many applications. More specifically the Series RLC circuits that we covered today are used in:

  • Variable tuner circuits
  • Low pass, high pass, band pass and band stop filters
  • Oscillators
  • Voltage multipliers
  • Pulse discharge circuits
     We will get back to these circuits when we get into AC current, which is actually the next topic of Electromagnetism!



Mathematical equations that I had in this post where drawn using quicklatex!  

Previous posts about Electromagnetism

Electric fields:

Getting into Electromagnetism -> electromagnetim, electric charge, conductors, insulators, quantization

Coulomb's law with examples -> Coulomb's law, superposition principle, Coulomb constant, how to solve problems, examples

Electric fields and field lines -> Electric fields, Solving problems around Electric fields and field lines

Electric dipoles -> Electric dipole, torque, potential and field

Electric charge and field Exercises -> examples in electric charges and fields

Electric flux:

Electric flux and Gauss's law -> Electric flux, Gauss's law

Applications of Gauss's law (part 1) -> applying Gauss's law, Gauss applications

Applications of Gauss's law (part 2) -> more Gauss applications

Electric flux exercises -> examples in electric flux and Gauss's law

Electric potential:

Electric potential energy -> explanation of work-energy, electric potential energy

Calculating electric potentials -> more stuff about potential energy, potential, calculating potentials

Equipotential surfaces and potential gradient -> Equipotential surface, potential gradient

Millikan's Oil Drop Experiment -> Millikan's experiment, electronvolt

Cathode ray tubes explained using electric potential -> cathode ray tube explanation

Electric potential exercises (part 1) -> applications of potential

Electric potential exercises (part 2) -> applications of potential gradient, advanced examples


Capacitors (Condensers) and Capacitance -> Capacitors, capacitance, calculating capacitance

How to solve problems around Capacitors -> combination, solving problems, simple example

Electric field energy and density -> Electric field energy, energy density

Dielectric materials -> Dielectrics, dielectric constant, permittivity and strength, how to solve problems

Electric capacitance exercises -> examples in capacitance, energy density and dielectrics

Current, resistance and EMF:

Electric current -> Electric current, current density

Electrical resistivity and conductivity -> Electrical resistivity, conductivity, thermal coefficient of resistivity, hyperconductivity

Electric resistance -> Resistance, temperature, resistors

Electromotive Force (EMF) and Internal resistance -> Electromotive force, internal resistance

Power and Wattage of Electronic Circuits -> Power in general, power/wattage of electronic circuits

Electric current, resistance and emf exercises -> exampes in all those topics

Direct current (DC) circuits:

Resistor Combinations -> Resistor combinations, how to solve problems

Kirchhoff's laws with applications -> Kirchhoff's laws, how to solve problems, applications

Electrical measuring instruments -> what are they?, types list, getting into some of them, an application

Electronic circuits with resistors and capacitors (R-C) -> R-C Circuit, charging, time constant, discharging, how to apply

RC circuit exercises -> examples in Kirchhoff, charging, discharging capacitor with/without internal resistance

Magnetic field and forces:

Magnetic fields -> Magnetism, Magnetic field

Magnetic field lines and Gauss's law of Magnetism -> magnetic field lines, mono- and dipoles, Flux, Gauss's law of magnetism

The motion of charged particles inside of a magnetic field -> straight-line, spiral and helical particle motion

Applications of charged particle motion -> CERN, Cyclotrons, Synchrotrons, Cavity Magetron, Mass Spectrometry and Magnetic lens

Magnetic force applied on Current-Carrying Conductors -> magnetic force on current-carrying conductors/wires, proofs

Magnetic force and torque applied on current loops (circuits) -> magnetic force on current loops, magnetic moment and torque

Explaining the Physics behind Electromotors -> tesla, history and explaining the physics behind them

Magnetic field exercises -> examples in magnetic force, magnetic flux, particle motion and forces/torque on current-carrying conductors

Magnetic field sources:

Magnetic field of a moving charged particle -> moving charge, magnetic field, force between parallel charged particles

Magnetic field of current-carrying conductors -> magnetic field of current, Biot-Savart law

Force between parallel conductors and the magnetic field of a current loop-> force between parallel conductors, magnetic field of current loop

Ampere's law and Applications -> Ampere's law, applications

Magnetic materials -> Magnetic materials, classification and types, material examples

Displacement current -> Displacement current, Extension of Ampere's law

Exercises in Magnetic field sources -> examples all around magnetic field sources

Electromagnetic Induction:

Electromagnetic Induction and Faraday's law -> Electromagnetic Induction, Experiments, Faraday's law 

Motional Electromotive Force (Emf) -> Motional Emf, Faraday's law and motional emf, generalization

Lenz's law and Induced Electric fields -> Lenz's law, Induced Electric Fields

Eddy Currents and Applications -> Εddy currents, applications (brakes, testing, others)

Maxwell's equations -> What they are, each equation analyzed separately

Electromagnetic Induction exercises -> examples all around Electromagnetic Induction

Mutual and Self Induction:

Mutual Inductance -> Mutual Inductance, applications

Self Induction -> Self Induction, Lenz's law, Inductance of a Coil 

Magnetic Energy Density -> Energy stored in a magnetic field (or inductor), Magnetic energy density, Coaxial Cable Inductance Example

R-L circuits -> R-L circuit energizing, de-energizing, Characteristic time constant

L-C circuits -> L-C circuit, oscillations, energy cases, applications

    And this is actually it for today's post and I hope that you learned from it! Without covering AC current, such circuits cannot be analyzed as in depth as the previous ones, but I still think that you got an overall idea. Next up in Physics are exercises/examples all around Mutual and Self Induction...including the 3 types of circuits that we talked about during the previous articles!

C ya!

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