# Does someone have a non-textbook analogy to explain elements of a circuit?

I was wondering if someone could explain with analogies what inductors, capacitors, transistors, diodes, and op amps do?

I understand the basic idea; I'm currently an Electrical Engineering student. I already know the textbook definitions, but I was just wondering if someone had some sort of analogy to sum up everything that isn't something from a textbook. :) I'd like to further improve my understanding, but to be honest, just learning stuff from classes and books is hard and painful and boring, because as a student it's still a challenge to see the BIG picture.

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• Steeven's answer looks pretty good to me, but can I chuck in that conduction current is not the only current. Have a look at Nader Engheta's Taming Light at the Nanoscale: " It is important to note that the displacement current does not consist of drifting charged particles as the conduction current does". For an analogy, imagine you're in a canoe at sea, and a big wave comes at you. Displacement current is like the upward flow of water that bears you up. There's then a downward flow. Light is like alternating displacement current, hence vacuum impedance. – John Duffield Oct 6 '15 at 12:02
• Often used in my mechanical engineering classes to help electrical engineering students: a mass-spring-damper system from mechanical engineering has the exact same differential equation as a inductor-capacitor-resistor system from electrical engineering. – Sanchises Oct 6 '15 at 16:36
• Transistors and op amps are usually considered off-topic on Phys.SE. – Qmechanic Oct 6 '15 at 18:03
• – Adam Haun Oct 6 '15 at 18:56
• Realize that non-textbook analogy is a bit of a tall order (and I'm not exactly sure what purpose a completely unheard-of answer would even serve--the SE kind of Q&A site is good at providing canonical answers, not at being a creativity contest). Pretty much everything said in the answers below is found a good number of textbooks. – Fizz Oct 8 '15 at 4:25

A water flow analogy is classic and quite good a long way into electric circuit understanding. Think of water flowing in a pipe system. This is somewhat hard to adjust into the AC-area, tough, but worth a mention for DC and a good starting point.

Analogies for some definitions:

• Charge $[\mathrm{C}]$ $\longrightarrow$ Amount of water $[\mathrm{kg}]$
• Current $[\mathrm{C/s}]$ $\longrightarrow$ Water flow rate $[\mathrm{kg/s}]$
• Potential $[\mathrm{J/C}]$ $\longrightarrow$ Pressure $[\mathrm{Pa}]$
• Voltage or Potential difference $[\mathrm{J/C}]$ $\longrightarrow$ Difference in pressure $[\mathrm{Pa}]$
• Resistance $[\mathrm{\Omega}]$ $\longrightarrow$ A narrow part of a pipe or a mill wheel.

(In this context, see this other answer: https://physics.stackexchange.com/questions/161650/could-someone-intuitively-explain-to-me-ohms-law/161701#161701)

Analogies for the circuit parts:

• Capacitor $\longrightarrow$ A total blocking of the pipe with an elastic membrane (No water ever passes the membrane, but build up pressure on one side makes it expand and "push" the opposing side with equal pressure but the force directed the opposite way)

• Diode $\longrightarrow$ A one-way ball-valve (A diode prevents charge in one direction but permits without resistance in the other direction)

• Battery $\longrightarrow$ A pump returning water back to a starting point.

• Operational amplifier (Op amp) $\longrightarrow$ A pump rising the pressure (placed at a point in the pipe circuit).

• Transistor $\longrightarrow$ An adjustable valve (transistors are "adjusters" where a small signal can make large changes in current/voltage)

• Inductor $\longrightarrow$ A heavy mill wheel (at first it doesn't move, but after some time it does and doesn't give resistance to the flow anymore.)

• I'd like to back up that mill wheel, we were talking about that a while back. See Daniel Reynolds electronics course. – John Duffield Oct 6 '15 at 11:47
• You might want to clarify that that's a capacitor in series, while a capacitor tied to ground is like a holding tank. I'm not sure how to describe an inductor tied to ground. – Carl Witthoft Oct 6 '15 at 12:07
• Besides I don't know what sister you talk about, the analogy for an opamp is off. They are so versatile that there really no analogy, and saying it is a pump makes it sound like they have some external energy source, while their energy is fed from the circuit they are a part of. If at all an analogy for its most common configuration is "Magic device that makes the output pipes pressure equal to the pressure difference between its two inputs". – PlasmaHH Oct 6 '15 at 13:45
• I don't see how this is a non-textbook analogy. – Danu Oct 6 '15 at 15:14
• I took non-textbook to mean "talk me through it in layman's terms", rather than "explain it using a completely novel analogy that has never before appeared in any textbook". – Oscar Bravo Oct 6 '15 at 15:52

I'll have a go... This is all off the top of my head and should be checked out by reading a textbook after.

First thing to note is that most of these components only make sense with an oscillating electrical signal, i.e., AC. They are not much use in a DC circuit [though I bet someone can think of one... see comments]

• Inductor: Basically a coil of wire. When a current starts to flow a magnetic field starts to rise. This takes work as you "charge up" the magnetic field so it resists the change of current. This makes a sharp spike in the signal, turn into a blunt hump. So it smoothes the signal (useful in a rectifier circuit).

• Capacitor: A charge storage device: Initially, it allows the current to flow in rapidly but as it charges, it resists the flow and presents an ever-increasing resistance to the flow until its potential is the same as the driving potential. The rising charge on the driven side induces a similar charge on the other side and so it transmits a changing current (even though it is open-circuit for DC). In series, can be used as a filter - it will have a very low impedance for signals at its resonant frequency. In parallel, it smoothes differentials bewteen the two rails (rectifiers again)

• Diode: One-way gate: Current can flow one way only.

• Transistor: The name means Transfer Resistor - which tells you nothing! Basically, you connect the Collector to the top rail (e.g. +5V), the Emitter to ground via a resistor and the Base is your signal input. With nothing connected to the base, the Emitter will float to a certain voltage given by simple Ohmic voltage division (say, +2V). When you inject a current into the Base, this will reduce the effective resistance of the Collector-Emitter junction and the Emitter will rise in voltage. Reduce the base current and the Emitter will fall again. So the output (Emitter) will simply follow what the input (Base) is doing. The clever part is that current in the output is coming from the Collector so can be a strong as you like - you have amplified the Base signal! (kerannng!)

• OP-amp: A voltage amplifier: This is a "complex" integrated circuit, made up of many of the above. Basically; small voltage difference across the input terminals = big $V_{diff}$ across the outputs. Understand all of the above well before worrying about how?

• From this "explanation" how a transistor works (common collector configuration) I derive that the emitter VOLTAGE is following the base CURRENT. To me, a somewhat "questionable" explanation. And - with an open base node - the Emitter will "float to a certain voltage given by simple Ohmic voltage division" ??? – LvW Oct 7 '15 at 7:31
• @LvW If you want to write a "better" explanation, at the level "required" by the OP, go right "ahead"! – Oscar Bravo Oct 7 '15 at 14:56
• Thank you very much! This is for AC mostly? That is what I am studying right now .. – Kenny Truong Oct 8 '15 at 1:31
• @Owen-Boyle, the correct and logical description of the BJT function is that the collector current Ic is determined/controlled by the base-emitter voltage Ic=f(Vbe). Hence, the amplification properties of a transistor are characterized by its transconductance gm. – LvW Oct 8 '15 at 9:23
• @LvW The first thing I do when replying/commenting on a post is to click on the user to get a feel for the person behind the question. The OP is a kid just starting out (no offence, KT!). I try to pitch my answer to what I guess might be appropriate for the reader, without compromising accuracy. I accept that my answer is a bit flaky and hand-wavy. Your comment adds rigour and some useful terminology. I hope the OP can profit from combining the two! – Oscar Bravo Oct 8 '15 at 12:46

learning stuff from classes and books is hard and painful and boring

Welcome to engineering. :-)

Analogies can be helpful at the very beginning, but ultimately there's no substitute for directly understanding the subject, especially when you get to more complex topics like op amps. Understanding the big picture is something that takes time and a broad knowledge base. Rest assured that your hard and painful work will pay off. Pay special attention to the basics -- circuit analysis, electronics, E&M fields, signal analysis. As you study and (more importantly) do homework, you will gain intuition about the behavior of electronic circuits, and also the deeper mathematical models behind them.

That being said, the water flow analogy is not terrible. If you have a strong mathematical background, the mechanical analogy (voltage/current/resistance = force/velocity/friction) might help. Of course, those are only as good as your understanding of hydraulics and mechanics.

What might help more is reading many explanations of the same subject. Sometimes schools pick terrible textbooks, so look for others at your library. There are also many starter explanations of these concepts online. (William Beaty has some good stuff.)