4QD-TEC: Electronics Circuits Reference Archive
Understanding Electricity - an analogy with water
Electricity seems very difficult for most people to understand. You can't properly see it, you can't properly feel it and you can't taste it or smell it so what is it? It travels quite well through metals and generally poorly through non-metals. It doesn't behave quite like anything else.
But this is not quite true: fluid flowing through a system of pipes can behave in many respects like electricity flowing in a circuit. The 'obvious' exception to this is that if you pierce a pipe full of water, the water comes out.
Try piercing the insulation and touching the conductor: you will soon find the electricity coming out! The main difference is that water will fill any space whereas electricity will 'fill' only a conductor. Yes - the analogy is not perfect, but it is helpful and understanding will come not only from the similarities but also from the differences.
In a water system we measure water pressure in feet (foot head). In an electrical system pressure is measured in volts. There is quite a close analogy between feet and volts in this context.
Water flow can be measured in many units but we are using feet, so let's keep to cubic feet per second to measure flow rate. The electrical equivalent is Amperes (amps).
In our water system volume is in cubic feet. Electrical equivalent is Coulombs, which is not that commonly used. So one Coulomb per second is one amp. Flow and volume correlate nicely - cubic feet per second is clearly a flow rate.
Resistance and Ohm's law
In an electrical system resistance is measured in Ohms. There is no name for the water equivalent. So what is it? In electricity, if a pressure of one volt will drive a flow of one amp then the resistance is one Ohm. Two volts would drive 2 amps through 1 ohm: pressure divided by current equals resistance.
In the water system resistance doesn't equate to anything useful as an aid to understanding. However 'resistance' is just that: the amount that the pipe 'resists' the flow of water. Double the water pressure and the flow will double. Double the length of the pipe and, if we didn't change the pressure, we would expect the flow to halve. This, in common sense terms, is exactly what Ohm's law says! Increase the pressure and the flow must increase. Increase the resistance and the flow must decrease. So Ohm's law is Pressure/Resistance = flow. Or Volts/Ohms = amps.
In a water system a tank has capacitance. You might think then that capacitance is cubic feet.... Wrong, we have seen that coulombs is cubic feet. Cubic feet/coulombs is the amount of water/electricity you put in the tank/capacitor to raise the pressure/voltage to a certain level.
Do you see then that capacitance in an electrical circuit equates to the base area of the water tank. Capacitance equates to square feet. In a water (electrical) system, volume (coulombs) is base area (capacitance) times feet head (voltage). In a capacitor, 1 coulomb stored in a 1 Farad capacitor raises the pressure to 1 volt. In a water system 1 cubic foot stored in a tank of base area 1 square foot raises the pressure to 1 foot. The analogy is quite close! Consider also that in electricity, the capacitance between two plates is directly proportional to their area so you might expect area and capacitance to be related.
Inductance is to electricity as Mass is to water. The analogy is accurate but the difference is that mass is a property of the water (or other fluid) whereas the inductance is not a property of the electricity but a property of the pipe work carrying the electricity. It is as if, by winding our pipe work in a spiral, we could change the mass of the fluid in it! The analogy breaks down totally here.
I find the analogy of little use here - because water is so heavy you really cannot envisage it changing direction and doing anything useful! For a.c. in a water system you have to think of waves on an ocean! Even that is interesting, since if you think of the pressure a few feet below the surface, it will have a steady (d.c.) level with a superimposed a.c. level due to the waves: this is exactly what happens in most electronic circuits: the a.c. level is superimposed on a steady d.c. level!
Phase lag & lead
In an a.c. electrical system college students are taught that in an inductor current lags voltage and, in a capacitor, current leads voltage. How current could lead I never did understand because 'leading' implies prediction. Maybe time travel is possible... Anyway, who remembers which is leading and which is lagging? There are mnemonics for remembering it and yes, I know about 'ELI the ICE man'. This is not understanding but learning by rote. It simply goes to prove how difficult the understanding is. Personally I don't remember - I work it out from understanding!
But consider a water tank (capacitor). To get pressure we have to fill the tank (current must flow) for some time to establish a head of pressure (voltage) - so current must flow into the capacitor before we can get voltage across it...
Consider a mass - an inductor. You have to push a mass (pressure, voltage across an inductor) quite hard before it starts to move (current starts to flow). So in an inductor, voltage (pressure) across the inductor comes first, current through it follows! Via the analogy, understanding is quite easy.
And Yes: I know there are Mnemonics for remembering this. Any emails about Eli the ice man will be binned! Mnemonics do not help you understand! Understanding is not the same as remembering. Mnemonics help you remember only!
If you want more reading on this analogy try
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Page's Author: Richard Torrens
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