Water and Electricity

Learning about electricity and circuits doesn't have to be complicated.

In fact, you probably already have a lot of good intuition about electronics from your everyday experiences.

While you can't easily see the tiny electrons speeding along a wire, you can imagine them as being similar to water flowing through pipes.

Let's start with a battery. A battery is a voltage source and represents potential energy. Think of it as a bucket of water; the higher you hold it above the ground, the more potential energy (or voltage) it has.

We'll use one of these symbols to designate a battery or generic voltage source respectively.

When we connect a large pipe, the water quickly flows from the voltage source to ground. We call this a short circuit, since the flow is immediately "shorted out" and doesn't do anything useful. All of the energy returns to ground directly. 

In an ideal world, we would expect the resulting flow, or current, to be infinite.

Let's slow things down a bit. 

The next important circuit element is a resistor. A resistor limits the flow of energy, much like a thin pipe or valve slows down and limits how much water can flow. 

The total flow (current, represented by the variable "i") has the relationship: V = i*R

When R has a large value of resistance (measured in Ohms), you can think of it as a small, very constricting pipe. When the resistance is low, current flows easily (a very wide pipe).

For example, if V equals 1.5 Volts (a standard AA battery) and R equals 3 Ohms we can see that the resulting current will be 0.5 Amps.

Now let's consider another fundamental element, the capacitor. 

The symbol for a capacitor looks like this.

In our water world, a capacitor behaves like a pipe with a flexible rubber stopper on the end -- water can only flow up to the point that the rubber is pushing back as hard as the water pressure.

This rubber-capped pipe doesn't seem like it would be all that useful. However, it is when the water doesn't just flow directly in one direction (direct current, or DC), but instead alternates between pushing and pulling (alternating current, or AC).

When the flow is DC (steady in one direction), the capacitor completely stops the flow just like the rubber cap. But if the flow is AC (pushing and pulling back and forth), the capacitor acts like a variable resistor. 

The faster the vibration back and forth, the easier it is to move the rubber diaphragm (less resistance). The slower the vibration, the closer it is to DC which has infinite resistance (nothing can flow).

We can plot the equivalent resistance of the capacitor depending on the frequency (f). This "frequency dependent resistance" is given a special name, reactance, and represented by the variable X.

So a capacitor is really just a frequency dependent resistor that limits low frequencies!

Mathematical equations may make your head hurt at first, but they are just a simple way of writing down an exact relationship between things.

For the capacitor, we can write down its effect as an equation like this.

That is, the capacitor's reactance is equal to one divided by (two * pi * frequency * capacitance).

But what if we want a component with the opposite effect -- something that lets low frequencies through but limits high frequencies?

Say hello to the inductor. Its symbol looks like a coil of wire because that's how inductors are made. We'll use the letter L because "i" is already used for current, and two meanings for the same letter would be confusing.

In our water analogy, the inductor is like a water wheel or flywheel; it takes some energy to start it spinning, but once it's in motion the heavy wheel tries to keep pushing water until it's slowed back down.

And if we make a graph of the inductor's reactance (its resistance as we change the frequency of pushing and pulling on the flow of current), it looks like this.

Notice how the inductor's reactance goes in the opposite direction of the capacitor's reactance.

If we write out the inductor's reactance equation, it looks similar to the capacitor but flipped.

Using an inductor and a capacitor together we can create resonant circuits that only let through a very narrow range of frequencies (like selecting a specific radio station, or boosting the bass sounds on your stereo).

We've seen that inductors and capacitors behave differently depending on the frequency. But other devices have properties that change with voltage instead of frequency.

The simplest voltage dependent component is the diode. A diode is like a valve where current can flow one way easily, but gets immediately stopped if it changes directions.

In addition to behaving like a one-way street, diodes also have properties that make them useful for non-linear effects (such as guitar distortion pedals) and lighting (like LEDs; everybody loves blinky lights!).

To illustrate that a diode emits or receives light, we can add some arrows to its symbol like this.

LED Photodiode

Finally, let's consider a transistor. You've probably heard about old transistor radios or the fact that transistors are at the center of what makes computers possible.

A transistor is just a special kind of control valve. By separating the control input (the knob) from the main flow of current, a small input can cause a large change. 

In terms of the water analogy, think of the transistor as a simple faucet; a small change to the knob can drastically change the total flow of water through the pipe.

Of course there are lots of different types and configurations of transistors for different applications, but in the end they're all just control valves -- tiny, precise control valves that can work together to do incredible things.

In fact, the computer you're using right now probably has over one billion transistors packed into a square smaller than a postage stamp. That's a lot of faucets!

In this note you've learned about:
- Voltage sources (V)
- Resistors (R) 
- Capacitors (C)
- Inductors (L)
- Diodes (D)
- Transistors (Q)

You now know about the basic building blocks of modern electronics -- congratulations!

And using the water analogy you should be able to form intuition about all kinds of interesting things related to the world of circuits.

If you want to continue learning more, have no fear. It's easier than you think, and electronics can open the door to all kinds of fun projects and professional opportunities.

So what are you waiting for? Find a fun electronics project, get the basic parts, and start making your own awesome circuits!

This note is currently private and only visible to you. To share this note with others, Publish it.
Adam Kumpf
Created: 15 Apr 2015
Updated: 16 Apr 2015