Analogy to Water

The classic analogy to electricity is to water. With that analogy there are reasonably good analogs to many of the measurements of devices of electronics.

• Wires vs Pipes
  Electric flow is constrained to wires. But the analogy breaks down if you let the water leave the pipe system. Instead we will think of closed loop systems.
• Switches vs Valves
  A switch can either block flow or allow it to go. A valve can work similarly. However opening a water valve half way is not similar to a switch.
• Voltage vs Water Pressure
  As voltage goes up, the electric current goes up. This is quite similar to water pressure.
• Current vs Water Flow
  Water flow rate is quite similar to electric current.
• Resistors vs Constrictions
  Either a constriction or a blockage in a water pipe will reduce the flow. This can be overcome with higher pressure. Similarly you can maintain an electrical current by increasing voltage. And you can measure a pressure difference across a constriction, similarly to measuring a voltage difference across a resistor.
• Battery vs Pump
  A pump can be used to keep water flowing in a system. For our purposes it is best to imagine a closed system, with no open pipes. The water pressure is low on the input side of the pump and high on the output side. A battery similarly provides a voltage difference across its terminals.
• Capacitors vs Diaphrams
  If you put a rubber diaphram across a pipe it will stop the flow. But changes in water pressure will show up as temporary flows. Similarly there will be an effective current thru a capacitor, but only so long as the voltage is changing.
• AC and DC
  DC is Direct Current. It is kind of a misnomer since what is really being specified is the voltage, not the current. The voltage in a DC system is basically constant.
  This is in contrast to an AC or Alternating Current system where the voltage is varying regularly between positive and negative.

Ohm's Law

I will now reveal to you something that was a military secret in World War II.

V=I*R

Amazing. It's amazing that was considered secret. But I've never been able to fathom the military mind.

But there some fairly obvious implications. Assume that the resistance is a constant. (Much of the time that is a good assumption.)

• Doubling the voltage across a resistor will double its current.
• If you know the voltage across a resistor and the current thru it you can calculate its resistance.

Power

P=V*I

Now I want you to combine those last two equations in order to express power as a function of resistance and voltage. What is striking about the result?

References

• http://caladan.nanosoft.ca/tutor1.php -- Contains a little more theory than I have presented but also spends more time on the analogy. If my presentation leaves you unsure, please read it. Also includes Kirchoff's Laws.
• http://bnrg.eecs.berkeley.edu/~randy/Courses/CS39C.S97/telegraph/electricity.html -- Also speaks a little about inductors. However I think his capacitor analogy is weak.