Upon first seeing a circuit diagram like the above, with its dizzying, labyrinthine interconnections and mysterious hieroglyphics, you can be forgiven for believing that electronics might forever be beyond comprehension. And it is true that while the field of electronics has a useful array of water-based metaphors for explaining where electrons are going, there are some strange things happening deep inside the devices that make modern life possible.
All that having been said, understanding circuits boils down to being able to trace the interactions of four basic forces: voltage, current, resistance, and power.
Voltage, measured in volts, is often analogized as being like water pressure in a hose. For a given hose with a set diameter and length, more water pressure is going to mean more water flow and less water pressure is going to mean less water flow. If two 100-gallon tanks, one empty and one full, are connected by a length of pipe with a shutoff valve at its center, the water in the full tank is going to exert a lot of pressure on the valve because it ‘wants’ to flow into the empty tank.
Voltage is essentially electrical pressure, or, more technically, a difference in electrical potential. The negative terminal of a battery contains many electrons which, because of their like charges, are repelling each other and causing a build up of pressure. Like the water in the 100-gallon tank they ‘want’ to flow through the conductor to the positive terminal.
Current, measured in amps, is the amount of electricity flowing past a certain point in one second, not unlike the amount of water flowing through a hose. If more pressure (i.e. ‘voltage’) is applied, then current goes up, and correspondingly drops if pressure decreases. Returning to our two water tanks, how could we increase water pressure so as to get more water to flow? By replacing the full 100-gallon tank with a full 1000-gallon tank!
But neither the water in the pipe nor the current in the wire flows unimpeded. Both encounter resistance, measured in ohms when in a circuit, in the form of friction from their respective conduits. No matter how many gallons of water we put in the first tank, the pipe connecting them only has so much space through which water can move, and if we increase the pressure too much the pipe will simply burst. But if we increase its diameter, its resistance decreases and more water can flow through it at the same amount of pressure.
At this point you may be beginning to sense the basic relationship between voltage, current, and resistance. If we increase voltage we get more current because voltage is like pressure, but this can only be pushed so far because the conductor exhibits resistance to the flow of electricity. Getting a bigger wire means we can get more current at the same voltage, or more means we can increase current to get even more current.
If only there were some simple, concise mathematical representation of all this! There is, and its called Ohm’s Law:
Here ‘E’ means voltage, ‘I’ means current, and ‘R’ means resistance. This equation says that voltage is directly proportional to the product of current and resistance. Some basic algebraic manipulations yield other useful equations:
I = E/R
R = E/I
From these we can see clearly what before we were only grasping with visual metaphors. Current is directly proportional to voltage: more pressure means more current. It is indirectly proportional to resistance: more resistance means less current. Knowing any two of these values allows us to solve for the other.
That last fundamental force we need to understand is power. In physics, power is defined as ‘the ability to do work’. Pushing a rock up a hill requires a certain amount of power, and pushing a bigger rock up a hill, or the same rock up a steeper hill, requires more power.
For our purposes power, measured in watts, can be represented by this equation:
P = IE
You have a given amount of electrical pressure and a given amount of electrical flow, and together they give you the ability to turn a lightbulb on. As before we can rearrange the terms in this equation to generate other useful insights:
I = P/E
E = P/I
From this we can deduce, for example, that for a 1000 watt appliance increasing the voltage allows us to draw less current. This is very important if you’re trying to do something like build a flower nursery and need to know how many lights will be required, how many watts will be used by each light, and how many amps and volts can be supplied to your building.
There you have it! No matter how complicated a power grid or the avionics on a space shuttle might seem, everything boils down to how power, voltage, current, and resistance interact.
The majority of my knowledge on this subject was comes from an excellent series of lectures given by a former Navy-trained electrician, Joe Gryniuk. His teaching style is jocular and his practical knowledge vast. Sadly, near video eighteen or so, the audio quality begins to degrade and makes the lectures significantly less enjoyable. Still highly recommended.