Do you know where the nearest power plant is?
Direct current is easy to understand because there’s a constant voltage that wants to push charge through a circuit in a single direction. But if DC is so simple, why does the world use AC to power our homes and offices?
Back in the late 1800’s this was not decided at all and depending on where you lived, you might actually have DC supplied to your home. The reason AC finally won is because of two reasons.
First, we don’t want high voltages in our houses. 240 volts in some parts of the world is high enough.
And second, electric companies are in business to make money and that means they don’t want to waste resources by losing power in the power lines. If power is being used anywhere, the electric company wants somebody else to be paying for it. They don’t want to be paying themselves to warm up birds perched on the lines.
Make sure to listen to the full episode for the full explanation of how AC solved these issues. You can also read the full transcript below.
Transcript
In the last episode, I started explaining some simple electronics that will help you become a better programmer. This episode builds on that by explaining the reasons why the world uses alternating current instead of direct current. They’re usually abbreviated as AC and DC.
Direct current is easy to understand because there’s a constant voltage that wants to push charge through a circuit in a single direction. That doesn’t mean that the charge will always flow in that direction such as when charging a battery, you sort of overpower the battery and force the charge to flow back into the battery. But in general, DC is fairly easy to deal with.
So if DC is so simple, why does the world use AC to power our homes and offices? Back in the late 1800’s this was not decided at all and depending on where you lived, you might actually have DC supplied to your home. The reason AC finally won is because of two reasons.
First, we don’t want high voltages in our houses. 240 volts in some parts of the world is high enough.
And second, electric companies are in business to make money and that means they don’t want to waste resources by losing power in the power lines. If power is being used anywhere, the electric company wants somebody else to be paying for it. They don’t want to be paying themselves to warm up birds perched on the lines.
Let me fill in some missing pieces here. First, you need to know a little about power. I’m talking about power in a specific technical electrical sense and not as in how we normally say something like, “That machine’s really powerful.”
Power is defined as voltage times current and is measured in watts. When you add time to this, you get a watt-hour which is a unit of energy. If you use one watt of power for an entire hour, then you’ve used one watt-hour of energy. The electric company will bill you in terms of kilowatt-hours which are 1000 watt-hours.
If you listen to last week’s episode, then you’ll also know that voltage is equal to current times resistance. What resistance am I talking about? The power lines are made of aluminum or copper because they conduct electricity very well. But they still have a small amount of resistance. And that resistance increases as the line gets longer. This means that the farther away you live from the power plant, the higher the resistance will be. It’s also the one thing that’s fairly constant. What I mean is that while, sure, the electric company can install new power lines made of some new material with a lower resistance, they don’t do this every day. Once power lines are installed, they remain for years.
Now if we combine the two formulas and rearrange things a bit, we get power equals current squared times resistance. That’s important because it means that current is much more important when calculating power than resistance. If you push 10 amps through a wire, the current will play a factor of 100 in how much power is used. And if you push 100 amps? Then it plays a factor of 10,000. A thousand amps plays a factor of a million.
Since current is so important, the electric company wants to lower it as much as possible. But they have a problem, because the energy demands of the public don’t go down just because they want to lower the current. And they can’t just raise the voltage to accommodate a lower current because that gets back to the first point about how most people don’t want high voltages in their homes.
Back in the 1800’s, one solution for this was to put the power plants closer to the homes. This allowed the electric company to send direct current a short distance, maybe less than a couple miles. The resistance was lower because of the short power lines. The energy need was lower because the power plant served fewer customers. And that meant that both the current and voltage could be kept low.
At the same time, another solution was to use AC because it allows the voltage to easily be increased for transmission over long distances and then easily brought back down to lower levels nearby the customer. This process uses transformers and I can explain more about transformers next week.
For now though, just know that while we can efficiently change DC voltages today, back in the 1800’s this was not so easy. But transformers were available and can change AC voltages with no moving parts.
Today long distance power lines can reach over 700 thousand volts. That high voltage allows the current to be lowered which reduces the power lost in the wires. We still use DC whenever the power source is nearby or batteries are involved such as inside computers. A computer will plug into an AC power source and convert that to DC for use within the computer.
And because we can efficiently transmit electricity over long distances, this allows us to build bigger power plants away from most people. The power plants can be hydroelectric powered by dams, or use wind power generated by giant windmills, or use large fields of solar panels to capture energy from the sun, or use nuclear fuel. We don’t need a power plant located just a mile or two away especially if it’s radioactive.