Electricity: the parts I couldn't understand

18 May 2013

I want to make robots. Not just a simulation running on a computer but real-life Actual® Stuff® with moving parts. The problem is that I’m a software engineer and hardware has a steep learning curve. One of the biggest surprises is that apparently I never understood electricity at all.

Sometimes it’s helpful to learn from a person who’s just one step ahead of you rather than a veteran. To that end I’ll do my best to explain a few concepts that didn’t make sense to me when taught the traditional way and what I’ve learned as a total amateur. I want to write this all down before I forget what was so hard about it.

Current

Electrons flow from negatively-charged stuff to positively-charged stuff. Electricity is the flow of electrons in a current. Sure, thanks. Let’s rephrase it: stuff that has too many electrons pushes those electrons away toward stuff with too few electrons. There’s going to be ‘current’ as those electrons get pushed away.

Here’s what I wish I’d somebody had mentioned to me: Electrical diagrams are all written backwards

TheyTraffic Beijing describe current flowing from positive to negative (red to black). Even though electrons are going in the opposite direction. This has confused me. Badly. I once exploded a car battery in high school because I knew enough chemistry to understand electron flow but didn’t know that ‘ground’ meant ‘where the electrons come from’. Ever since then I’ve struggled to get this straight. Here’s how I now think of it: electrical current is like the wave of movement in a traffic jam. The cars are all pointing forward but that’s not the direction of the current. Whenever the front-most car sees a gap it moves forward, then the one behind it takes its place, etc. From an outside observer it appears that there is a wave of open space moving backward. The electrons are the cars, the wave is the ‘current’.

So if you see an arrow on a circuit diagram it represents the wave moving backwards on a freeway – not the cars themselves.

Circuits

A circuit is a loop in which electrons can flow. They won’t move unless you connect a power source to the circuit because the whole point of a power source is to continuously push too many electrons into a circuit so they all have to move.

Resistance

OneThis resistor dims the LED of the most basic ways to modify an electrical circuit is to add resistance to one part. Here it helps to think of a water pipe that is narrow in the middle. The water will be able to move fast or slow depending on how much resistance it feels traveling through the middle. There’ll be higher water pressure one whichever side the water is coming from but the water will always move at the same speed on both ends of the pipe. The pressure is irrelevant – the more you constrain the middle the slower the whole pipe will be able to move water. It’s like this with an electrical circuit. If you add a resistor to one part the whole circuit will appear to slow down. You can plug in a lightbulb to the circuit on either side of the resistor and it will receive the same (diminished, thanks to the resistor) current. So a resistor doesn’t just resist current at the point where it’s installed – it slows the whole circuit down.

Induction

Induction is where magnetism connects one flow of electricity to another. Induction is useful when you have two circuits that you don’t want physically touching but you need the current in one to affect the other. Induction motors work this way; you provide current to the fixed part of the motor and that current (sharing a magnetic field with the rotating part of the motor) controls the rotation. The principle behind this is that an inductor resists changes in current. Said another way: It’s hard to increase the current of a circuit that contains a inductor and it’s also hard to decrease it afterward. This resistance to change comes from a sort of magnetic turbulence in the inductor whenever the current isn’t steady.

Capacitance

A capacitor is basically just a really short-term battery. Electrons pile up on one side of a capacitor and they’re much-needed on the other side – but they can’t cross because there’s a small gap. When a connection is made across the gap then the electrons all flow instantly from one side to the other (and through the rest of the circuit). This is how camera flashes work – they store up electricity for a few seconds and then let it all out at once. Note: I do not understand how capacitors are discharged. There are two terminals on a capacitor that you can hook up to your circuit but how do you trigger the discharge? If somebody can explain that to me I’ll update this page.

I could write about LEDs, motors, batteries, etc. but those all roughly made sense when I tried studying them. It’s the foundational stuff that confused me and I hope my perspective on these concepts might help you out too.


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