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.


Please if you found this post helpful or have questions.