Almost everything in this skill is one idea applied at different speeds. A voltage pushes, something resists, and a current results — that is direct current (DC). Let the push reverse many times a second and you get alternating current (AC), where components that did nothing in a steady circuit suddenly dominate it, because they respond to change rather than to level. Magnetism is what links the two worlds: a changing field makes a voltage, which is how power reaches you at all. Read this for the shape — what causes what, which quantity is shared and which divides, and where the everyday intuitions quietly fail.
Each chapter opens with the short version. Tap one to read the detail.
Charge, current, and the direction nobody means literally
~2 min
Whether a material conducts comes down to how tightly its atoms hold their outer electrons. Current then has two directions in circulation at once: electrons move one way, and the convention everybody draws points the other.
Ohm's law, stated three ways on purpose
~2 min
Voltage, current and resistance are one relationship you can enter from any side. The handbook states it three ways deliberately, because which form you need depends on which quantity is missing.
Series and parallel: what is shared, what divides
~2 min
Series shares current and divides voltage; parallel shares voltage and divides current. One consequence is worth committing to memory, because it is the check that catches most arithmetic errors.
Induction: only change makes a voltage
~2 min
A magnetic field sitting still induces nothing. A voltage appears only while the field through a circuit is changing, and the polarity of that voltage always fights the change that caused it.
Alternating current and the numbers that are quoted
~2 min
An AC wave averages to zero over a cycle, which is useless for describing how much work it does. The figure everyone quotes instead is the effective, or RMS, value — 0.707 of the peak.
Reactance and impedance: opposition that depends on frequency
~2 min
Capacitors and inductors oppose alternating current by an amount that changes with frequency, and they shift current out of step with voltage. Combine that with resistance and you get impedance, the AC replacement for R.
The p-n junction, and turning AC into DC
~2 min
Joining p-type and n-type material makes a junction that conducts easily one way and barely at all the other. That asymmetry is a diode, and pointing diodes at an AC supply is how rectifiers produce DC.
What actually injures people
~2 min
Regulators frame the danger as four outcomes — shock, electrocution, fire and explosion — and most injuries trace to a short list of ordinary failures rather than exotic faults.
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