Chapter 1
The Basic Trio: Power, Current, and Voltage
Everything in this book is built to answer one question: how much energy moved, and where did it go? To get there you need current, voltage, and the sign conventions that keep their directions honest. Eight exercises below.
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What you need before you start
Current is the rate charge flows past a point.
i(t) = dqdt A = C/s
Voltage is the work needed to move a unit of charge between two points.
v12 = dwdq V = J/C
Power is their product — and its sign depends entirely on how you drew the arrows.
p = v i W = J/s
Passive sign convention — current enters the + terminal.
Then pabs = vi and the element is a load.
Active sign convention — current leaves the + terminal. Then pdel = vi and the element is a source.
Active sign convention — current leaves the + terminal. Then pdel = vi and the element is a source.
In any closed circuit, Σpabs = 0. Nothing else in Chapter 1 is as useful as this.
voltage
current
power