Basic Electricity and Electronics
Core concepts
Voltage is electrical pressure, current is charge flow, and resistance opposes current. Ohm's law gives V = I x R; power is P = V x I = I squared x R = V squared / R. In series, current is common, voltages add and resistances add. In parallel, voltage is common, currents add, and total resistance is the reciprocal sum; for two resistors, R = R1 x R2 / (R1 + R2). Kirchhoff's voltage law sums loop voltages to zero, and the current law sums currents at a node to zero.
AC signals have period, frequency, peak, peak-to-peak and RMS values. For a sine wave, RMS = 0.707 x peak and average = 0.637 x peak; 115 V aircraft AC is an RMS value. Capacitive reactance falls as frequency rises; inductive reactance rises with frequency. Impedance combines resistance and reactance. Transformers trade voltage for current by turns ratio while conserving power. Diodes rectify, transistors switch or amplify, and batteries in series add voltage while parallel cells add capacity.
Traps
The most common errors are treating a peak value as RMS, adding parallel resistances directly, and assuming current is the same in parallel branches. Watch for questions about reactance versus frequency and whether a transformer steps voltage up or current up.
Worked example
A 28 V DC bus feeds a 7 ohm load. I = V / R = 4 A; P = V x I = 112 W. If the same circuit drew 2 A, the power would be 56 W - power scales with current for a fixed voltage.