Electricity and Electronics
16% of the exam 2 min
The largest General subject in this site's weighting at 16%: the circuit laws, series and parallel math, AC behavior, transformers, batteries, circuit protection and measurement, semiconductors and logic.
The four laws you will use on every circuit question
Ohm's law ties voltage, current and resistance together: E = I × R. Watt's law gives power: P = I × E, which can also be written I² × R or E² ÷ R. Kirchhoff's current law says the current flowing into a junction equals the current flowing out, and his voltage law says the voltage drops around any closed loop add up to the source voltage. Almost every General electricity calculation is one of these four, rearranged. Under electron flow theory current moves from negative to positive; conventional flow, used on many schematics, draws it the other way.
Series, parallel, and the capacitor reversal
Resistors in series add. Two resistors in parallel combine as product over sum, equal resistors in parallel are one value divided by their number, and any parallel total is smaller than the smallest branch, which is the fastest way to throw out distractors. Capacitors behave the opposite way: capacitances add in parallel and combine by reciprocals in series, so a series string is smaller than its smallest capacitor.
AC: reactance, transformers and power factor
Inductive reactance is XL = 2πfL, so it rises with frequency. Capacitive reactance is XC = 1 ÷ (2πfC), so it falls as frequency rises. The total opposition in an AC circuit is impedance. A transformer's voltage follows its turns ratio, and it needs a changing magnetic field, which is why steady DC does not pass through it (Faraday's law); Lenz's law says the induced current opposes the change that caused it. True power equals apparent power times power factor, and can never exceed apparent power.
Batteries, protection and measurement
On a lead-acid battery the FAA handbook reads 1.300 to 1.275 specific gravity as a high state of charge, 1.275 to 1.240 as medium and 1.240 to 1.200 as low, with no temperature correction between 70 °F and 90 °F. White deposits around nickel-cadmium vent caps are potassium carbonate, cleaned with distilled water. The two battery types are never serviced with the same tools, because each electrolyte contaminates the other. Circuit breakers and fuses are sized to protect the wire. An ammeter goes in series, a voltmeter in parallel, and an ohmmeter only on a de-energized circuit; an open circuit reads infinite resistance.
Where people lose points
✗ "Two resistors in parallel: just add them."
✓ That is series. Parallel totals are smaller than the smallest branch.
✗ "Capacitors in parallel combine like resistors in parallel."
✓ Opposite. Capacitances add in parallel.
✗ "Capacitive reactance rises with frequency."
✓ Inductive reactance rises; capacitive reactance falls.
✗ "A circuit breaker protects the radio it feeds."
✓ It is sized to protect the wire from overheating.
✗ "Leave power on so the ohmmeter has a reference."
✓ Never. An ohmmeter supplies its own current; power must be off.
Numbers to memorize
| Ohm's law | E = I × R |
| Power | P = I × E = I²R = E²/R |
| Two resistors in parallel | (R1 × R2) ÷ (R1 + R2) |
| Inductive reactance | XL = 2πfL (rises with frequency) |
| Capacitive reactance | XC = 1 ÷ 2πfC (falls with frequency) |
| True power | Apparent power × power factor |
| Lead-acid, high charge | 1.300 to 1.275 specific gravity |
| Hydrometer temperature correction | Not needed from 70 °F to 90 °F |
| Ni-Cad vent deposits | Potassium carbonate; distilled water |
| Resistor gold band | ±5% tolerance (silver ±10%) |
Test yourself
No answers here on purpose — retrieving them is the practice. Drill this domain if any of these stall you.
- State Ohm's law and the three forms of Watt's law.
- Compute three 12-ohm resistors in parallel, then in series.
- Explain why a transformer will not work on steady DC.
- Decode the color bands brown, black, red, gold.
- Say how to connect an ammeter, a voltmeter and an ohmmeter.