Electrical Theory and Fundamentals
Electrical Theory and Fundamentals tests Ohm's law, power, series and parallel circuits, AC behavior, grounding and bonding, conductors, and voltage drop. Electronic systems technicians apply these ideas to low-voltage power, control circuits, and equipment interfaces.
Core rules
- Use V = I x R and P = V x I = I squared x R. Keep units consistent and distinguish RMS from peak for AC.
- Series resistances add; parallel branches share current and add conductance. A opened series element stops the circuit; an opened parallel branch may leave others working.
- AC impedance includes resistance and reactance. Power factor matters for loads with motors or supplies; real power is V x I x power factor for sinusoidal loads.
- Voltage drop uses the round-trip conductor length. Low-voltage 24 V circuits are less tolerant of absolute drop than 120 V circuits, so conductor size and distance matter.
- Grounding is a safety connection to earth; bonding is an intentional equipotential connection between metal parts. Neither is a substitute for a neutral return conductor.
Worked example
A 24 V camera draws 0.5 A over a one-way distance of 100 ft using 18 AWG copper. At about 6.4 ohms per 1,000 ft, the loop resistance is 200 ft x 0.0064 = 1.28 ohms. Voltage drop is 0.5 A x 1.28 ohms = 0.64 V, or 2.7 percent of 24 V. That is usually acceptable, but a 12 V camera at the same current would drop 5.3 percent, so 24 V or a larger conductor is preferable.
Traps
Mixing peak and RMS values; ignoring power factor; using a chassis or conduit as a current-carrying conductor; sizing conductors from ampacity alone without checking voltage drop; and assuming a parallel circuit loses all loads when one branch opens.