Licensed Electrical Contractor Electrical Theory & Fundamentals 3 — Questions and Answers
Question 1: What is the formula for calculating electrical power in a DC circuit?
- P = V / I
- P = V × I (Correct answer)
- P = V + I
- P = V² × I
Correct answer: P = V × I
Electrical power (watts) equals voltage (volts) multiplied by current (amperes): P = V × I.
Question 2: What is the phase relationship between voltage and current in a purely inductive circuit?
- Current leads voltage by 90°
- Voltage and current are in phase
- Current lags voltage by 90° (Correct answer)
- Current lags voltage by 45°
Correct answer: Current lags voltage by 90°
In a purely inductive circuit, current lags voltage by 90° because the inductor opposes changes in current.
Question 3: What is the term for the total opposition to current flow in an AC circuit, combining resistance and reactance?
- Conductance
- Admittance
- Impedance (Correct answer)
- Susceptance
Correct answer: Impedance
Impedance (Z) is the total opposition to AC current flow, combining resistance (R) and reactance (X), expressed in ohms.
Question 4: A 120V source is connected to a 20-ohm resistor. What is the current flowing through the circuit?
- 2,400 A
- 100 A
- 6 A (Correct answer)
- 0.17 A
Correct answer: 6 A
Using Ohm's Law: I = V/R = 120/20 = 6 amperes.
Question 5: What property of a material determines how well it conducts electricity?
- Permeability
- Resistivity (Correct answer)
- Permittivity
- Reluctance
Correct answer: Resistivity
Resistivity (ρ) is an intrinsic material property that determines electrical resistance; lower resistivity means better conductivity.
Question 6: In a series RLC circuit at resonance, what is true about the impedance?
- Impedance is at its maximum value
- Inductive and capacitive reactances cancel, leaving only resistance (Correct answer)
- Capacitive reactance dominates
- Resistance drops to zero
Correct answer: Inductive and capacitive reactances cancel, leaving only resistance
At resonance, XL = XC so they cancel each other out, and impedance equals only the circuit resistance (minimum impedance).
Question 7: What is the effect of doubling the voltage in a circuit while keeping resistance constant?
- Current halves and power stays the same
- Current doubles and power quadruples (Correct answer)
- Current doubles and power doubles
- Current stays the same and power doubles
Correct answer: Current doubles and power quadruples
By Ohm's Law, doubling V doubles I; since P = V²/R or P = VI, power increases fourfold (quadruples).
Licensed Electrical Contractor Electrical Theory & Fundamentals 3
What is the formula for calculating electrical power in a DC circuit?