Electrical Knowledge Flashcards

Which of the following best describes the relationship between voltage, current, and resistance in a DC circuit?

Answer: Current equals voltage divided by resistance

Ohm's Law states that I = V/R, meaning current (I) equals voltage (V) divided by resistance (R). This is one of the most fundamental principles in electrical theory.

A circuit has a voltage of 120V and a resistance of 30 ohms. What is the current flowing through the circuit?

Answer: 4 amps

Using Ohm's Law: I = V/R = 120/30 = 4 amps. Dividing voltage by resistance gives the current in amperes.

What does a normally open (NO) contact do in a relay circuit?

Answer: Passes current only when the relay coil is energized

A normally open (NO) contact is open (no current flow) when the relay coil is not energized. When the coil is energized, the contact closes, allowing current to pass.

Which wire color is typically used for the grounding conductor in standard US residential wiring?

Answer: Green or bare copper

In US wiring standards (NEC), green or bare copper wire is used for the equipment grounding conductor. White or gray is neutral, black is the hot lead.

What type of circuit has two or more resistors connected end-to-end, sharing the same current path?

Answer: Series circuit

In a series circuit, components are connected end-to-end so the same current flows through each component. Total resistance is the sum of all individual resistances.

A transformer steps voltage up from 120V to 480V. What is the turns ratio of primary to secondary?

Answer: 1:4

Turns ratio = V_primary / V_secondary = 120/480 = 1:4. A step-up transformer has more turns on the secondary winding than the primary.

Which of the following is the correct definition of electrical power?

Answer: Power equals voltage multiplied by current

Electrical power (P) = V × I (voltage times current), measured in watts. Combined with Ohm's Law, it can also be expressed as P = I²R or P = V²/R.

What is the purpose of a fuse in an electrical circuit?

Answer: To protect the circuit by interrupting current when it exceeds a safe level

A fuse is an overcurrent protection device. When current exceeds the fuse's rated value, the fusible element melts and opens the circuit, preventing damage or fire.

In a 3-phase system, what is the phase angle difference between each phase?

Answer: 120 degrees

In a balanced 3-phase system, each phase is separated by 120 electrical degrees. This equal spacing provides smooth power delivery and efficient motor operation.

Which instrument is used to measure electrical resistance in a circuit?

Answer: Ohmmeter

An ohmmeter measures electrical resistance in ohms. It must be used on a de-energized circuit. Modern digital multimeters include ohmmeter function.

What happens to total resistance when resistors are connected in parallel?

Answer: Total resistance decreases below the smallest individual resistance

In a parallel circuit, total resistance is always less than the smallest individual resistor. The formula is 1/Rt = 1/R1 + 1/R2 + ... because multiple paths reduce overall opposition.

What does GFCI stand for, and what does it protect against?

Answer: Ground Fault Circuit Interrupter; protects against ground faults that could cause electrocution

GFCI (Ground Fault Circuit Interrupter) detects small current imbalances (as little as 5mA) between hot and neutral, indicating current leaking to ground, and trips quickly to prevent electrocution.

A motor nameplate shows 460V, 3-phase. If connected to 208V, what will likely happen?

Answer: The motor will overheat and draw excessive current

A motor designed for 460V connected to 208V will be severely under-volted. It will draw excessive current trying to develop rated torque, causing overheating and potential winding damage.

What is the function of a contactor in an industrial motor control circuit?

Answer: To switch high-current motor circuits using a low-power control signal

A contactor is an electrically controlled switch used for switching high-current motor circuits. A small control current energizes the coil, which closes the power contacts to start the motor.

Which of the following describes capacitive reactance?

Answer: Opposition to current flow by a capacitor, which decreases as frequency increases

Capacitive reactance (Xc = 1/(2πfC)) opposes current flow and decreases as frequency increases. At DC (0 Hz), a capacitor blocks current entirely; at high frequencies it acts nearly as a short.

What is the purpose of lockout/tagout (LOTO) procedures when working on electrical equipment?

Answer: To ensure equipment is de-energized and cannot be re-energized while workers are servicing it

LOTO procedures (OSHA 29 CFR 1910.147) require de-energizing equipment and applying personal locks before maintenance to prevent accidental re-energization, which could cause electrocution or injury.

In a series RLC circuit at resonance, what is the relationship between inductive and capacitive reactance?

Answer: They are equal and cancel each other out

At resonance, XL = XC, so they cancel. The circuit impedance equals only the resistance, current is maximum, and the circuit operates at its resonant frequency (f = 1/(2π√LC)).

What type of electrical fault occurs when a hot conductor contacts the ground or equipment enclosure?

Answer: Ground fault

A ground fault occurs when an energized (hot) conductor makes contact with grounded metal or earth. This creates an unintended current path and is a serious shock and fire hazard.

What does the term 'power factor' describe in an AC circuit?

Answer: The ratio of real power to apparent power, indicating how effectively power is being used

Power factor (PF = Real Power / Apparent Power = cos θ) indicates how effectively electrical power is converted to useful work. A PF of 1.0 is ideal; reactive loads lower the power factor.

Which of the following components is used to convert AC power to DC power?

Answer: Rectifier

A rectifier converts AC (alternating current) to DC (direct current) using diodes. Half-wave rectifiers use one diode; full-wave rectifiers use four diodes in a bridge configuration.