Explore key concepts in the measurement of power, electrical energy, and power factor with questions designed to assess your understanding of methods, units, and related calculations. This quiz helps reinforce foundational knowledge for anyone interested in electrical engineering, energy auditing, and electricity usage analysis.
Which instrument is most commonly used to directly measure the real (active) power in a single-phase AC circuit supplying a resistive-inductive load?
Explanation: A wattmeter directly measures the real power consumed in an AC circuit, making it ideal for this purpose. An ammeter only measures current and does not provide direct power readings. Although an energy meter records energy over time, it is not used for instantaneous power measurement. A voltmeter measures voltage, which also cannot directly give power without knowing current and phase angle.
What is the standard SI unit of electrical energy as recorded by a household energy meter, such as the one measuring monthly consumption?
Explanation: The kilowatt-hour is the standard practical unit for measuring electrical energy consumption, especially on utility bills. Watt is a unit of power, not energy. Joule is the SI base unit of energy but is rarely used for household electricity measurements due to its small size. Volt-ampere is a unit of apparent power, not energy.
In an AC circuit, if the apparent power is 100 VA and the real power is 80 W, what is the power factor of the circuit?
Explanation: Power factor is defined as the ratio of real power to apparent power, so 80 W divided by 100 VA equals 0.8. A value greater than 1 (such as 1.25 or 180) is not possible for power factor. The value 0.2 is too low and would indicate much more reactive power than real power, which is not the case here.
A device rated at 500 W runs continuously for 4 hours. How much electrical energy does it consume during this period?
Explanation: Energy is calculated as power multiplied by time, so 500 watts times 4 hours equals 2000 watt-hours, or 2 kilowatt-hours. 0.5 kilowatt-hours results from 500 W used for 1 hour. 2000 kilowatts is a power output, not energy, and the number of joules should be much higher; 125 joules is much too small for the given scenario.
What is a common purpose of adding capacitors to an industrial AC power system with a low power factor?
Explanation: Capacitors are typically added to counteract the lagging reactive power created by inductive loads, thereby improving the overall power factor. Adding capacitors does not increase real power output, as real power is determined by the load. While capacitors can slightly affect voltage, their main purpose is not voltage boosting. They also do not measure energy consumption; meters do that.