## Power & Energy MCQ (Interview-Exam) Question-Answer

**Q.1** Which of the following is not an expression power?

**A.** P=I²R

**B.** P=I/R

**C.** P=VI

**D.** P=V²/R

**Ans : **P=I/R

**Explanation: ** Power is the product of voltage and current. Writing I in terms of V, we get P=V2/R and writing V in terms of I, we get P=I2r.

**Q.2** Which of the following statements are true?

**A.** Power is proportional to current only

**B.** Power is proportional to both the voltage and current

**C.** Power is proportional to voltage only

**D.** Power is neither proportional to voltage nor to the current

**Ans : **Power is proportional to both the voltage and current

**Q.3** Kilowatt-hour(kWh) is a unit of?

**A.** Power

**B.** Resistance

**C.** Current

**D.** Energy

**Ans : **Energy

**Q.4** A 250V bulb passes a current of 0.3A. Calculate the power in the lamp.

**A.** 50W

**B.** 90W

**C.** 75W

**D.** 25W

**Ans : **75W

**Explanation: ** V = 250v and I = 0.3A. P=VI. Which implies that, P=250*0.3=75W.

**Q.5** A current of 5A flows in a resistor of 2 ohms. Calculate the energy dissipated in 300 seconds in the resistor.

**A.** 15000kJ

**B.** 15J

**C.** 15kJ

**D.** 1500J

**Ans : **15kJ

**Explanation: ** P=I²R =5²*2=50W.

E= Pt=50*300=15000J=15kJ.

**Q.6** Calculate the power in the 20 ohm resistance.

**A.** 200kW

**B.** 2W

**C.** 2000kW

**D.** 2kW

**Ans : **2kW

**Explanation: ** V = 200v and Resistance( R) = 20ohm. P=V²/R= 200²/20=2000W=2kW.

**Q.7** Calculate the power across each 10 ohm resistance.

**A.** 1kW, 1kW

**B.** 100kW, 100kW

**C.** 100W, 100W

**D.** 1000kW, 1000kW

**Ans : **1kW, 1kW

**Explanation: ** This is parallel connected circuit, hence the voltage across each of the resistors is the same. P =(V²)/R=(100²)/10 = 1000W=1kW. Since both the resistors receive the same amount of voltage, the power in both is the same.

**Q.8** Calculate the power across each 20 ohm resistance.

**A.** 500W, 500W

**B.** 500kW, 500kW

**C.** 1000W, 1000W

**D.** 1000kW, 1000kW

**Ans : **500W, 500W

**Explanation: ** It is a circuit connected by a series so the current in each resistance is the same. To find the current: I=V/R=200/20=5A.

To find the power: P=I²R=5²*20=500W. Since both resistors have a resistance of 20 ohms, the strength of both resistors is the same.

**Q.9** The SI unit of power is?

**A.** J/s(joules per second)

**B.** J/h(joules per hour)

**C.** kW(kilo-watt)

**D.** Ws(watt-second)

**Ans : **J/s(joules per second)

**Explanation: ** Power = energy/time

SI unit of power = SI unit of energy/SI unit of time = joule/second.

**Q.10** Calculate the work done in a resistor of 20 ohm carrying 5A of current in 3 hours.

**A.** 15J

**B.** 15kWh

**C.** 1.5J

**D.** 1.5kWh

**Ans : **1.5kWh

**Explanation: ** P=I²R=5²*20=500W=0.5kW.

To find Work done: W=Pt=0.5*3=1.5kWh.

**Q.11** A bulb has a power of 200W. What is the energy dissipated by it in 5 minutes?

**A.** 1000J

**B.** 1kJ

**C.** 60J

**D.** 60kJ

**Ans : **60kJ

**Explanation: ** Power = 200w and time = 5min. E=Pt => E= 200*5= 1000Wmin=60000Ws= 60000J= 60kJ.

**Q.12** Which among the following is a unit for electrical energy?

**A.** kWh(kilowatt-hour)

**B.** C(coloumb)

**C.** V(volt)

**D.** Ohm

**Ans : **kWh(kilowatt-hour)

**Explanation: ** I=V/R. Total resistance R = 20+40=60ohm. V=120V. I=120/60=2A.

**Q.13** Calculate the energy dissipated by the circuit in 50 seconds.

**A.** 50J

**B.** 100kJ

**C.** 50kJ

**D.** 100j

**Ans : **50kJ

**Explanation: ** V = 100 and R = 10. Power in the circuit= V2/R = 1002/10 = 1000W.

Energy= Pt= 1000*50 = 50000J = 50kJ.

**Q.14** Out of the following, which one is not a source of electrical energy?

**A.** Battery

**B.** Generator

**C.** Solar cell

**D.** Potentiometer

**Ans : **Potentiometer

**Q.15** Calculate the energy in the 10 ohm resistance in 10 seconds.

**A.** 40kJ

**B.** 4kJ

**C.** 400J

**D.** 4000J

**Ans : **40kJ

**Explanation: ** P=V2/R. P=2002/10= 4000W. E=Pt = 4000*10=40000Ws = 40000J = 40kJ.

**Q.16** Which among the following is an expression for energy?

**A.** V²Rt

**B.** V²t²/R

**C.** V²It

**D.** V²t/R

**Ans : **V²t/R

**Q.17** A current of 2A flows in a wire offering a resistance of 10ohm. Calculate the energy dissipated by the wire in 0.5 hours.

**A.** 72kJ

**B.** 72kJh

**C.** 72Wh

**D.** 7200J

**Ans : **72kJ

**Explanation: ** I (current) = 2A and Resistance(R) = 10ohm. Power = I²R = 2²*10=40. Energy = Pt = 40*0.5*60*60 = 72000J=72kJ.

**Q.18** A battery converts___________.

**A.** Chemical energy to electrical energy

**B.** Chemical energy to mechanical energy

**C.** Electrical energy to chemical energy

**D.** Mechanical energy to electrical energy

**Ans : **Chemical energy to electrical energy

**Q.19** Practically, if 10kJ of energy is supplied to a device, how much energy will the device give back?

**A.** Less than 10kJ

**B.** More than 10kJ

**C.** Equal to10kJ

**D.** Zero

**Ans : **Less than 10kJ

**Q.20** Calculate the energy in the 5 ohm resistor in 20 seconds.

**A.** 2.15kJ

**B.** 21.5J

**C.** 21.5kJ

**D.** 2.15J

**Ans : **21.5kJ

**Explanation: ** I=220/(5+10)=14.67A. P=I²R = 14.67²*5=1075.8W. E=Pt = 1075.8*20 = 21516J=21.5kJ.

## Power & Energy in Electrical Engineering

**1. Definition of Power:**

- Power (P) is the rate at which energy is transferred or converted. It is measured in watts (W) and is the product of voltage (V) and current (I) in an electrical circuit (P = VI).

**2. Units of Power:**

- The watt (W) is the standard unit of power in the International System of Units (SI). Larger units include kilowatts (kW), megawatts (MW), and gigawatts (GW).

**3. Definition of Energy:**

- Energy is the ability to do work. In electrical engineering, energy is often measured in watt-hours (Wh) or kilowatt-hours (kWh). It is the product of power and time (Energy = Power × Time).

**4. Units of Energy:**

- The watt-hour (Wh) is the standard unit of energy. Larger units include kilowatt-hours (kWh), megawatt-hours (MWh), and gigawatt-hours (GWh).

**5. Relationship between Power and Energy:**

- Power and energy are related by time. Power is the rate at which energy is used or produced. Energy is the cumulative result of power over a specific period.

**6. AC and DC Power:**

- Power in electrical systems can be alternating current (AC) or direct current (DC). AC power is characterized by periodic changes in direction, while DC power flows consistently in one direction.

**7. Power Factor:**

- Power factor is a measure of how effectively electrical power is converted into useful work output. It is the ratio of real power (in watts) to apparent power (in volt-amperes).

**8. Efficiency:**

- Efficiency is a crucial aspect in electrical systems, representing the ratio of useful power output to the total power input. Efficient systems minimize energy losses.

**9. Renewable Energy:**

- In the context of power and energy, renewable sources like solar, wind, hydro, and geothermal play a significant role in generating sustainable electricity, reducing dependence on non-renewable resources.

**10. Transmission and Distribution:**

- Efficient transmission and distribution of electrical power involve minimizing losses over long distances. High voltage transmission lines are used to reduce power loss during transportation.

**11. Energy Storage:**

- The development of energy storage technologies, such as batteries, capacitors, and pumped storage, is crucial for balancing supply and demand in electrical grids.

**12. Smart Grids:**

- Smart grids leverage advanced communication and control technologies to enhance the efficiency, reliability, and sustainability of electrical power systems.