Energy use in daily life Transitional Reasoning Practice

Questions

1. 1.

   A household electric clothes dryer operates at a constant electrical power rating of 2000 W during a 50-minute drying cycle. In this period, the appliance delivers 4.5 MJ of useful thermal energy to evaporate moisture from wet laundry, while the remaining energy is lost to the surroundings and exhaust air. Which statement correctly evaluates the energy efficiency, useful power delivery, and wasted energy of the clothes dryer during this cycle?

   • A) The dryer operates at an efficiency of 75%, delivering useful power at 1500 W and dissipating 1.5 MJ of energy as waste to the surroundings.
   • B) The dryer operates at an efficiency of 25%, delivering useful power at 500 W and dissipating 4.5 MJ of energy as waste to the surroundings.
   • C) The dryer operates at an efficiency of 60%, delivering useful power at 1200 W and dissipating 2.4 MJ of energy as waste to the surroundings.
   • D) The dryer operates at an efficiency of 80%, delivering useful power at 1600 W and dissipating 1.2 MJ of energy as waste to the surroundings.
2. 2.

   A household vacuum cleaner draws an electrical input power of 800 W from the mains supply. When operating with a clean filter, it provides an effective useful suction power of 160 W. When the filter becomes heavily clogged with dust, the useful suction power drops to 80 W while the motor continues to draw 800 W from the mains. Which statement correctly evaluates the energy efficiency of the vacuum cleaner and the effect of the clogged filter during a 30-minute cleaning session?

   • A) The efficiency of the vacuum cleaner decreases from 25% to 12.5%, causing the total electrical energy drawn from the mains to drop from 1440 kJ to 720 kJ.
   • B) The efficiency of the vacuum cleaner is 80% when clean and 90% when clogged, meaning that 144 kJ less energy is converted into useful work during the 30-minute session.
   • C) The efficiency of the vacuum cleaner decreases from 20% to 10%, resulting in 144 kJ more energy being dissipated as waste heat and sound while the total electrical energy drawn remains 1440 kJ.
   • D) The efficiency of the vacuum cleaner decreases from 20% to 10%, causing the total electrical energy consumed from the mains to double from 1440 kJ to 2880 kJ.
3. 3.

   A household refrigerator has a compressor with an electrical power rating of 150 W that cycles on and off throughout the day. With an intact door seal, the refrigerator operates on a 20% duty cycle (running for 4.8 hours each day) to keep its contents chilled. When the magnetic door seal becomes degraded, heat enters faster from the surroundings, increasing the duty cycle to 35% (running for 8.4 hours each day) to maintain the same internal temperature. Which statement correctly evaluates the increase in daily electrical energy consumption and the extra energy consumed over a 30-day month due to the degraded seal?

   • A) The daily electrical energy consumption increases by 0.36 kWh, resulting in an additional 10.8 kWh consumed over a 30-day month because the 15% increase in duty cycle corresponds directly to 0.36 kWh of wasted energy.
   • B) The daily electrical energy consumption increases by 0.54 kWh, resulting in an additional 16.2 kWh consumed over a 30-day month because the compressor must operate for 3.6 more hours each day.
   • C) The daily electrical energy consumption increases by 1.26 kWh, resulting in an additional 37.8 kWh consumed over a 30-day month because the degraded seal requires the compressor to run continuously during peak hours.
   • D) The daily electrical energy consumption increases by 0.90 kWh, resulting in an additional 27.0 kWh consumed over a 30-day month because the degraded seal causes the compressor's electric power draw to rise from 150 W to 225 W.
4. 4.

   A cordless household appliance is recharged using a mains charger that draws a constant power of 50 W for 2.0 hours. When fully charged, the battery stores 270 kJ of electrical energy, which is subsequently discharged over 15 minutes of cleaning where the motor delivers a constant useful mechanical power of 180 W. Which statement correctly evaluates the energy efficiency of the charging process and the overall efficiency from the mains input to useful mechanical output?

   • A) The charging process has an efficiency of 75%, and the overall efficiency from mains input to useful mechanical output is 45%.
   • B) The charging process has an efficiency of 75%, and the overall efficiency from mains input to useful mechanical output is 60%.
   • C) The charging process has an efficiency of 80%, and the overall efficiency from mains input to useful mechanical output is 40%.
   • D) The charging process has an efficiency of 60%, and the overall efficiency from mains input to useful mechanical output is 30%.
5. 5.

   A homeowner investigates the energy performance of reheating a bowl of soup that requires exactly 150 kJ of thermal energy to reach the desired temperature. A microwave oven with an electrical power rating of 1200 W takes 180 seconds to complete the heating, whereas an electric countertop oven with a power rating of 1500 W requires 900 seconds to deliver the same useful heat because it must also heat the oven cavity and surrounding air. Based on these measurements, which statement correctly evaluates the energy consumption and efficiency of the two cooking methods?

   • A) The countertop oven is more energy efficient because its higher power rating enables a greater rate of heat transfer to the food.
   • B) Both appliances have equal energy efficiency because the useful thermal energy absorbed by the soup is identical in both cases.
   • C) The microwave oven consumes 216 kJ of electrical energy with an efficiency of approximately 69%, whereas the countertop oven consumes 1350 kJ with an efficiency of approximately 11%.
   • D) The microwave oven consumes 66 kJ of total electrical energy, making it waste 1200 kJ less energy than the countertop oven.
6. 6.

   A household washing machine has an electric motor rated at 300 W to rotate the drum and an internal heating element rated at 1800 W to heat the water. In a 90-minute Warm Wash program, the heater operates for the first 20 minutes while the motor runs continuously for the full 90 minutes. In an alternative 60-minute Cold Wash program, the heating element remains off throughout the cycle and the motor runs continuously for 60 minutes. Which statement correctly evaluates the electrical energy consumed by these two programs and the role of water heating in household energy use?

   • A) The electric motor consumes more energy than the heating element during the Warm Wash because it operates for four and a half times longer.
   • B) The Cold Wash consumes 0.30 kWh while the Warm Wash consumes 1.05 kWh, demonstrating that heating water accounts for the majority of the cycle's energy consumption despite the heater's short operating time.
   • C) The Cold Wash saves only 0.15 kWh compared to the Warm Wash because the energy difference is determined solely by the motor running for 30 minutes less.
   • D) The Warm Wash consumes 3.15 kWh of electrical energy because both the heating element and the motor draw their full power ratings for the entire 90-minute duration.
7. 7.

   A commuter uses an electric scooter for a 12 km daily trip, traveling at a steady speed of 6.0 m/s for a duration of 2000 s. During the ride, the scooter draws an average electrical power of 250 W from its battery while maintaining a constant forward thrust of 30 N to overcome resistive forces. Based on the useful mechanical work performed and the total electrical energy consumed, which statement correctly evaluates the energy performance of the scooter?

   • A) The scooter performs 500 kJ of useful work and operates at an efficiency of 60%, dissipating 200 kJ as waste heat.
   • B) The scooter operates at an efficiency of 83% because the motor consumes 360 kJ of electrical energy while delivering 300 kJ of useful work.
   • C) The scooter operates at an efficiency of 72%, delivering 360 kJ of useful work while dissipating 140 kJ as thermal energy.
   • D) The scooter delivers 180 kJ of useful work and operates at an efficiency of 36%, wasting 320 kJ of electrical energy.
8. 8.

   A domestic electric pump is used daily to raise 1800 kg of water from a ground-level reservoir to a rooftop storage tank at a vertical height of 10 m. The pump operates at an electrical input power of 750 W and takes 400 s to complete the filling process each day. Taking the acceleration due to gravity as 10 m/s², which statement correctly evaluates the energy delivered to the water and the overall efficiency of this daily pumping operation?

   • A) The useful work done on the water is 18 kJ, resulting in an operating efficiency of 6% with 282 kJ of energy dissipated.
   • B) The useful work done on the water is 300 kJ, indicating an ideal 100% efficiency because all electric power is converted into hydrostatic potential energy.
   • C) The useful work done on the water is 180 kJ, but the efficiency is 40% because 180 kJ of the total 300 kJ input is lost as heat.
   • D) The useful work done on the water is 180 kJ, resulting in an operating efficiency of 60% while 120 kJ of electrical energy is dissipated as waste heat and sound.
9. 9.

   A student investigates replacing a 100 W incandescent desk lamp with a 10 W LED lamp used for 5 hours each day. Both lamps produce identical useful visible light power of 5 W, while the remaining input power is dissipated as thermal energy to the room. Which statement correctly evaluates the energy efficiency of the two lamps and the reduction in wasted thermal energy over a 30-day period?

   • A) The LED lamp is five times as efficient as the incandescent lamp, reducing wasted thermal energy by 14.25 kWh over the 30-day period.
   • B) The LED lamp is ten times as efficient as the incandescent lamp, reducing wasted thermal energy by 13.5 kWh over the 30-day period.
   • C) The LED lamp is ten times as efficient as the incandescent lamp, reducing wasted thermal energy by 1.5 kWh over the 30-day period.
   • D) Both lamps have the same efficiency because they deliver identical useful light power, reducing wasted thermal energy by 15.0 kWh over the 30-day period.
10. 10.

    A consumer investigates two electric kettles, X and Y, used daily to boil identical amounts of water from room temperature. Kettle X has an electric power rating of 2000 W and takes 3.0 minutes to finish boiling, whereas Kettle Y has a power rating of 1500 W and requires 4.5 minutes. Based on the total electrical energy consumed for this task, which statement correctly evaluates the energy efficiency and operating cost per boil?

    • A) Kettle Y is more energy-efficient and costs less to run per boil because an appliance with a lower power rating always consumes less total energy.
    • B) Kettle X is more energy-efficient and costs less to run per boil because it consumes less total electrical energy to complete the task.
    • C) Both kettles incur identical running costs and have equal efficiency because the useful thermal energy delivered to the water is the same.
    • D) Kettle Y consumes less total electrical energy because heating time increases whenever energy consumption decreases.

Answers and step-by-step solutions appear after you finish the test.