Power Calculations Quiz

Power Calculations

• Power (P): Rate at which energy is transferred. Measured in watts (W) or kilowatts (kW).
• Energy (E): Amount of work done. Measured in joules (J)
• Time (t): Duration of energy transfer. Measured in seconds (s), minutes (min), or hours (hr).

Practice Problems

• Problem 1: Calculate the power of a drill transferring 2000 J of energy in 5 seconds.

  • P = E / t = 2000 J / 5 sec = 400 W = 0.4 kW

• Problem 2: Find the power of a runner transferring 3500 J of energy in 5 minutes.

  • Convert minutes to seconds: 5 minutes * 60 seconds/minute = 300 seconds
  • P = E / t = 3500 J / 300 sec = 11.67 W = 0.01167 kW

• Problem 3: Determine energy released from a 2 W flashlight for 4.5 hours.

  • Convert hours to seconds: 4.5 hours * 3600 seconds/hour = 16200 seconds
  • E = P * t = 2 W * 16200 sec = 32400 J

• Problem 4: Calculate energy used by a 1500 W microwave oven for 20 minutes.

  • Convert minutes to seconds: 20 minutes * 60 seconds/minute = 1200 seconds
  • E = P * t = 1500 W * 1200 sec = 1,800,000 J

• Problem 5: Find the efficiency of a toaster converting 2000 J of electrical energy to 200 J of heat.

  • Efficiency = (Useful Energy Output / Total Energy Input) * 100%
  • Efficiency = (200 J / 2000 J) * 100% = 10%

• Problem 6: A light bulb consuming 30 J of energy per second, transferring 3 J as light and 27 J as heat energy. Calculate the efficiency.

  • Efficiency = (3J / 30J) * 100% = 10%

• Problem 7: A kettle taking in 2000 J per second, transferring 1500 J as heat and 500 J as wasted energy. Calculate its efficiency.

  • Efficiency = (1500 J / 2000 J) * 100% = 75%

• Problem 8: Calculate energy used by a 600 W refrigerator operating 12 hours a day for a week. (Find E for 84 hours.)

  • E = P × t = 600W × 84 hrs × 3600 seconds/hr = 1.8144 × 10^8 J

• Problem 9: Calculate electrical energy and cost for a 750 W dryer used for 15 minutes daily for a year.

  • Convert to hours, then calculate total energy for year.
  • Multiply energy by cost per kWh.

• Problem 10: Calculate cost of three light bulbs with different wattage levels operating for 2.5 hours. Find total power then cost.

• Problem 11: Calculate cost of 1100 W heater operating 3 hours daily for 30 days. Calculate total energy and use rate.

• Problem 12: Calculate the cost to run a lawn mower transferring 5 kW-h of energy at $0.10/kW-h.

• Problem 13: Calculate the energy consumed by a 2 kW washing machine used for 2 hours and cost at $0.05/kW-h.

Key Units

• Watt (W): Joules per second (J/s).
• Kilowatt (kW): 1000 watts.
• Kilowatt-hour (kW-h): Commonly used for energy billing.
• Joule (J): Unit of energy.
• Seconds (s), Minutes (min), Hours (hr): Units of time.

Problem Solving Strategy

1. Identify known quantities (E, P, t)
2. Convert units
3. Apply the formulas (E = P x t; P = E / t)
4. Report answers with correct units.

Test your understanding of power calculations with this quiz. Solve practical problems that involve calculating power, energy, and time based on given data. This will help reinforce your knowledge of energy transfer concepts.