Students will learn how to properly think about efficiency and how to calculate the efficiency of electrical devices.
; Power in electricity is the voltage multiplied by the current
; the electrical energy used is equal to the power dissipated multiplied by the time the circuit is running
; Efficiency is the Power out divided by the Power input
Assuming same time periods:
Conservation of Energy & Electrical Efficiency:
Electrical energy consumed can be determined by multiplying power by time . Recall the equations for mechanical and thermal energy/work . An important idea is the efficiency of an electrical device: the fraction of electrical energy consumed that goes into doing useful work , expressed as a percentage.
You use a 100 W electric motor to lift a 10 kg mass 5 m, and it takes 20 s.
The electrical energy consumed is
The work done is against gravity, so we use
The efficiency is
Watch this Explanation
Time for Practice
- The useful work a light bulb does is emitting light (duh). The rest is “wasted” as heat.
- If a standard incandescent 60 W bulb is on for 1 minute and generates 76 J of light energy, what is its efficiency? How much heat energy is produced?
- If the efficiency of a CFL (compact fluorescent) bulb is 20%, how many joules of light energy will a 60 W bulb produce?
- Most microwave ovens are 1000 W devices. You heat 1 cup (8 fl. oz., or 30 ml) of room temperature water for 30 seconds in a microwave.
- What current does the oven draw?
- If the water heats up to , what is the heating efficiency of the oven?
- The 2010 Toyota Prius battery is rated at 201.6 V with a capacity of 6.5 Ah.
- What is the total energy stored in this battery on a single charge?
- If you used the battery alone to accelerate to 65 mph one time (assuming no friction), what percentage of the battery capacity would you use?
Answers to Selected Problems
- a. 2.1%, 3524 J b. 720 J
- a. 8.3A b. 29%
- a. b. 11.2%