13.6. Energy Efficiency http://www.ck12.org
Explore More
- The useful work a light bulb does is emitting light (duh). The rest is “wasted” as heat.
a. 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?
b. If the efficiency of a CFL (compactfluorescent) 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
( 20 ◦C)for 30 seconds in a microwave.
a. What current does the oven draw?
b. If the water heats up to 90◦C, what is the heating efficiency of the oven? - The 2010 Toyota Prius(m= 1250 kg)battery is rated at 201.6 V with a capacity of 6.5 Ah.
a. What is the total energy stored in this battery on a single charge?
b. 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. 4. 7 × 106 Jb. 11.2%