Conservation of Energy and Electrical Efficiency
You use a 100 W electric motor to lift a 10 kg mass 5 m, and it takes 20 s.
- 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
(20∘C)for 30 seconds in a microwave.
- What current does the oven draw?
- 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.
- 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?
- a. 2.1%, 3524 J b. 720 J
- a. 8.3A b. 29%
4.7×106 Jb. 11.2%