The Friday after Thanksgiving, shopaholic Mona heads to Eagle Mall to do her Christmas shopping. It is raining, and in her haste to find a parking space she forgets to turn off her lights. She has an older car and the lights will not turn off automatically.
Mona’s car battery is rated at 12-volt, 70 amp-hours, each head light is rated at 80 watts and each tail light is rated at 40 watts. If all of these lamps are on, how long does Mona have to shop before her battery is dead as a door nail?
Answer: Two head lights draw 160 watts, two tail lights draw 80 watts and, therefore, the total power dissipation is 240 watts. DC power in watts = (voltage in volts) (current in amperes). Hence, 240 watts/12 volts = 20 amperes. Then, 70 amp-hours = (20 amperes) (time in hours). Therefore, time = 3.5 hours.
Conclusion: Mona has three and a half hours to shop.