A person in good physical condition can put out 100 W of useful power for several hours at a stretch, perhaps by pedaling a mechanism that drives an electric generator. Neglecting any problems of generator efficiency and practical considerations such as resting time: (a) How many people would it take to run a 4.00-kW electric clothes dryer? (b) How many people would it take to replace a large electric power plant that generates 800 MW?

Video Transcript

This is College Physics Answers with Shaun Dychko. A person can output about 100 watts and we are going to figure out the number of people it would take to run a clothes dryer which is 4 kilowatts and 'kilo' is a prefix meaning times 10 to the 3. So we have 4.00 times 10 to the 3 watts per clothes dryer multiplied by 1 person for every 100 watts provided, the watts cancel and we are left with the number of people per clothes dryer and that is 40 people. And to have the people output the same power as a power plant— an 800 megawatt power plant, 'mega' meaning times 10 to the 6— we would take 1 person for every 100 watts times 800 times 10 to the 6 watts per power plant and that is 8 times 10 to the 6 people per power plant I guess, strictly speaking if you want to have the units correct. There we go! 8 million people would need to replace a single power plant.