The tutor begins a series on the idea of a human-powered household.

It’s not a new concept, that human energy could be harnessed to produce electricity. However, the specific numbers aren’t always discussed.

The exercise bike says I can burn 300 calories in 20 minutes. It’s a hard pace, but I can do it. Three hundred dietary calories is really 300 000 physics calories. A physics calorie is 4.186J; the 300 000 calories is worth (4.186)(300 000) = 1 255 800J.

Watts=J/s=1255800/(20*60)=1046.5W

Apparently, for that twenty minutes, I can produce power at a rate of 1046.5W. (Not bad; one horsepower is 746W:)

Let’s turn to the vacuum cleaner, which is 12A. Its power rating is therefore

Power=Amps*Volts = 12*120 = 1440W

Apparently, even my brave 1046W effort is not enough to power the vacuum cleaner. However, if the exercise bike were connected to a generator, I could build up a reserve, then draw from it to power the vacuum cleaner.

Assuming 85% efficiency, my 1046W gets reduced to (0.85)(1046)=889W. To use the vacuum cleaner for an hour, I’d first have to ride for 1440/889=1.6 hours or 1hr, 36 minutes. After that ride, I’d still have to vacuum: I’d definitely be thinner:)

HTH:)

Source:

Serway, Raymond A. Physics for Scientists and Engineers (W.M.P.). Toronto: Saunders College Publishing, 1986.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.