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On Dec 13, 2014, at 12:41 PM, Dan Beeker <debeeker@comcast.net> wrote:
On Dec 13, 2014, at 12:00 PM, phys-l-request@www.phys-l.org wrote:
Keep in mind that such a thing would be at best a stunt,
and has the potential to mislead students (and visitors
et cetera). That's because a bicycle generator is not
a particularly practical way to generate electricity
This is exactly the reason one would want to have a bicycle powered generator demonstration. Most everyone thinks it is a good idea until you actually get on one and find out how little practical utility such a thing has. It takes a lot of effort (I was going to say work : ) to run even a 100 watt load for any reasonable length of time. We find having lights to indicate the relative effort required to convert a given power level is much more effective than just a power meter - people seem to understand bright translates to more power. The down side is, to my knowledge, no mass produced bicycle is designed to run a generator and it requires a lot of alteration to make one functional as a generator. Our department has such a beast and we have invested a lot of time and money into making it functional, practical and safe. Moving parts must be covered. Most generators will output rather high voltages if not limited, so there is a shock hazard. There is also the problem of lights burning
out from over zealous peddlers without some kind of regulation. If you limit voltage then the current is high meaning larger cabling and the danger of serious arcing and sparking if you have a bad connection - think arc welder. The bicycle must be stable. You don't want it tipping over on someone and it must be easy to get on/off as well as somewhat comfortable to sit on. All that said, once you have a functional demo it becomes an attractor. Most people that haven't seen one are quite surprised at how little useful electrical power is converted. In our case it has been a worthwhile effort albeit a tremendous amount of work.
As far as making useful measurements, as John Denker says, it is probably not the best way to learn the relationships between torque, power, rpm etc. First, it is very difficult to maintain constant torque using foot pedals. I would find other ways to teach rotational parameters then use the bicycle generator as an example of how simple(?) physics makes analysis of more complicated systems possible. Like many real world things, if you can show the physical principles and tackle the complications of something people have an actual interest in, it makes physics more relevant and certainly makes our jobs as teachers much easier .
Dan
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