As an example of my question, imagine I have a small generator attached to a bicycle pedals. If this generator is connected to one light bulb, how would it compare with the generator being connected to multiple bulbs?

My assumption is that, as you add more bulbs, either;

  1. The resistance on the pedals would remain the same, but you would now have to pedal faster in order to light the bulbs to the same brightness, or;
  2. It would become harder to pedal when the extra bulbs are added, but the same rpm would be needed to produce the same brightness.

Which (if either) of my assumptions is the correct one?


As you add bulbs in parallel, the load impedance drops and to light all the bulbs you have to pedal harder. When the load impedance becomes less than the output impedance of the generator, then more work is expended in heating the generator coils than in lighting the bulbs. In the limit of near-zero load impedance (short circuit), all the pedal work is lost as I^2 x R losses in the generator windings. The pedal effort then does not change.

If you add bulbs in series, then to light all the bulbs you again have to pedal harder. However, in this case the load impedance grows bigger and less current is allowed to flow through the load. The pedal effort declines and the bulb string grows dimmer. In the limit of infinite load impedance, the current goes to zero and the pedal effort goes to zero too.

  • $\begingroup$ Thanks for your answer. So, if instead of bulbs you had a motor how would this work? If you turned the motor off, I guess the mechanical resistance of the generator would be close to 0? And if you turned the motor up to its highest power, I guess it would require you to pedal much harder? $\endgroup$ – PhysicsGuy123 Apr 28 '19 at 18:00
  • $\begingroup$ that's basically correct- but there are other details worth mentioning which I can't right at the moment- Niels $\endgroup$ – niels nielsen Apr 28 '19 at 18:10
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    $\begingroup$ If you turn the motor up to its highest power, then unless the input power is increased it will stall or slow down so that input matches output. $\endgroup$ – Solar Mike Apr 28 '19 at 20:01
  • $\begingroup$ @PhysicsGuy123 Motors don't actually have a power setting. $\endgroup$ – user253751 Apr 30 '19 at 23:26
  • $\begingroup$ @immibis, yes, but most motors will be a part of a circuit which contains a switch which can be turned up or down to increase or decrease the power output of the motor. $\endgroup$ – PhysicsGuy123 May 12 '19 at 18:04

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