Question

in a physics lab, a student connected the leads of a Genecon hand crank generator to a lightbulb rack. He experimented with the changing the number of bulbs he was lighting by unscrewing them just enough to break the electrical connection. He observed that it was hardest to turn the handle of the generator when there were three bulbs as opposed to just one bulb on the rack. Why is this?

Answer 1

In hand crank generator, voltage is generated by rotating a coil in magnetic field. Now as the coil rotates , it will experience force which will try to prevent it rotation according to Lenz's law. The force is basically Lorenz force,

$F = I (d \overrightarrow l \times \overrightarrow B)$

which is proportional to current I. Now when 3 bulbs are connected in parallel, the equivalent resistance decreases and current increases. Another way to see this is, to light up all the bulbs specified current is required. So in case 3 bulbs we need 3 times the current required for 1 bulb. So the opposing Lorenz force is 3 times larger for the case of 3 bulbs. Since the opposing force is large, it is hard to turn the handle of the generator.

We know that magnetic force doesn't do work. So to power up the bulbs we need to supply energy somehow. In case of hand crank generator our mechanical energy required to turn the handle is converted to electrical energy. Now 3 bulbs require 3 times power than 1 bulb. This is why it is harder to turn the handle for 3 bulb than for the case of 1 bulb since this is the only way we can supply more power.