Shown above is a modified exercise bicycle, which has been fitted with a (DC) generator (of the type found in Ford automobiles of the late 1950s and early 1960s). Inside the box to which the generator is connected is a load resistor, and a circuit that switches on the indicator lamps according to how much power is being dissipated in the load (and the lamps). The two large meters show the voltage developed across the load, and the current passing through it. The numbers next to the lamps show the power dissipated, in watts. A switch, located above where the cable connects to the box, allows you to disconnect the load from the generator, so that you can demonstrate how much easier it is to pedal the bicycle when the load is disconnected, than it is when the load is connected. Please note: Do NOT disconnect the load while someone is pedaling the bicycle. Depending on how hard the person is pedaling, the sudden, dramatic decrease in load could cause injury. If you wish to flip the switch while someone is pedaling, before the person starts pedaling, disconnect the load. Have the person start pedaling without the load, then flip the switch to connect it, and make sure that the person is ready before you do so. Pedaling suddenly becomes much more difficult.

The pages for demonstrations 52.36 -- Falling weight generator, and 72.12 -- AC/DC generator, describe in detail how generators convert mechanical energy (or work) into electrical energy (or work). As one turns the shaft of the generator, it turns a set of armature coils that sit within the magnetic field of a set of permanent magnets. The changing magnetic flux through the coils as they turn, induces an EMF in them, which, if a load is connected across the coils, causes a current to flow through them and the load. This demonstration shows the conversion of mechanical work on the part of the person riding the bicycle (which itself comes from the conversion of chemical energy to mechanical work) into electrical energy in the load and lamps in the black box, and also provides a way to introduce the concept of power and its relation to work and energy. The current in the armature coils produces a magnetic field that opposes the field of the permanent magnets, which causes repulsion of the coils from the magnets in one direction, and attraction in the opposite direction, and thus a torque that opposes the external torque applied to the shaft of the generator. (See demonstration 72.09 – Lenz’s law.) As a result, this demonstration also gives the students who ride it a sense of how much effort it requires to expend a particular amount of energy per unit time, and how difficult it might be to keep doing so for a given time.

As noted above, on the front face of the box are a large voltmeter and a large ammeter. As the rider pedals, these read the voltage across the load and lamps, and the current through them. The power dissipated in the load and lamps is the product of the voltage and the current, or VI, for which the units are [(N·m)/C](C/s), or J/s. One joule/s equals one watt (W), which is the unit for the numbers next to the lamps. That is, starting from zero, as one increases the pedaling effort, the lights indicate output of just above 0 W, then 28 W, then 63 W, etc., up to 510 W, above which the lamp on top of the box lights. Next to the lamp that indicates an output of 379 watts is a label that reads “1/2 HP,” for “1/2 horsepower.” In the 1770s, James Watt established the unit of the horsepower (hp) as the power required to pull a 150-pound weight out of a 220-foot-deep well in one minute. (He had invented a steam engine, which he intended to sell. Since his engine would replace the horses that his potential customers used, he thought it prudent to rate its power output relative to that of a horse. See https://spark.iop.org/why-one-horsepower-more-power-one-horse.) This works out to 33,000 foot-pounds per minute, or 550 foot-pounds per second. If we convert foot-pounds to newton-meters (joules), we find that this equals 746 W. One-half horsepower, then, is 746/2 = 373 W. Hence the “1/2 HP” label at the lamp that corresponds to a power output of 379 W, which is close to this value.

If you have a student sit on the bicycle, with the switch off (so that the load is disconnected), the student finds it easy to turn the pedals. If you now flip the switch to connect the load (after warning the student and confirming that the student is ready), the student now feels significant resistance; the faster the student is pedaling, the greater the resistance. This is because with the load disconnected, the only work the student is doing is the work necessary to spin the flywheel of the bicycle and turn the unloaded generator. Though the flywheel is somewhat massive, and the gear ratio from the wheel to the generator is rather high, it is not difficult to spin them. When the load is connected, by turning the pedals the student is now driving current through the load, and thus doing work and producing power. The faster the student tries to pedal, the greater the work the student does per unit time, and the greater the power the student produces. As noted above, while the student pedals with the load connected, the meters and lamps indicate the voltage, current and power the student is producing at a given instant.

If, with the load connected, the student pedaled at a constant rate for a specific time interval, then the total energy expended by the student, or the work done, would be the power output multiplied by the number of seconds in the time interval. If the student pedaled at a varying rate, then the total energy expended, or work done, would be the integral of the power at each instant over the time interval of interest. In all of this, we are ignoring the fact that the efficiency of the generator is not 100%.