The magneto generator: an introductory physics experiment.

A simple Atwood machine can be used to drive electric generators that have pulleys attached to the armature. This allows for quantitative experiments on the behavior of electric generators. Computer interfaced sensors can be used to measure the position and velocity of the falling mass in the Atwood machine as well as the voltage output of the generator. From this data a number of important physical principles can be observed. The position and velocity data can be used to calculate the changes in gravitational potential energy and kinetic energy. The difference between these is energy that either goes into rotational kinetic energy in the armature, heat generated by friction or generating electrical power. The generators can be operated with or without a load such as a light bulb. With no load the energy difference is accounted for by friction and rotational kinetic energy in the armature. With the load on the generator some of the energy goes into generating electrical power so the falling mass is observed to accelerate to a smaller velocity. From this students learn that a generator is an energy transformation device and not an energy creation device. The graph of velocity vs. time is not a straight line but curves with decreasing slope. This is due to the back-torque produced by the current in the generator. The voltage vs. time graph shows the alternating voltage produced by this type of generator and the voltage amplitude increases with time as predicted by Faraday's law of induction. A graph of rms voltage vs. velocity does not give a straight line, however, due to the fact that the armature of the generator possesses capacitive and inductive reactance. Given the importance of electromagnetic induction in physics and technology this is a highly appropriate experiment for the introductory physics course.

* Shaw, J. Department of Chemistry and Physics, Northwest Missouri State University.