Engineering Expert Witness Blog

      As an engineering expert I’ve often witnessed energy change forms, something our example coffee mug has been experiencing as it moves from a shelf to the floor.   The mug’s various energies were proven to be mathematically equivalent, expressed as, 4.9 kg•meter² /second² , which is read as, “kilogram meter squared per second squared.”   This mouthful led to the renaming of the measurement to the Joule, in honor of James Prescott Joule, a scientist who successfully demonstrated the interrelationship between different forms of energy.   We’ll focus on one of his experiments, the Joule Apparatus, today.

      Back in the 1840s Joule built his Apparatus, a device which demonstrated the interrelationship between different forms of energy.

The Joule Apparatus

      The Joule Apparatus consisted of a weight suspended by string over a pulley, which in turn was wound around a winding drum.   As long as the drum remained stationary, the weight remained motionlessly suspended.   While motionless, the weight’s potential energy lay latent within it, just as it had in our example coffee mug resting on a shelf.

      But when the pressure keeping the winding drum stationary was released, the weight was set free to fall, and its potential energy began converting to kinetic.   In the process, the string the weight was attached to unwound from the drum, which caused the drum to turn and along with it the paddle wheel it was attached to.

      Joule’s Apparatus followed energy through many forms.  From the quiet of potential energy to the kinetic energy demonstrated by the falling weight.   The kinetic energy in turn was converted into mechanical energy, made manifest by the interaction between the moving drum and paddle wheel.   The rotating paddles agitated the water, causing its temperature to rise.    Observing this, Joule concluded that the mechanical energy of the spinning paddle wheel had been converted into heat energy, which temperature measurement proved was transferred into the water.   Joule’s experiment thus proved the link between potential, kinetic, mechanical, and heat energies.

      Joule’s work paved the way to make possible the later development of a host of modern mechanical devices that also converted heat energy into mechanical energy, or vice versa.   These devices include a car’s engine and your kitchen’s refrigerator.

      Next time we’ll see how Joule demonstrated a link between electrical and other forms of energy, including mechanical and heat.   We’ll then use his discoveries to convert our falling coffee mug’s kinetic energy into yet another form.

Copyright 2015 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog

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