Re: emf and batteries

[John Clement <clement@HAL-PC.ORG>]

> The term emf is an historical fly in amber.  Until the early 19th
> century the terms force and energy were often interchanged.  The Latin
> for force is "vis" and the Latin for kinetic energy is "vis viva" or
> living force.  The
> term emf referred to  energy, or more specifically an intensive energy -
>
> electrical potential energy per unit charge.

The real fly is that students often do not have good understanding of
ratios.  They may show some understanding when dealing with concrete objects
such as volume or mass, but EMF and potential difference are the ratios of
two abstract quantities which are not really understood.

The word force in EMF is often used by instructors to help explain the
concept because EMF has connections with force.  For example a concrete
visualization is to say that the battery pushes the charge through the light
bulb.  One can have a physical picture of marbles or water being pushed
through a tube, and then losing energy by "friction" in the filament.  So
immediately force is invoked, but the EMF is not force, so the student has a
misconception.

Even considering EMF is impossible until students have a one way model with
and understanding of a complete circuit.  The recent TPT article is apropos.

John M. Clement
Houston, TX