Water Analogy for Electricity
Date: Fall 2012
I am writing science materials for third graders and want to explain the flow of electrical current through a wire by comparison to water in a hose. But there is something I need to be clear on to make sure I communicate correctly: If the resistors on a circuit are light bulbs that emit energy as both heat and light, how does one describe what is lost from the overall circuit? The potential or kinetic energy of the electrons that are moving through the wire? Not that third graders need that detail, but I need it to make sure I am thinking rightly about this!
The potential energy of the electrons drops as they pass through dissipative devices such as resistors. If you are comparing to water in a hose, the water pressure drops through dissipative elements.
Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
Potential Energy is lost. Almost all energy within a circuit is potential energy. Voltage, or potential difference, is potential energy per unit charge. As electrons have negative charge, they flow from lower to higher electric potential. An increase of electric potential (i.e. a rise of voltage) is a drop in electric potential energy for electrons.
Dr. Ken Mellendorf
Thank you for your e-mail.
I have thought of electricity in a wire as analogous to water in a (pipe). Resistors (or light bulbs) are analogous to restrictions in the pipe; the rapid flow through such restrictions results in a pressure difference, heating, and the expenditure of power. The pressure is analogous to electrical voltage and the water flow is analogous to electrical current flow. In some ways, the comparison becomes more difficult: I have no easy analogy with water for light from a bulb. A hole in the pipe causes leakage while electric current leakage requires some sort of alternate path such as another wire. A hole is comparable to a wire. I am not sure how far you want to go with this.
I am giving you an engineer's perspective; I expect that a physicist's perspective will be different. Perhaps the best explanation will take advantage of both.
In the case of electrons and light bulbs, it would be the potential energy (or electron volts eV) of the combined electrons bombarding electrons of the filaments to produce light energy in the form of photons.
In the case of water molecules, you can think of some kind of apparatus that would convert kinetic energy to produce work such as a hydro-electric plant.
The energy lost is due to the drop on electrical potential, or
voltage, across the resistor. It is this potential/voltage that is the
driving force of current. The energy is not "kinetic energy" -- it
would be incorrect to think of electrical current as fast-moving
little balls with mass and momentum. The energy comes from the
electrical field (voltage) being established -- the electrons are
simply responding to the field according to the laws of physics.
Hope this helps,
If you are using the pressure in the water as the V and the volume of water as the I, for the R you can try using a finger pinch midway through the hose. The flow is impeded by the resistance. Think of the R as frictional resistance of the water against the walls of the hose; push back at the pinch or if there is a stick plugging up the hose. Both volume and pressure of the water will be affected. The overall power has decreased.
Heat given off due to resistance of e- current flow causes loss of energy, both I and V. The overall power decreases.
Peter E. Hughes, Ph.D. Milford, NH
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Update: November 2011