Letter P

PARALLEL - A way of connecting components so they each have the same voltage difference across them. On a circuit diagram, you would draw the components parallel to each other and connect them all back to the same point. Compare with SERIES. For more information about parallel circuits click.

PARTICULATE MATTER (PM) - Unburned fuel particles that form smoke or soot and stick to lung tissue when inhaled. A chief component of exhaust emissions from heavy-duty diesel engines.

PASSIVE SOLAR SYSTEM - A solar heating or cooling system that uses no external mechanical power to move the collected solar heat. For more information about passive solar system click.

PHOTENTIAL ENERGY - Stored energy.

PHOTOCELL - A device that produces an electric reaction to visible radiant energy (light).

PHOTOVOLTAIC CELL - A semiconductor that converts light directly into electricity. Same as solar cell.

PRIMARY BATTERY - A battery is made up of a number of individual cells. Typically a six volt battery is made up of four 1.5 volt cells. The cross section through a typical cell is shown.