eigenvalue:

eigenvector:

electrodynamic: In contrast to electrostatics, this is the study of the interactions between electrical charges which are allowed to move. (see also electromagnetic interaction)

electron:

electrostatic: In contrast to electrodynamics, this is the study of the interactions between stationary electrical charges. (see also electromagnetic interaction)

energy: Technically defined as the "ability to do work", this concept is traditionally thought of as difficult to grasp initially. The concept has to do with the colloquial idea of effort (see work). Since it takes some effort to move an object, we say a moving object has kinetic energy. Similarly, it takes some effort to lift an object; we would like to define an energy to associate with lifting or dropping an object --- this is the potential energy. The "ability to do work," then is our ability to get energy out of an object. An object loses energy by giving it to other objects. If a moving truck hits a parked car, the car gains some energy and the truck loses some; the truck has "done some work" on the car. (See work) (Energy is expressed in units of joule which were named for James Prescott Joule.)
    Since it is also possible to have work done by objects which are warm, heat can be said to be a form of energy. In fact, Einstein's famous equation, E=mc², expresses that mass can also be considered a form of energy, although it is less obvious that this is the case.

energy, kinetic:

energy, potential:

energy band: (Context: condensed matter physics; See band, energy)

energy level: (Context: condensed matter physics)

energy gap: (Context: condensed matter physics) The energy gap is the amount of energy necessary for an electron in the valence band to jump to the conduction band.

equinox: As the orientation of the Earth (relative to the sun) varies through the year, the sun appears to move across the sky a little higher in the sky or a little lower.  During the shorter days of winter, the sun is lower; during the longer days of summer, the sun is higher.  Halfway between the extremes (see solstice) are the times when the night is as long as the day.  The word equinox is rooted in the Latin aequinoctium, from aequi- (equal) and nox (night).  The equinox marks the half-way point between the longest day of summer and the shortest day of winter.