For a heat engine, the ratio of work done by the engine to heat intake. Efficiency is never 100%.

Elastic collision

A collision in which both kinetic energy and momentum are conserved.

Electric generator

A device that converts mechanical energy to electrical energy by rotating a coil in a magnetic field; sometimes called a “dynamo.”

Electromagnetic induction

The property by which a charge moving in a magnetic field creates an electric field.

Electromagnetic spectrum

The spectrum containing all the different kinds of electromagnetic waves, ranging in wavelength and frequency.

Electromagnetic wave

A transverse traveling wave created by the oscillations of an electric field and a magnetic field. Electromagnetic waves travel at the speed of light, m/s. Examples include microwaves, X rays, and visible light.


A negatively charged particle that orbits the nucleus of the atom.


A unit of measurement for energy on atomic levels. 1 eV = J.


A conserved scalar quantity associated with the state or condition of an object or system of objects. We can roughly define energy as the capacity for an object or system to do work. There are many different types of energy, such as kinetic energy, potential energy, thermal energy, chemical energy, mechanical energy, and electrical energy.


The disorder of a system.


The state of a nonrotating object upon whom the net torque acting is zero.

Equilibrium position

The stable position of a system where the net force acting on the object is zero.