Eddy - Changing, curling path in turbulent flow of a fluid.

Efficiency - The ratio of useful work input, or the percentage of the work put into a machine that is converted to useful work output.

Elapsed Time - The time that has passed, or elapsed since the beginning of the time measurement.

Elastic - Term applied to a material that returns to its original shape after it has been stretched or compressed.

Elastic Collision - A collision in which colliding objects rebound without lasting deformation or the generation of heat.

Elasticity - The property of a body or material buy which it experiences a change in shape when a deforming force acts on it and by which it returns to its original shape when the deforming force is removed.

Elastic Limit - The distance of stretching and compressing beyond which an elastic material will not return to its original state.

Electrical Force - A force one electric charge exerts on another. When the charges are both positive or both negative, the force is repulsive; when the charges are unlike, the force is attractive.

Electrical Resistance - The resistance of a material to the flow of an electric current through it; measured in ohms.

Electrically Polarized - Term applied to an atom or molecule in which the charges are aligned so that one side is slightly more positive or negative than the opposite side.

Electric Charge - See charge.

Electric Current - The flow of electric charge; measured in amperes.'

Electric Field - A force field that fills the space around every electric charge or group charges. Another electric charge intorduced into this region will experience an electric force acting on itself.

Electric Potential - The electric potential energy per charge at a location in an electric field; measured in volts and often called voltage.

Electric Power - The rate at which electric energy is converted into another form, such as light, heat, or mechanical energy.

Electromagnet - A magnet whose field is produced by an electric current. Usually in the form of a wire coil with a piece of iron inside the coil.

Electromagnetic Induction - The phenomenon of inducing a voltage in a conductor by changing the magnetic field around the conductor.

Electromagnetic Spectrum - The range of electromagnetic waves extending from radio waves to gamma waves.

Electrostatics - The study of electric charges at rest.

Element  - A substance made of only one kind of atom. Carbon, hydrogen oxygen, and nitrogen are some examples.

Ellipse - An oval-shaped curve that is the path taken by a point that moves such that the sum of its distances from two fixed points is constant.

Energy - That property of an object or a system which enables it to do work; measured in joules.

Equilibrium - In general, a state of balance. In particular: (a) The state of a body on which no net force acts. (b) The state of a body on which no net torque acts. (c) The state of a liquid in which the processes of evaporation and condensatioare taking place at equal rates.

Escape Speed - The minimum speed necessary for an object to escape permanently from a gravitational field which holds it.

Evaporation - The change of state from a liquid to gas that takes place at the surface of a liquid.

Eyepiece - The lens of a telescope that is closer to the eye; enlarges the real image formed by the first lens

Fact - A close agreement by competent observers of a series of observations of the same phenomena.

Fahrenheit Scale - The temperature scale in common use in the United States. The number 32 is assigned to the freezing point of water, and the number 212 to the boiling point of water (at standard atmospheric pressure).

Family - A group of elements in the same column of the periodic table. Elements within a family have similar chemical properties and have the same number of electrons in the outer shell.

Faraday's Law - The statement that the induced voltage in a coil is proportional to the product of the number of loops and the rate at which the magnetic field changes within those loops. In general, the statement that an electric field is induced in any region of space in which a magnetic field is changing with time. The magnitude of the induced electric field is proportional to the rate at which the magnetic field changes.

Farsighted - Term applied to a person who has trouble focusing on nearby objects because the eyeball is so short that images form behind the retina.

Field - See force field.

First Postulate of Special Relativity - The statement that all the laws of nature are the same in all uniformly moving frames of reference.

Fission - See nuclear fission.

Fluid - Anything that flows; in particular, any liquid or gas.

Focal Length - The distance between the center of a lens and either focal point.

Focal Plane - A plane that passes through either focal point of a lens and is perpendicular to the principal axis. For a converging lens, any incident parallel beam of light converges to a point somewhere on a focal plane. For a diverging lens, such a beam appears to come from a point on a focal plane.

Focal Point - For a converging lens, the point at which a beam of light parallel to the principal axis converges. For a diverging lens, the point from which such a beam appears to come.

Focus - For an ellipse, one of the two points for which the sum of the distances to any point on the ellipse is a constant. A satellite orbiting the earth moves in an ellipse which has the earth at one focus.

Force - Any influence that tends to accelerate an object; commonly, a push or pull; measured in newtons.

Forced Vibration - The vibration of an object that is made to vibrate by another vibrating object that is nearby. The sounding board in a musical instrument amplifies the sound through forced vibration.

Force Field - What fills the space around a mass, electric charge, or magnet, so that another mass, electric charge, or magnet introduced to this region will experience a force. Examples of force fields are gravitational fieldselectric fields, and magnetic fields.

Free Fall - Motion under the influence of the gravitational force only.

Freezing - The change in state from liquid to solid.

Frequency - The number of vibrations per unit of time; measured in hertz.

Friction - The force that acts to resist the relative motion (or attempted motion) of objects or materials that are in contact.

Fulcrum -The pivot point of a lever.

Fusion  See nuclear fusion.

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g - The symbol for gram.

Generator - A machine that produces electric current by rotating a coil within a stationary magnetic field.

Gravitational Field - A force field that fills the space around every mass. Another mass in this region will experience a gravitational force.

Grounding - Allowing charges to move freely along a connection between a conductor and the ground.

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- The symbol for hour.

Half life - The time required for half the atoms of a radioactive isotope of an element to decay.

Heat - The energy that is transferred from one material to another because of a temperature difference between the materials. Once the energy is absorbed by matter, it is internal energy.

Hertz - The SI unit of frequency. One hertz (Hz) equals one vibration per second.

Hooke's Law - The statement that the amount of stretch or compression of an elastic material is directly proportional to the applied force.

Huygens' principle - The statement that every point on any wave front can be regarded as a new point source of secondary waves.

Hypothesis - An educated guess; a reasonable explanation of an observation or experimental result that is not fully accepted as factual until tested over and over again by experiment.

Hz - The symbol for hertz.

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Impulse - The product of force multiplied by the time interval during which the force acts. The impulse is equal to the change in momentum.

Incoherent - Type of light beam in which the waves are out of phase with each other.

Induced - Term applied to electric charge that has been redistributed on an object because of the presence of a charged object nearby. Also, term applied to a voltage, electric field, or magnetic field that is created due to a change in motion though a magnetic field or electric field.

Induction -The charging of an object without direct contact. See also electromagnetic induction.

Inelastic - Term applied to a material that does not return to its original shape after it has been stretched or compressed.

Inelastic Collision - A collision in which the colliding objects become distorted and generate heat during the collision.

Inertia - The resistance of any material object to change in its state of motion.

Infrared - Electromagnetic waves of frequencies lower than the red of visible light.

Infrasonic -  Term applied to sound of pitch too low to be heard by the human ear, that is, of pitch below 20 hertz.

In Parallel - Term applied to portions of an electric circuit that are connected at two points and provide alternative paths to the current between those two points.

In Phase - Term applied to two or more water waves whose crests arrive at a place at the same time, so that their effects reinforce each other.

In Series - Term applied to portions of an electric circuit that are connected in a row so that the current that goes through one must go through all of them.

Instantaneous Speed - The speed at any instant of time.

Insulator - A material that is a poor conductor of heat and that delays the transfer of heat, also a poor conductor of electricity.

Interference Pattern  - A pattern formed by the overlapping of two or more waves that arrive in a region at the same time.

Internal Energy - The total energy inside a substance.

Inversely - When two values change in opposite directions, so that if one is doubled, the other is reduced to one half, they are said to be inversely proportional to each other.

Ion - An atom with a net electric charge, due to the loss or gain of electrons.

Iridescence - The phenomenon whereby the interference of light waves of mixed frequencies reflected from the top and bottom of thin films causes a spectrum of colors.

Iris - The colored part of the eye, which surrounds the black opening through which light passes and which regulates the amount of light entering the eye.

Isotope - A form of an element having a particular number of neutrons in the nuclei of its atoms. Different isotopes of a particular element have the same atomic number but different atomic mass numbers.