E–H
20.1 A–D
 
20.2 E–H
 
20.3 I–L
 
 
20.4 M–P
 
20.5 Q–T
 
20.6 U–Z
 
E–H
E
Efficiency
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.
Electron
A negatively charged particle that orbits the nucleus of the atom.
Electronvolt
A unit of measurement for energy on atomic levels. 1 eV = J.
Energy
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.
Entropy
The disorder of a system.
Equilibrium
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.
F
Faraday’s Law
A law, || = , which states that the induced emf is the change in magnetic flux in a certain time.
First Law of Thermodynamics
Essentially a restatement of energy conservation, it states that the change in the internal energy of a system is equal to the heat added plus the work done on the system.
Focal length
The distance between the focal point and the vertex of a mirror or lens. For concave mirrors and convex lenses, this number is positive. For convex mirrors and concave lenses, this number is negative.
Focal point
The point of a mirror or lens where all light that runs parallel to the principal axis will be focused. Concave mirrors and convex lenses are designed to focus light into the focal point. Convex mirrors and concave lenses focus light away from the focal point.
Force
A push or a pull that causes an object to accelerate.
Free-body diagram
Illustrates the forces acting on an object, drawn as vectors originating from the center of the object.
Frequency
The number of cycles executed by a system in one second. Frequency is the inverse of period, f = 1/T. Frequency is measured in hertz, Hz.
Frictional force
A force caused by the roughness of two materials in contact, deformations in the materials, and a molecular attraction between the materials. Frictional forces are always parallel to the plane of contact between two surfaces and opposite the direction that the object is being pushed or pulled.
Fundamental
The standing wave with the lowest frequency that is supported by a string with both ends tied down is called the fundamental, or resonance, of the string. The wavelength of the fundamental is twice the length of the string, .
G
Gamma decay
A form of radioactivity where an excited atom releases a photon of gamma radiation, thereby returning to a lower energy state. The atomic structure itself does not change in the course of gamma radiation.
Gamma ray
An electromagnetic wave of very high frequency.
Gold foil experiment
An experiment by Ernest Rutherford that proved for the first time that atoms have nuclei.
Gravitational constant
The constant of proportionality in Newton’s Law of Gravitation. It reflects the proportion of the gravitational force and , the product of two particles’ masses divided by the square of the bodies’ separation. N · m2/kg2.
Gravitational Potential Energy
The energy associated with the configuration of bodies attracted to each other by the gravitational force. It is a measure of the amount of work necessary to get the two bodies from a chosen point of reference to their present position. This point of reference is usually chosen to be a point of infinite distance, giving the equation . Objects of mass m that are a height h above the surface of the earth have a gravitational potential energy of .
Ground state
In the Bohr model of the atom, the state in which an electron has the least energy and orbits closest to the nucleus.
H
Half-life
The amount of time it takes for one-half of a radioactive sample to decay.
Harmonic series
The series of standing waves supported by a string with both ends tied down. The first member of the series, called the fundamental, has two nodes at the ends and one anti-node in the middle. The higher harmonics are generated by placing an integral number of nodes at even intervals over the length of the string. The harmonic series is very important in music.
Heat
A transfer of thermal energy. We don’t speak about systems “having” heat, but about their “transferring” heat, much in the way that dynamical systems don’t “have” work, but rather “do” work.
Heat engine
A machine that operates by taking heat from a hot place, doing some work with that heat, and then exhausting the rest of the heat into a cool place. The internal combustion engine of a car is an example of a heat engine.
Heat transfer
A transfer of thermal energy from one system to another.
Hertz (Hz)
The units of frequency, defined as inverse-seconds (1 Hz = 1 s–1). “Hertz” can be used interchangeably with “cycles per second.”
Hooke’s Law
For an oscillating spring, the restoring force exerted by the spring is directly proportional to the displacement. That is, the more the spring is displaced, the stronger the force that will pull toward the equilibrium position. This law is expressed mathematically as F = –kx, where F is the restoring force and x is the displacement. The constant of proportionality, –k, is the spring constant.
Hypotenuse
The longest side of a right triangle, opposite to the right angle.
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