Lesson: Chapter - 20
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,
c = 3.00 × 108
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 =
1.6 × 10-19
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, |e|
= ?F/?t,
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,
?1 = 2L.
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
m1m2/r2,
the product of two particles’ masses divided by the square of the bodies’
separation.
G = 6/67 × 10,sup>-11
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 U = -Gm1m2/r.
Objects of mass m that are a height
h above the surface of the earth
have a gravitational potential energy of
Ug = mgh.
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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