Fundamentals and Units of Measurement 1649namic temperature of the triple point of pure water.
Note: the term degree (°) is not used with the term
kelvin as it is with other temperature scales.
Ordinary temperature measurements are made with
the celsius scale on which water freezes at 0°C and boils
at 100°C. A change of 1°C is equal to a change of
1 kelvin, therefore 0°C = 273.15 K: 0°C = 32°F.
Luminous Intensity (IL ). Luminous intensity is the
luminous flux emitted per unit solid angle by a point
source in a given direction. The candela (cd) is the unit
of luminous intensity. One candela will produce a lumi-
nous flux of 1 lumen within a solid angle of 1 steradian.
The international definition of the candela is the
luminous intensity, perpendicular to the surface, of
1 »600 000 m^2 of a black body at the temperature of
freezing platinum under a pressure of 101 325 N/m^2
(pascals).
Molecular Substance (n). Molecular substance is the
amount of substance of a system that contains as many
elementary entities as there are atoms in 0.012 kg of
carbon 12.
The mole is the unit of molecular substance. One
mole of any substance is the gram molecular weight of
the material. For example, 1 mole of water (H 2 0)
weighs 18.016 g.
Plane Angle (D). The plane angle is formed between
two straight lines or surfaces that meet. The radian (rad)
is the unit of plane angles. One radian is the angle
formed between two radii of a circle and subtended by
an arc whose length is equal to the radius. There are 2S
radians in 360q.
Ordinary measurements are still made in degrees.
The degree can be divided into minutes and seconds or
into tenths and hundredths of a degree. For small
angles, the latter is most useful.
(48-3)Solid Angle (A). A solid angle subtends three dimen-
sions. The solid angle is measured by the area,
subtended (by projection) on a sphere of unit radius by
the ratio of the area A, intercepted on a sphere of radius
r to the square of the radius (A/r^2 ).
The steradian (sr) is the unit of solid angle. The
steradian is the solid angle at the center of a sphere that
subtends an area on the spherical surface, which is equal
to that of a square whose sides are equal to the radius of
the sphere.Energy (E). Energy is the property of a system that is a
measure of its ability to do work. There are two main
forms of energy—potential energy and kinetic energy.- Potential energy (U ) is the energy possessed by a
body or system by virtue of position and is equal to
the work done in changing the system from some
standard configuration to its present state. Poten-
tial energy is calculated with the equation
(48-4)
where,
M is the mass,
g is the acceleration due to gravity.
h is the height.
For example, a mass M placed at a height h
above a datum level in a gravitational field with an
acceleration of free fall (g), has a potential energy
given by U = mgh. This potential energy is
converted into kinetic energy when the body falls
between the levels.- Kinetic energy (T) is the energy possessed by
virtue of motion and is equal to the work that
would be required to bring the body to rest. A body
undergoing translational motion with velocity, v,
has a kinetic energy given by
(48-5)
where,
M is the mass of the body,
v is the velocity of the body.
For a body undergoing rotational motion(48-6)
where,
I is the moment of inertia of the body about its axis
of rotation,
Z is the angular velocity.
The joule (J) is the unit of energy. The mechanical
definition is the work done when the force of 1 newton
is applied for a distance of 1 m in the direction of its
application, or 1 Nm. The electrical unit of energy is the
kilowatt-hour (kWh), which is equal to 3.6 × 10^6 J.
In physics, the unit of energy is the electron volt
(eV), which is equal to (1.602 10 ± 0.000 07) × 10^19 J.H 2 =2 atomsu1.008 atomic weightO=2 atomsu16 atomic weight
H 2 O=18.016 gOne degree of arc (1q S
180=---------Rad1Rad=57.2956qUMgh=T=0.5Mv^2T=0.51IZ^2