d. Electron Volt. The unit of energy used in radiology is the electron volt (eV).
An electron volt is the amount of potential energy released when an electron is
accelerated by a potential difference of one volt. A keV is equal to one thousand
electron volts and a MeV is equal to one million electron volts.
e. Energy Transformation. Energy is constantly changing from one form to
another. All changes in the universe involve the transformation of energy. Various
forms of energy, such as mechanical energy (in steam engines), chemical energy (in
storage batteries), radiant energy (from the sun), nuclear energy (in nuclear reactors),
etc., are convertible, one to another. A boiler-steam engine generator hookup in a
power plant illustrates this conversion. In this sequence, chemical energy stored in the
fuel changes to heat energy that causes the water to boil, producing steam. The kinetic
energy in the steam molecules is converted to mechanical energy as the engine moves.
The mechanical energy is transformed into electrical energy as the steam engine drives
the generator. Electrical energy is converted into light, sound, heat, and motion through
such ordinary devices as electric bulbs, buzzers, toasters, motors, and so forth.
The quantity of matter in an object is often called its mass. Although frequently
used interchangeably with weight, the term mass is usually more specifically used to
designate the degree of inertia contained within an object. Inertia may be defined as
the tendency of a resting body to remain at rest, or a moving body to remain in motion in
a straight line.
a. To set a resting body in motion or to stop a moving body, a force must be
applied. The greater the mass of an object, the greater the force needed to speed it up
or slow it down. For example, it takes more force to stop an automobile in a given time
than it takes to stop a bicycle when both are moving at the same speed.
b. When a force succeeds in moving a resting body, it is said that work is
performed. The amount of work done is determined by the quantity of the force used
and the distance the body moves in a straight line at a constant speed. Work is
expressed by the following equation:
Work = force X distance
c. The unit of work in the metric system is the joule (newton-meter). A joule is
the amount of energy needed for a force of one newton to act through a distance of one
meter. One watt is the unit of power equal to the work done at the rate of one absolute
joule per second.