Transcript Blue Circuit Board

Electrostatic and Electrodynamics
Your Name
At the completion of this chapter, the student
should be able to do the following:
• Define electrification and provide examples
• List the laws of electrostatics
• Identify units of electric current, electric potential, and
electric power
The primary function of an x-ray imaging system is to convert electric energy into
electromagnetic energy. Electric energy is supplied to the x-ray imaging system
in the form of well-controlled electric current. A conversion takes place in the xray tube, where most of this electric energy is transformed into heat, some of it
into x-rays.
Electrostatics is the study of stationary electric charges.
Because of the way atoms are constructed, electrons often are free to travel from
the outermost shell of one atom to another atom. Protons, on the other hand, are
fixed inside the nucleus of an atom and are not free to move. Consequently, nearly
all discussions of electric charge deal with negative electric charges—those
associated with the electron.
On touching a metal doorknob after having walked across a deep-pile carpet
in winter, you get a shock (by contact). Such a shock occurs because
electrons are rubbed off the carpet onto your shoes (by friction), causing you
to become electrified. An object is said to be electrified if it has too few or
too many elect
.
Electrification can be created by:
• contact
• friction
• induction
Electrification by
induction and contact
The electrostatic force is very strong when objects are close but decreases
rapidly as objects separate. This inverse square relationship for electrostatic
force is the same as that for x-ray intensity.
Electric charges have potential energy. When positioned close to each other,
like electric charges have electric potential energy because they can do work
when they fly apart. Electrons bunched up at one end of a wire create an
electric potential because the electrostatic repulsive force causes some
electrons to move along the wire so that work can be done.
The unit of electric potential is the volt (V).
Electric potential is sometimes called voltage; the higher the voltage, the
greater is the potential to do work. In the United States, the electric potential in
homes and offices is 110 V. X-ray imaging systems usually require 220 V or
higher.
Electrodynamics is the study of electric charges in motion.
We recognized electrodynamic phenomena as electricity. If an electric potential
is applied to objects such as copper wire, then electrons move along the wire.
This is called an electric current, or electricity.
The direction of electric current is important. In his early classic experiments,
Benjamin Franklin assumed that positive electric charges were conducted on
his kite string. The unfortunate result is the convention that the direction of
electric current is always opposite that of electron flow. Electrical engineers
work with electric current, whereas physicists are usually concerned with
electron flow.
It was the 1740's when he started working with electricity. He conducted many
different experiments to try to understand more about it. His most famous
experiment being his kite flying one in June of 1752. Franklin believed that
lightning was a flow of electricity taking place in nature. To test his hypothesis,
he tied a metal key to a child's kite and flew the kite during a thunderstorm. The
key became charged with electricity, and Ben had proof that lightning is really a
string of electricity. His kite experiment and his others helped him develop many
of the words and terms that we still use today when dealing with electricity:
charge, discharge, conductor, minus, plus, electrician, electric shock, and others
A conductor is any substance through which electrons flow easily.
Most metals are good electric conductors; copper is one of the best.
Water is also a good electric conductor because of the salts and other
impurities it contains
An insulator is any material that does not allow electron flow.
Glass, clay, and other earthlike materials are usually good electric
insulators.
A semiconductor is a material that under some conditions behaves as
an insulator and in other conditions behaves as a conductor. Mterials:
germanium and silicon.
Superconductivity is the property of some materials to exhibit no
resistance below a critical temperature (Tc).
Superconducting materials such as niobium and titanium allow electrons to
flow without resistance
Electric circuit:
• The path over which the electrons flow
Electric current is measured in amperes (A). The ampere is proportional to the
number of electrons flowing in the electric circuit. One ampere is equal to an
electric charge of 1 C flowing through a conductor each second.
Tube current
How many electron travel from cathode to anode
Modifying a conducting wire by reducing its diameter (wire gauge), increasing
its length or inserting different material (circuit elements) can increase its
resistance.
Increasing electric resistance results in a reduced electric current
Electric potential is measured in volts (V), and electric resistance is
measured in ohms (Ω). Electrons at high voltage have high potential energy
and high capacity to do work. If electron flow is inhibited, the circuit
resistance is high.
OHM'S LAW
where V is the electric potential in volts, I is the electric current
in amperes, and R is the electric resistance in ohms. Variations
of this relationship are expressed as follows:
Resistor
Inhibits flow of electrons
Battery
Provides electric potential
Capacitor
Momentarily stores electric
charge
Ammeter
Measures electric current
Voltmeter
Measures electric potential
Switch
Turns circuit on or off by
providing infinite resistance
Transformer
Increases or decreases
voltage by fixed amount (AC
only)
Rheostat
Variable resistor
Diode
Allows electrons to flow in only
one direction
Series circuit
Parallel circuit
Electric current, or electricity, is the flow of electrons through a conductor. These
electrons can be made to flow in one direction along the conductor, in which
case the electric current is called direct current (DC).
D/C current
Current in which electrons oscillate back and forth is called alternating current
(AC).
A/C current frequency 60 Hz
A/C current
Electric power is the rate at which electric energy is transferred by an electric
circuit. The SI unit of power is the watt.
Electric power, like mechanical power, is represented by the letter P in
electrical equations. The term wattage is used colloquially to mean "electric
power in watts."
P=VxI