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 x- ray 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 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. An insulator is any material that does not allow electron flow. 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 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

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. Increasing electric resistance results in a reduced electric current Modifying a conducting wire by reducing its diameter (wire gauge), increasing its length or inserting different material (circuit elements) can increase its resistance.

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:

ResistorInhibits flow of electrons BatteryProvides electric potential Capacitor Momentarily stores electric charge AmmeterMeasures electric current VoltmeterMeasures electric potential Switch Turns circuit on or off by providing infinite resistance Transformer Increases or decreases voltage by fixed amount (AC only) RheostatVariable 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 = V x I