Chapter 4 - Basic Physics for Radiography Flashcards
What is matter?
Matter is defined as anything that occupies space and has shape or form. The three basic forms of matter are solids, liquids, and gases. The quantity of matter that makes up any physical object is called its mass.
Laws of Conservation
Matter can be neither created nor destroyed, but it can change form.
Energy can be neither created nor destroyed, but it can change form.
Atoms
The fundamental particles that compose atoms are neutrons, protons, and electrons.
Electron Shells
Shell Number Shell Symbol Maximum Number of Electrons
1 K 2
2 L 8
3 M 18
4 N 32
5 O 50
6 P 72
7 Q 98
ionization
This process, which is called ionization, produces an atom with an electric charge.
Energy
Energy is defined as the ability to do work. It occurs in several forms and can be changed from one form to another. Some familiar forms of energy include heat, light, and electricity. Scientists have categorized energy in various ways. One method classifies energy into the following types: mechanical, chemical, thermal, nuclear, electric, and electromagnetic.
Mechanical energy
Mechanical energy can be further classified as either kinetic energy or potential energy. Kinetic energy is energy of motion, the ability of a moving object to do work. For example, a bowling ball in motion has energy to knock down the pins. Potential energy can be thought of as “stored” energy. When a bowling ball has been lifted, the work required to raise it is “stored” in the ball because of its position. When the ball is released, its potential energy is also released and is converted into kinetic energy.
Chemical energy
Chemical energy is released through chemical changes in atoms or molecules. An example of chemical energy is fire.
Thermal energy
Thermal energy is commonly called heat and is the result of atomic motion. As temperature rises, electrons move faster in their orbits and the orbits expand, which causes the electrons to move farther from the nucleus. This phenomenon explains why matter expands in size when heated and contracts when cooled.
Nuclear energy
Nuclear energy is the energy released by radionuclides. This is the energy used to produce electricity in a nuclear power plant or the explosion of a nuclear bomb.
Electric energy
Electric energy, or electricity, is the ability of electric charges to do work.
Electromagnetic energy
Electromagnetic energy is the important energy we deal with every day in radiology. This energy consists of light, x-rays, radio waves, microwaves, and other forms of energy. These energies have both electric and magnetic properties, changing the field through which they pass both electrically and magnetically (Fig. 4.5). These changes in the field occur in the form of a repeating wave, a pattern that scientists call a sinusoidal form or sine wave (Fig. 4.6).
wavelength
More important in radiology is the distance from one crest to the next, or wavelength (Fig. 4.8).
frequency
The frequency of the wave is the number of times per second that a crest passes a given point (Fig. 4.9).
velocity (speed)
Electromagnetic energy moves through space at the velocity (speed) of approximately 186,000 miles/s. All electromagnetic energy moves at the same velocity.
photon
The smallest possible unit of electromagnetic energy is the photon, which may be thought of as a tiny “bundle” of energy.
Sine Wave Energy
*Sine waves with shorter wavelengths (higher frequency) have more energy.
*X-rays with shorter wavelengths are more penetrating.
Characteristics of X-Rays
*Have no mass
*Are highly penetrating and invisible
*Are electrically neutral
*Produced over a wide range of energies and wavelengths
*Travel in straight lines at the speed of light
*Can ionize matter
*Produce biologic changes in tissues
*Produce secondary and scatter radiation
electric circuit
Three electric factors are part of an electric circuit: resistance, current, and potential difference.
Resistance
Resistance is any property of the circuit that opposes or hinders the flow of current. The unit used to measure resistance is the ohm, represented by the Greek letter omega (Ω). Resistance depends on several factors: the material of the conductor, its length, and its diameter. The longer the conductor, the more resistance it will provide. Resistance is decreased when the wire diameter is greater.
Current
Current is the quantity of electrons flowing in a circuit. The ampere, abbreviated A, is the unit used to measure the rate, or volume, of current flow in the circuit. In your home the electric circuit to the toaster may require 8 amps and the circuit to a lamp may require only 1 amp. In radiology, every body part will require a different amperage setting on the generator.
Potential difference
Potential difference is the force, or speed, of the electron flow in the current. The volt, abbreviated V, is the unit used to measure potential difference. In your home the electric circuit to the stove or dryer that contains 220V has twice the amount of electricity as the 110V circuit going to your television. In radiology, every body part will require a different voltage setting on the generator.
kilovolt peak (kVp)
For this reason, it is convenient to use the kilovolt peak (kVp), equal to 1000V, to measure the voltage across an x-ray tube,
milliampere (mA)
equal to 1/1000 of an ampere (0.001A), to measure x-ray tube current.
rectification
AC can be converted so that it flows in one direction only. This process is called rectification.
electromagnetic induction
When a conductor is placed in a magnetic field and there is movement between the lines of magnetic force and the conductor, electric current will flow in the conductor. This principle can be demonstrated by moving a circuit in and out of the force field surrounding a magnet (Fig. 4.15). The same result is obtained by moving the magnet in relation to the conductor (Fig. 4.16). This process is called electromagnetic induction.
