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RAD 350 Chapter 4 Electricity, Magnetism and Electromagnetism
I. The primary purpose of an x-ray tube is to convert electric energy into
electromagnetic energy or x-rays
-energy conversion is around us everywhere
-electricity created the heat in a hair dryer or toaster
-chemical energy cranks the starter motor to start your car
-an electrical motor blows the cold air your AC makes
II. Electrostatics – Study of stationary electric charges
Electric charges are POSITIVE or NEGATIVE
Matter has mass and ENERGY equivalent; it can also have electric charge
-Electrification – building up electrical charges – balloon/hair; walking on
carpet and touching a doorknob; can be caused by contact, friction or INDUCTION
where all cause movement of NEGATIVE CHARGES and the result is like a
capacitor- storage of energy UNTIL something causes it to discharge – lightening is
a prime example where the negative charges continue to mount in the sky until
they become so charged, they discharge toward the Earth – which is a ground –
where charges go to dissipate
-smallest unit of electrical charge is the electron
-unit of electrical charge is measured in coulombs (C)
III. Electrostatic Laws:
-unlike charges attract – like charges repel
-Coulomb’s Law: Electrostatic force is DIRECTLY proportional to the product
of the electrostatic charges and INVERSELY proportional to the square of the
distance between them
-electric charge is distributed uniformly on the surface of an object – BUT
MORE concentrated at pointed areas
-electric potential is measured in VOLTS
REVIEW Table 4-2 for electrical symbols!!!
IV. ELECTRODYNAMICS – electric charges in MOTION
Terms:
-conductor – substance that electrons flow freely through
-insulator – substance that does NOT permit electron flow
-semiconductor – substance that under certain conditions permits flow
-resistance – increasing resistance will inhibit electron flow (hair
blower)
OHM’s Law: V=IR where V is electric potential (volts), I is current (amps) and
R is resistance in ohms. Triangle example in class.
Circuits
-series
-parallel
Figure resistance in BOTH types of circuits
Current flow:
DC = flows in only ONE direction
AC= flows both directions (positive and negative – NOT polarity) and
posses magnetic properties
Power = Measured in WATTS (W) P=IR
V. Magnetism and why it’s important in electricity (AC current only!)
- ANY charged particle in motion creates a magnetic field
- The magnetic lines are ALWAYS closed loops
-Magnetic permeability – ability of a substance to attract magnetic field
intensity lines
-Magnets are classified according to their origin
-natural – earth/loadstone
-permanent magnets – usually made of iron – NOT really permanent as
they lose magnetic property over time!
-electromagnets – wire wrapped around an iron core
-ONLY work when current is flowing – FIRE DOORS
-All magnets have a north and south pole (+ & -)
-ALL MATTER can be classified by it’s magnetic susceptibility
-diamagnetic material WEAKLY repelled
-FERROMAGNETIC – strongly attracted to a magnet and can themselves
become magnetized (via induction)
-paramagnetic – somewhere between diamagnetic and ferromagnetic
(like MRI contrast – GADO)
- the degree a substance can be magnetized is called magnetic susceptibility
-Magnetic force is proportional to the product of the POLE STRENGTHS
divided by the square of the distance between them.
-The poles have much MORE magnetic attraction than the sides.
-The SI unit of magnetic force is the Telsa (T) and Gauss (G); 1 T = 1,000 G
-discuss the 5G line for monitors and MRI units