Download Chapter 5 Electrostatics

Chapter 5 Electrostatics
• Photon Energy
– Photon energy is DIRECTLY proportional to its
frequency
– Photon energy INCREASES as wavelength
DECREASES
– Photon energy INCREASES as kVp INCREASES
Electricity, Magnetism and the x-ray
tube relationship
• Primary purpose of an x-ray tube is to convert
electric (A/C current) into ELECTROMAGNETIC
ENERGY!
Laws of electrostatics
• Unlike charges ATTRACT
• Like charges REPEL
• Electrostatic force is DIRECTLY proportional to
the PRODUCT of the charges and the SQUARE
or the distance between them (kind of like Inv.
Sq. law and laws of magnetics)
• When objects are “electrified, the charges are
distributed throughout the object – BUT are
greatest at POINTED spots
Electric Terms
• Electrodynamics – electron flow
• Conductors – Matter that conducts the
current
• Insulators – Matter that INHIBITS current flow
• SUPERCONDUCTOR – Like an MRI unit – low
resistance (niobium/titanium)
• Semi-conductor = depending on the
conditions, can be either a
conductor/insulator
Electronic Terms, con’t
• Electric Potential = Volt (V)
• Resistance = Increasing electric resistance
(Ohm’s) = LESS current flow in amps (A)
• Ohm’s Law = voltage across the circuit is equal
to the CURRENT X RESISTANCE (V + IR) where
I= current (amps), R= resistance in ohm’s, V=
POTENTIAL in volts
• WATT (W) = measurement of electrical power
Current
• Direct Current (DC) = electron flow in only one
direction
• Alternating current (AC) SINUSOIDAL form
each way (+ & -)
– Magnetism and AC are very closely related (+/- OR
N-S pole charges)
– Magnets are classified according to their origin
(natural, permanent, electromagnet)
Magnetic Laws
• Every magnet has TWO poles
• Like poles REPEL, unlike ATTRACT
• Magnetic lines are ALWAYS closed
– Magnetic force and the DISTANCE from the
magnet are INVERSLEY proportional to the square
of the distance – BUT the attraction is GREATEST
at the POLES rather than the sides of the magnet.
4 Fundamental forces of nature
•
•
•
•
Gravity
Strong nuclear forces
Weak nuclear forces/interaction
Electromagnetic force(s)
The Battery
• Discovered (invented) by “Volta” who made a
“voltaic “ pile (copper, zinc and wet paper) =
1st crude batter and SOURCE of EMF!
Charges in motion
• Can create a magnetic field (flux lines, gauss,
tesla)
• The book takes a lot of time with the “right
hand rule” spend your time on other items:
– Solenoid – coil of wire that MAGNIFIES the action
of the current flow via COILS
– Electromagnet – ferromagnetic material wrapped
in a coil of wire
FARADAY
• Found that a coil of wire & a mangetic field that is
MOVING can cause current (AMPS) to flow in the
wire coil – INDUCTION OF CURRENT – Farady’s
law
• Magnitude of induced current depends on:
– Strength of magnetic field
– Velocity of the magnetic field as it moves past the
conductor (coil)
– ANGLE of the conductor (coil) to the magnetic field
– NUMBER of TURNS in the conductor
LENTZ’ Law
• Induced current flows in a direction such that
it opposes the action that induces it
– Two ways to induce current:
• Self induction – single coil of wire with a current
running through it – OPPOSES the magnetic forces it
produces
• Mutual induction – inducing a current flow through a
secondary coil by passing a varying current through the
primary coil – VERY IMPORTANT FOR TRANSFORMERS!
Electric Generator
• Simple electric generator: coil of wire placed
in a strong magnetic field between tow poles
of a magnet
Electric Motor
• Electric energy applied to the “current loop”
to produce a mechanical motion – rotation of
the loop in the magnetic field – like the stator
windings on the x-ray tube – able to have the
anode turn w/o any current on the inside of
the tube – INDUCTION motor
TRANSFORMERS
• Change the intensity of the AC voltage and
current by MUTUAL INDUCTION
– Voltage and AMPS may be changed (stepped up or
down) by changing the numbers of coils on each
side of a transformer
Transformer Energy Losses
• Resistance to current = heat loss
• AC reversal currents = hysteresis (self
induction current flow opposes magnetic
field)
• Magnetic current fluctuation from + to - =
“Eddy currents”
Types of Transformers
• Closed core
• Autotransformer
• Shell type