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Transcript
Rad T 110
Electromagnetic Radiation
The Wave Equation
• Velocity = frequency x wavelength
– Remember, photons all travel at the speed of
light, 186,000 miles/second so velocity is in
effect a constant
• Consequently, any changes to either
frequency or wavelength have to be
accompanied by a change in the other.
• So, if frequency
increases then
wavelength
decreases.
• Conversely, if
frequency decreases
then wavelength
increases.
What is frequency?
• Frequency is defined as the number of
wave forms per second.
• In the case of x-ray photons about 1019
cycles per second.
• Frequency is a representation of energy.
• As beam energy is increased, frequency
increases.
Beam Energy
• Beam energy is determined by kVp.
• Therefore, the higher the kVp the higher
the photon frequency.
• The higher the kVp
– The higher the beam quality
– The harder the beam
– The higher the penetrability of the beam
Electromagnetic Spectrum
Wave-particle Duality
• Higher energy photons act like particles.
• Low energy photons act like waves.
Inverse Square Law
• Addresses the issue of the divergent
beam.
• As the beam travels it diverges or spreads
out. The further it travels the more spread
out it becomes.
• However, given that the same number of
photons are in the beam their
concentration or intensity will be
diminished.
Single Phase Electric Current
• Full
Voltage ripple
• Half
Three phase Electric Current
• 6 pulse
Voltage ripple
• 12 pulse
Electromagnetic Induction
• Faraday’s Law
– An electric current is induced in a circuit if part
of the circuit is in a changing (alternating)
magnetic field.
– Also, a magnetic field can be generated by
alternating magnetic fields.
Transformers and Mutual Induction
• Transformers work with AC current through the
application of Faraday’s Law.
– An AC electric current induces a varying magnetic
field in a piece of iron (electromagnet).
– Another piece of wire is wrapped around the iron. In
this wire an electric current is induced. Why,
because the magnet field is moving throughout the
iron.
– So, without an AC current you would not induce a
varying magnetic field.
Step-down transformers
• These transformers also work on the principle of
mutual induction; however, their goal is to
decrease the incoming voltage.
• So, the secondary side has fewer windings than
the primary side.
• Any device you have at home that runs on
batteries and AC has a step-down transformer.
It is usually the little black box that gets hot.
Step Up Transformers
• These transformers are intended to
increase voltage.
• Remember, incoming voltage is relatively
low (<480 V) is in order to create kVp you
are going to have to dramatically increase
the voltage.
• This is done by having more wires on the
secondary side of the transformer than the
primary side.
• The primary side of
the transformer is
were the voltage
comes in.
• The step-up
transformer can also
be called a highvoltage or hightension transformer.
Closed core transformer
Types of Transformers in X-ray
Circuits
• Autotransformer
– Has only one winding and works on the
principle of self-induction (because of the
single winding).
• Step-up transformer, high voltage or high
tension
• Step-down or filament transformer
– Both of these are generally shell type
transformers and work using mutual induction.