Electrical Currents (1) Flashcards
What makes materials conductive?
free electrons (electrons from outermost/valence shell of an atom)
examples of good conductors
- copper
- gold
- silver
- aluminium
- iron
- graphite
Define electrical current
The movement of electrons through a conductor
What is an ampere?
(Amp)
unit used to describe electrical current.
1amp = number of electrons moving past a given point in 1 second.
Define Current flow
The unidirectional flow of electrons through a conductor.
I
Define Electrical Resistance
Opposition to the flow of electrons within an electrical circuit.
Resistance increases as the length of the conductor increases.
varies with the:
- cross section of the conductor
- length of conductor
- type of material
Describe Ohm’s Law
1826 by Georg Simon Ohm
1 volt (V) will force 1 amp of current flow (I) through 1 ohm (R) of resistance.
V = I x R
volt is energy to move a charge/start flow.
Direct Current
- battery
- continuous
- same direction
- penetrates
- locate subsurface discontinuities
- may vary in value
Alternating Current
- reverses at rate of 60 cycles per second
- readily available
- most common source
- limited to surface breaking discontinuities (skin effect)
- aid in particle mobility
- can be rectified
Half Wave Rectified Alternating Current
- reverse half cycle is blocked
- pulsating uni directional current
- maximum sensitivity to subsurface defects
- promotes high mobility particles
Full Wave Rectified Alternating Current / Single Phase Full Wave Direct Current
- reverse half cycle turned around to flow in same direction as first half of the cycle
- pulsating dc
- not used often in MPI
Three Phase Full Wave Rectified, Alternating Current
- three phase full wave DC
- little pulsation
- tests large equipment
Three Phase Alternating Current
- three separate currents flowing, separate sine curves, 60 cycles/sec
- used in a generator that starts AC 3x in 1/60 sec
- rectified often to be 3phase rectified AC or 3phase full wave DC
