Electrical Currents (1)

Question 1

What makes materials conductive?

Answer 1

free electrons (electrons from outermost/valence shell of an atom)

Question 2

examples of good conductors

Answer 2

• copper
• gold
• silver
• aluminium
• iron
• graphite

Question 3

Define electrical current

Answer 3

The movement of electrons through a conductor

Question 4

What is an ampere?

Answer 4

(Amp)
unit used to describe electrical current.
1amp = number of electrons moving past a given point in 1 second.

Question 5

Define Current flow

Answer 5

The unidirectional flow of electrons through a conductor.

I

Question 6

Define Electrical Resistance

Answer 6

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

Question 7

Describe Ohm’s Law

Answer 7

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.

Question 8

Direct Current

Answer 8

• battery
• continuous
• same direction
• penetrates
• locate subsurface discontinuities
• may vary in value

Question 9

Alternating Current

Answer 9

• 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

Question 10

Half Wave Rectified Alternating Current

Answer 10

• reverse half cycle is blocked
• pulsating uni directional current
• maximum sensitivity to subsurface defects
• promotes high mobility particles

Question 11

Full Wave Rectified Alternating Current / Single Phase Full Wave Direct Current

Answer 11

• reverse half cycle turned around to flow in same direction as first half of the cycle
• pulsating dc
• not used often in MPI

Question 12

Three Phase Full Wave Rectified, Alternating Current

Answer 12

• three phase full wave DC
• little pulsation
• tests large equipment

Question 13

Three Phase Alternating Current

Answer 13

• 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