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GENERAL INFORMATION
PART 1
Component testing may require a specific resistance value
or a test for INFINITY or CONTINUITY. Infinity is an OPEN
condition between two electrical points, which would read as
no resistance, or OL (Open Line) on a VOM. Continuity is a
closed condition between two electrical points, which would
be indicated as very low resistance (000.000) or “ZERO” on
a VOM.
ELECTRICAL UNITS
Ampere
The rate of electron flow in a circuit is represented by the
AMPERE. The ampere is the number of electrons flowing past
a given point at a given time. One AMPERE is equal to just
slightly more than 6.241x10
18
electrons per second.
With alternating current (AC), the electrons flow first in one
direction, then reverse and move in the opposite direction.
They will repeat this cycle at regular intervals. A wave diagram,
called a “sine wave” shows that current goes from zero to
maximum positive value, then reverses and goes from zero
to maximum negative value. Two reversals of current flow
is called a cycle. The number of cycles per second is called
frequency and is usually stated in “Hertz”.
Volt
The VOLT is the unit used to measure electrical PRESSURE,
or the difference in electrical potential that causes electrons to
flow. Very few electrons will flow when voltage is weak. More
electrons will flow as voltage becomes stronger. VOLTAGE
may be considered to be a state of unbalance and current flow
as an attempt to regain balance. One volt is the amount of
Electromotive Force (EMF) that will cause a current of 1 ampere
to flow through 1 ohm of resistance.
-
+
AMPERE - Unit measuring rate of
current flow (number of electrons
past a given point)
OHM - Unit measuring resistance
or opposition to flow
VOLT - Unit measuring force or
difference in potential
causing current flow
Conductor of a
Circuit
Figure 6. Electrical Units
Ohm
The OHM is the unit of RESISTANCE. In every circuit there
is a natural resistance or opposition to the flow of electrons.
When an EMF is applied to a complete circuit, the electrons
are forced to flow in a single direction rather than their free or
orbiting pattern. The resistance of a conductor depends on
(a) its physical makeup, (b) its cross-sectional area, (c) its
length, and (d) its temperature. As the conductor’s temperature
increases, its resistance increases in direct proportion. One (1)
ohm of resistance will permit one (1) ampere of current to flow
when one (1) volt of EMF is applied.
OHM’S LAW
A definite and exact relationship exists between VOLTS, OHMS
and AMPERES. The value of one can be calculated when the
value of the other two are known. Ohm’s Law states that in
any circuit the current will increase when voltage increases but
resistance remains the same, and current will decrease when
resistance increases and voltage remains the same.
VOLTS
(E)
AMPS
(I)
OHMS
(R)
Figure 7. Ohm’s Law
If AMPERES is unknown while VOLTS and OHMS are known,
use the following formula:
AMPERES =
VOLTS
OHMS
If VOLTS is unknown while AMPERES and OHMS are known,
use the following formula:
VOLTS = AMPERES x OHMS
If OHMS is unknown but VOLTS and AMPERES are known, use
the following:
OHMS
=
VOLTS
AMPERES
Section 1.2
Measuring Electricity
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