Section 2
MEASURING ELECTRICITY
Component testing may require a specific resis- tance value or a test for INFINITY or CONTINUITY. Infinity is an OPEN condition between two electrical points, which would read as no resistance on a VOM. Continuity is a CLOSED condition between two elec- trical points, which would be indicated as very low resistance 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 elec- trons flowing past a given point at a given time. One AMPERE is equal to just slightly more than six thou- sand million billion electrons per second (6.25 x 1018).
With alternating current (AC), the electrons flow first in one direction, then reverse and move in the oppo- site 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 consid- ered to be a state of unbalance and current flow as an attempt to regain balance. One volt is the amount of EMF that will cause a current of 1 ampere to flow through 1 ohm of resistance.
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
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 OHMS
(I)(R)
Figure 2-6. – Ohm's Law
Conductor of a
Circuit
- 
oHM - Unit measuring resistance or opposition to+flow
aMpeRe - Unit measuring rate of
current flow (number of electrons past a given point)
volt - Unit measuring force or difference in potential causing current flow
If AMPERES is unknown while VOLTS and OHMS are known, use the following formula:
AMPERES = VOLTSOHMS
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 | ||
|
Figure 2-5. – Electrical Units
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