Using
Ohm's Law and
the System Internationale (SI)
Metric System,
electrical measuring units can be derived.
System Internationale (SI) Metric
System
Electrical
units of measurement
are based on
the International (metric)
System, also known
as the SI System.
Units of electrical measurement
include the following:
Ampere
Appendix
A provides more
information concerning the
metric system,
metric prefixes, and powers of 10 that are used in electrical measuring
units.
Voltage
Voltage,
electromotive force (emf), or potential difference, is described as
the
pressure or force that causes electrons to move in a conductor. In
electrical formulas and equations, you
will see voltage symbolized with a
capital E, while on
laboratory equipment or schematic diagrams, the voltage
is often represented with a capital V.
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Current
Electron
current, or amperage,
is described as
the movement of
free
electrons through a conductor. In
electrical formulas, current is symbolized
with a capital I, while in the
laboratoryor on schematic diagrams, it is common
to use a
capital A to indicate amps or amperage (amps).
Resistance
Now that we have discussed the concepts of voltage
and current, we are ready
to discuss a third key
concept called resistance. Resistance is
defined as
the
opposition to current
flow. The amount
of opposition to
current
flow produced
by a material
depends upon the
amount of available
free
electrons it
contains and the
types of obstacles
the electrons encounter
as they attempt to move through the material. Resistance is measured in ohms
and is
represented by the symbol (R) in
equations. One ohm
is defined as
that amount of
resistance that will
limit the current
in a conductor
to one
ampere
when the potential
difference (voltage) applied
to the conductor is
one volt. The shorthand notation for ohm is the
Greek letter capital omega (W).
If a voltage is applied to a conductor, current flows. The amount of current flow
depends upon the resistance
of the conductor.
The lower the
resistance, the
higher
the current flow
for a given amount of voltage.
The higher the resistance,
the lower the current flow.
Ohm's Law
In 1827, George Simon Ohm discovered that there was
a definite relationship between
voltage, current, and resistance in an electrical circuit. Ohm's Law defines this
relationship and
can be stated in three ways.
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1. Applied voltage
equals circuit current
times the circuit
resistance. Equation
(1-2)
is a mathematical respresentation of this concept.
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E = I x R or
E = IR
2. Current is equal to the applied voltage divided by
the circuit resistance. Equation
(1-3) is a mathematical representation of this
concept.
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3. Resistance of
a circuit is
equal to the
applied voltage divided
by the circuit
current.
Equation (1-4) is a mathematical representation of
this concept.
R=E/I
where
I = current (A)
E = voltage (V)
R = resistance (W)
If any two of the component values are known, the
third can be calculated.
Conductance
The word "reciprocal" is sometimes used to mean "the
opposite of." The opposite,
or
reciprocal, of resistance is called conductance. As described above, resistance
is the
opposition to current flow. Since resistance and
conductance are opposites,
conductance can be defined as the ability to conduct current. For
example, if a wire
has a high conductance, it will have low resistance, and vice-versa. Conductance
is
found by taking the reciprocal of the resistance. The unit used to specify conductance
is called "mho," which is ohm
spelled backwards. The symbol for "mho" is the Greek
letter omega inverted (
). The symbol for conductance
when used in a formula is
G. Equation (1-5)
is the mathematical representation of
conductance obtained
by
relating the definition of conductance (1/R) to Ohm's Law, Equation (1-4).
G=1/R
Power
Electricity is generally used to do some sort of
work, such as turning a motor or
generating heat. Specifically, power is the rate at which work
is done, or the rate
at which heat is generated. The unit
commonly used to
specify electric power
is the watt.
In equations, you
will find power abbreviated with the
capital letter P, and watts,
the units of measure for power, are abbreviated with the
capital
letter W. Power is
also described as
the current (I) in
a circuit times
the voltage (E) across the circuit. Equation (1-6) is a mathematical
representation
of this concept.
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Using Ohm's Law for the value of
voltage (E),
E = I x R
and using substitution laws,
P = I x ( I x R)
power
can be described
as the current
(I) in a
circuit squared times
the
resistance (R) of the circuit. Equation
(1-7) is the mathematical representation
of this concept.
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