BASIC ELECTRICAL THEORY 3

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UNITS OF ELECTRICAL MEASUREMENT

 

System International (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:

 

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.

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 laboratory
or 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 (?). 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.

1. Applied voltage equals circuit current times the circuit resistance. Equation (1-2) is a
mathematical representation of this concept.

E = I x R or E = IR                                                                                                                                (1-2)

2. Current is equal to the applied voltage divided by the circuit resistance. Equation
(1-3) is a mathematical representation of this concept.

I= E / R                                                                                                                                                   (1-3)

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 (or ? )= E/I

(1-4)
where

I = current (A)
E = voltage (V)
R = resistance (?)

If any two of the component values are known, the third can be calculated.
Example 1: Given that I = 2 A, E = 12 V, find the circuit resistance.
Solution:
Since applied voltage and circuit current are known, use Ohm’s Law to solve for
resistance.

ohms law

Example 2:   Given E = 260 V and R = 240? , what current will flow through a circuit?
Solution:
Since applied voltage and resistance are known, use Ohm’s Law to solve for
current.

ohms law2

Example 3:   Find the applied voltage, when given circuit resistance of 100?  and circuit current
of 0.5 amps.
Solution:
Since circuit resistance and circuit current are known, use Ohm’s Law to solve for
applied voltage.

E = IR

E = (0.5 A)(100 ?) = 50 V

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).

conductance

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.

P = I x E or P = IE                                                                                                                                                                                     (1-6)

 

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.

inductance

 

inductance2

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