|
|
|
The materials are classified as conductors or insulators. The conductive materials are those materials
that allow the flow of electrical current. In contrast, others. The opposition to the motion of electrons
that a material does, is called electrical resistance. That varies depending on the material used, so we
talk about specific resistance or resistivity (simbolo ρ). For its characteristics of
flexibility as well as conductivity and economy, the most common material for electrical conductors is copper.
Copper has a specific resistance of 0,0175 Ohm x mm2 / m. In other words, a copper wire
of a meter long and section of a mm2 has an electrical resistance of 0,0175 Ω at
its ends. The resistance of a conductor is directly proportional to the its length and inversely proportional
to the section. Thus, the mathematical formula for calculating the electrical resistance of a
conductor is: R = ρ * l / s.
To prevent energy losses by Joule effect we are try to maintain the value of conductors electrical resistance
within acceptable limits, sizing the cables in a manner to maintain a proper ratio in efficiency and cost.
In some applications, however, it is necessary to introduce proper electric resistance. The most common use
of resistors is to get a voltage drop, or to limit the intensity of the circulating electric current, or even
to obtain an electrical signal of variable amplitude (for example, the volume of a radio), etc. At this purpose,
resistors are manufactured in various making and shapes. Resistors are, by far, the most numerous passive
components in every electronic device, and their use can be arranged in series, parallel, or a combination of both.
In the electric scheme below, it's proposed a simple interactive circuit where you can enter values: volt for the
DC generator; ohm for the electrical resistance, remembering to use the point and not the comma for decimals as used
in some countries. Activating the circuit by the "on" button, you will see the same circuit with the value of the current
that would flow with the data entered (the resistance of conductors are ignored). To exercise, your task is to
calculate the value of the current, according to Ohm's law as stated in the main page of the section, and compare
the result with that shown in the example.
Series arrangement:
In the series arrangment of two or more resistors, the total resistance, or equivalent resistance, is the sum of single resistor value.
| R1 | | R2 | | | | Rn |
 |
 |
|
|
 |
|
|
Rt= R1 + R2 + Rn |
|
|
|
Parallel arrangement:
In the parallel arrangment, the general rule tell us that the equivalent resistance is the reciprocal of the reciprocal sum of single resistor value.
In some particuilar cases the equivalent resistance is obtained in a simple manner. In the case of two equal resistor used in parallel manner, for an
example, the equivalent resistance is half the value of a single resistor value. Another way to calculate the equivalent resistance of two resistors
arranged in parallel is to divide the product of the two resistor value by their sum.
 |
 | |
| |
 | |
| R1 |  | R2 |
| Rn |
|
1
Re= ----------------------
1/R1 + 1/R2 + 1/Rn |
|
|
 | |
| |
 | |
|
In the interactive electric scheme below, you can automatically calculate the current flowing in it. Simply insert, in the corresponding field,
the voltage [V] and resistors value [R1 e R2], and turn the switch in on position by clicking the "on" button.
|
|
|
Series-Parallel arrangement:
In a combination of resistors arranged in series and parallel, calculating the equivalent resistance is more difficult, or best, more elaborate.
You should proceed, like an equation, simplifying the terms untill you reach a series or parallel arrangment equivalent circuit.
The circuit below shows an hypothetical resistors net arranged in a series-parallel combination.
The simplifying proceeding to follow is shown step by step.
| | | | | | | |
|
|
|  |
|
| |
| R1 | | |
|
R2 * (R3+R4)
Rx=----------------
R2 + (R3+R4) | |
|
|
| |
|
| | |
| | R5 | | | | | |
|
|
|
|
|
|
| 
