Resistance

Resistors  are one of the basic building blocks of electrical circuits.  Resistance occurs in  all materials, but resistors  are discrete components manufactured  to create an exact amount of intended resistance in a circuit.   Resistors are made of a mixture  of clay and carbon, so they are part conductor part insulator.   Because of this, they conduct electricity, but only with a set amount of resistance added.   The value of the resistance is carefully controlled.  Most resistors have four  color bands. The first band reveals the first digit of the value.  The second band reveals the second digit of the value.  The third band is used to multiply the value digits.  The fourth band  tells the tolerance of  the accuracy of the total value.  If no fourth band is present, it is assumed that the tolerance is plus or minus 20%. 

Resistance color code

Here are the digits represented by  the colored bands found on a resistor: 

Ohm’s law states this mathematical formula: Voltage is equal to resistance multiplied by the current flow, or  

                 E=IR. 

As with any algebraic formula, it is possible to rearrange the terms in order to solve the equation for a  specific unit of measurement. Two  algebraic equivalents of the formula would be:

                  

                 I=E/R   

                 R=E/I 

A very handy  magic triangle  is available that makes it easy to remember  the different permutations of  this formula.   

      

               E =IR

Cover the value to be determined with your finger,  and the relationship of  the other two are already in the proper form.   (Example: you need to know the amount of current flowing in  a circuit with 100Ω of resistance and 100 volts of pressure.   Cover I, the  symbol for current, and the remaining two symbols, E and R, appear in their correct relationship E/R.) 

         It's unit is ohm (Ω).

  

Resistance in series: 

                   A series of something  generally means connected along a line,  or  in  a row, or  in an order of some sort.  In electronics,  series resistance means that the resistors are connected one after the  other,  and that there is only  one path for current to flow through. 

Here is an example of  resistance in series: 

            R(T) = R¹+ R²+R³.   OHM'S

LAW OF SERIES CIRCUIT

1) Individual resistances add  up to  the total circuit resistance. 

2) Current through the circuit is the same at every point. 

3) Individual  voltages throughout  the circuit add up  to the total voltage. 

Resistance in parallel:

                         

There is another way to place more than one resistance into a circuit rather than in series.  Here  is a standard type of parallel circuit. 

            

In this example, each resistor has its own discrete path  to the voltage source, and if one of the pathways is opened, the other will still operate.  In a parallel circuit, the voltage in  each part of  the circuit remains constant, but the current varies in accordance with where a  reading is taken. This is the opposite of the way a series circuit operates.  

 There are many different ways to organize a parallel circuit.  In the  practical world, most wiring  is done in  parallel so that  the voltage to any one part of the network is the same as the voltage  supplied to  any other part of  it.   Having a  constant voltage is very important because electrical devices are designed to operate from a specific pressure.  It  would be  impractical to change that voltage at will throughout the electrical service. 

Although  the wiring  running between the lights is arranged differently, these  lamps have the same electrical connection as the lamps depicted in the previous schematic drawing.  No  matter how convoluted the wiring in a  lighting system may be, all of the circuits involved are  still in parallel, and all of the outlets have the same 120v service.

 LAWS OF PARALLEL CIRCUITS :

1) The reciprocals of all the  individual resistances add up to the reciprocal of the total circuit resistance.

            1/RT  = 1/R1  + 1/R2  + 1/R3  … 

2) Voltage  through  the  circuit  is the  same at every point.

 3)  Individual current  draws throughout the circuit add up to the total current  draw.  

                  IT  =.  I1 + I2  + I3......