The Resistor

When there is current in a material then the free electrons move through the material and while also coliding occasionally with atoms. Whenever such a collision happens then the electron will lose some of its energy in the process and slowing down the electron. The more collisions there are with atoms thre more the flow of current is resrticted. 
This property of a material that restricts the flow of elctron movement is called resistance, usually designated by 'R'. 

Resistance therefore is the opposition to current.

Whenever current flow is occurring through a material that has resistance, heat is produced by the collisions. A wire with a small resitance will become warm if a sufficient amount of current is applied.

Ohm: The Unit of Resistance

Resistance, R, is expressed in units of ohms. 
One ohm of restiance exists whenere there is one Ampere of current in the material with one Volt applied accross the material.

R=V/I

Conductance

The reciprocal of resistance if conductince, symbolised by G and measured in Siemens, symbolised by S.

G= 1/R

Resistors

Some components are specifically designed to have a certain amount of resistance; these devices are called Resistors.
The main use of a Resistor is to limit 

1. Limit current
2. Divide voltage
3. Generate heat

All resistors can be classified as either a Fixed Resistor or a Variable Resistor.

Fixed Resistors

Fixed resistors come in a large range of values that are determined by the manufacturer and cannot be easily changed to different value of resistance.

Fixed resistors can come in many forms and shapes and, different materials. The most commonly used resistor is the Carbon-type resistor. A mixture of insulating filler is added to the Carbon material to determine the resistance value of a fixed carbon resistor.

In terms of shape and form of Carbon resistors, there are a few standard types

• Carbon-composition rod
• Metal film chips
• Chip resistor array
• Resistor network 
• Radial-lead for PC board insertion

Resistor Color Codes

All resistors have a certain level of tolerance (inaccuracy) that a manufacturer will specifify; standard resistors will have either a 5% or a 10% tolerance rating, which is indicated directly on the resistor by means of using color bands. Gold bands indicates a 5% tolerance rate while Silver indicates 10% tolerance can be expected.
All resistors also their reistance value indicated as group of color bands on the resistor itself. 

How to read Resistor Color Codes:

If there are 4 less color bands on the resistor:

1. Start at the oppsite side of the resistor where the band is neither Silver nor Gold.
2. The first band's color is the 1stdigit of the resistor.
3. The second band's color is the second digit of the reststor
4. The third band's color indicates the number of zeros following the 2nd digit.
5. The fourth band's color indicates the tolerance level of the resistor. If there is no 4th band then it can be assumed that the resistor has tolerance rating of 20% or worse.

If there are 5 color bands on the resistor:

1. Start at the oppsite side of the resistor where the band is neither Silver nor Gold.
2. The first band's color is the 1st digit of the resistor.
3. The second band's color is the 2nd digit of the reststor
4. The third band's color indicates the 3rd digit.
5. The fourth band's color indicates the number of zeros following the 3rd digit.
6. The fifth band's color indicates the tolerance level of the resistor.

The color-to-digit relationship for a resistor is indicated here:

• BLACK: 0
• BROWN: 1
• RED: 2
• ORANGE: 3
• YELLOW: 4
• GREE: 5
• BLUE: 6
• VELVET: 7
• GREY: 8
• WHITE: 9

• GOLD: 5% tolerance
• SILVER: 10% tolerance
• No 4th band/color: 20%

Variable Resistors

Variable resistors are designed to have their resistance values be changable either manually or automatically.