Explain Resistor 

Electricity flows through a material carried by electrons, tiny charged particles inside atoms. Broadly speaking, materials that conduct electricity well are ones that allow electrons to flow freely through them. In metals, for example, the atoms are locked into a solid, crystalline structure (a bit like a metal climbing frame in a playground). Although most of the electrons inside these atoms are fixed in place, some can swarm through the structure carrying electricity with them. That’s why metals are good conductors: a metal puts up relatively little resistance to electrons flowing through it. Plastics are entirely different. Although often solid, they don’t have the same crystalline structure. Their molecules (which are typically very long, repetitive chains called polymers) are bonded together in such a way that the electrons inside the atoms are fully occupied. There are, in short, no free electrons that can move about in plastics to carry an electric current. Plastics are good insulators.

Illustrating a resistor with an old bulb:

In an old-style light bulb, for example, electricity is made to flow through an extremely thin piece of wire called a filament.The wire is so thin that the electricity really has to fight to get through it. That makes the wire extremely hot—so much  that it gives off light. Without resistance, light bulbs like this wouldn’t function. Of course the drawback is that we have to waste a huge amount of energy heating up the filament. Old-style light bulbs like this make light by making heat and that’s why they’re called incandescent lamps, newer energy-efficient light bulbs make light without making much heat through the entirely different process of fluorescence.
The heat that filaments make isn’t always wasted energy. In appliances like electric kettles,  and toasters, electric iron there are bigger and more durable versions of filaments called heating elements. When an electric current flows through them, they get hot enough to do the specific job they are been designed for.
Resistance is also useful in things like transistor radios and TV sets. It helps  in turning the volume nub .  The  knob is actually part of an electronic component called variable resistor. If you turn the volume down, you’re actually turning up the resistance in an electrical circuit that drives the TV’s loudspeaker. When you turn up the resistance, the electric current flowing through the circuit is reduced. With less current, there’s less energy to power the loudspeaker—so it sounds much quieter.

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If you break one open, and scratch off the outer coating of insulating paint, you might see  ceramic rod running through the middle with copper wire wrapped around the outside. A resistor like this is described as wire-wound. The number of copper turns controls the resistance very precisely: the more copper turns, and the thinner it becomes, the higher the resistance. In smaller-value resistors, designed for lower-power circuits, the copper winding is replaced by a spiral pattern of carbon. Resistors like this are much cheaper to make and are called carbon-film. Generally, wire-wound resistors are more precise and more stable at higher operating temperatures.

Suppose you’re trying to force water through a pipe. Different sorts of pipes will be more or less obliging, so a bigger one will resist the water less than a thinner one and a shorter pipe will offer less resistance than a longer one. If you fill the pipe with, say, pebbles or sponge, water will still trickle through it but much more slowly. In other words, the length, cross-sectional area (the area you see looking into the pipe to see what’s inside), and things inside the pipe all affect its resistance to water.
Electrical resistors are very similar—affected by the same three factors. If you make a wire thinner or longer, it’s harder for electrons to wiggle through it. And, as we’ve already seen, it’s harder for electricity to flow through some materials (insulators) than others (conductors).
the resistance  of a material increases as its length increases (so longer wires offer more resistance) and increases as its area decreases (thinner wires offer more resistance). The resistance is also related to the type of material from which a resistor is made.

Resistor Colour Codes

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1.   The final band is called the tolerance and it tells you how accurate the resistance value you’ve just figured out is likely to be. If you have a final band colored gold, it means the resistance is accurate to within plus or minus 5 percent. So while the officially stated resistance is 1000 ohms, in practice, the real resistance is likely to be anywhere between 950 and 1050 ohms.
2.   If there are five bands instead of four, the first three bands give the value of the resistance, the fourth band is the decimal multiplier, and the final band is the tolerance. Five-band resistors quoted with three digits and a multiplier, like this, are necessarily more accurate than four-band resistors, so they have a lower tolerance value.
3.   On most resistors, you’ll see there are three rainbow-colored bands, then a space, then a fourth band colored brown, red, gold, or silver.
4.   Turn the resistor so the three rainbow bands are on the left.
5.   The first two of the rainbow bands tell you the first two digits of the resistance.
6.    Calculating Resistor Values. For example, a resistor has the following coloured markings; Yellow Violet Red = 4 7 2 = 4 7 x 102 = 4700Ω or 4k7 Ohm. The fourth and fifth bands are used to determine the percentage tolerance of the resistor.

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