Resistors



Electronics is basically the manipulation of 3 things, namely:
(1)Voltage
(2)Current
(3)Resistance

VOLTAGE:
The voltage between two points is a short name for the electrical force that would drive an electric current between those points. Specifically, voltage is equal to energy per unit charge. In the case of static electric fields, the voltage between two points is equal to the electrical potential difference between those points.
Voltage can be measured using a VOLTMETER.Its SI unit is VOLT(V).
It is like the pressure availaible in a hose pipe.
CURRENT:
Electric current means, depending on the context, a flow of electric charge (a phenomenon) or  the rate of flow of electric charge (a quantity).This flowing electric charge is typically carried by moving electrons, in a conductor such as wire; in an electrolyte, it is instead carried by ions, and, in a plasma, by both.
Current can be measured using an AMMETER.Its SI unit is AMPERE(A).
It is like the amount of water flowing out of a hose pipe.
For a steady flow of charge through a surface, the current I in amperes can be calculated with the following equation:
where Q is the electric charge transferred through the surface over some time t. If Q and t are measured in coulombs and seconds respectively, I is in amperes.
More generally, electric current can be represented as the rate at which charge flows through a given surface as:

RESISTANCE:
The electrical resistance of an object is a measure of its opposition to the passage of an electric current. An object of uniform cross section will have a resistance proportional to its resistivity and length and inversely proportional to its cross-sectional area.
The resistance of an object can be defined as the ratio of voltage to current:
For a wide variety of materials and conditions, the electrical resistance R is constant for a given temperature; it does not depend on the amount of current through or the potential difference (voltage) across the object. Such materials are called Ohmic materials. For objects, such as resistors, made of ohmic materials the definition of the resistance, with R being a constant for that resistor, is known as Ohm's law.
In the case of a nonlinear conductor (not obeying Ohm's law), this ratio can change as current or voltage changes; the inverse slope of a chord to an I–V curve is sometimes referred to as a "chordal resistance" or "static resistance".
POWER:
Electric power is the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the WATT(W).


Click here for animation of water analogy



Direct current
In direct current resistive circuits, electrical power is calculated using Joule's law:
where P is the electric power, V the potential difference, and I the electric current.
In the case of resistive (Ohmic, or linear) loads, Joule's law can be combined with Ohm's law (I = V/R) to produce alternative expressions for the dissipated power:
where R is the electrical resistance.

Alternating current

Using the Pythagorean Theorem, the relationship among real, reactive and apparent power is:
(apparent power)2 = (real power)2 + (reactive power)2
Real and reactive powers can also be calculated directly from the apparent power, when the current and voltage are both sinusoids with a known phase angle between them:
(real power) = (apparent power)cos(θ)
(reactive power) = (apparent power)sin(θ)
The ratio of real power to apparent power is called power factor and is a number always between 0 and 1.

Back to resistance….

DC resistance

The resistance of a given resistor or conductor grows with the length of conductor and decreases for larger cross-sectional area. The resistance R of a conductor of uniform cross section, therefore, can be computed as

AC resistance

If a wire conducts high-frequency alternating current, then the effective cross sectional area of the wire is reduced because of the skin effect. If several conductors are together, then due to proximity effect, the effective resistance of each is higher than if that conductor were alone. These effects are so small for low frequency of ordinary household AC that they should ordinarily be treated as if it were DC resistance.

Resistance can be measured using an OHMMETER.The SI unit of resistance is OHM(Ω).



THE VALUE OF A CARBON COMPOSITION RESISTOR CAN BE CALCULOATED BY USING A COLOUR CODE:

 

RESISTOR FORMULAE:
Resistors in a parallel configuration each have the same potential difference (voltage). To find their total equivalent resistance (Req):
The parallel property can be represented in equations by two vertical lines "||" (as in geometry) to simplify equations.
The current through resistors in series stays the same, but the voltage across each resistor can be different. The sum of the potential differences (voltage) is equal to the total voltage. To find their total resistance:
A resistor network that is a combination of parallel and series can be broken up into smaller parts that are either one or the other. For instance,

The power dissipated by a resistor (or the equivalent resistance of a resistor network) is calculated using the following:


If the average power dissipated is more than the resistor can safely dissipate, the resistor may depart from its nominal resistance and may become damaged by overheating. Excessive power dissipation may raise the temperature of the resistor to a point where it burns out, which could cause a fire in adjacent components and materials. There are flameproof resistors that fail (open circuit) before they overheat dangerously.
There are two resistor body colors which you should know what they are if you are thinking of repairing electronic circuits. These body colors are white, and blue (and sometimes composite green depending on where you live) and are used to indicate non-flammable and/or fusible resistor types. It is important to know NOT to replace these with ordinairy type resistors. The non-flamable types are there for a reason (they don't burn when overheated) and just replacing it with a normal type resistor may create a fire-hazard or worse. The fusible types are usually white with one black band in the middle of the body. So if you ever are looking for the 'fuses' check these out. They are less than 0.1 ohm, carbon.
One can use this sentence to remeberr the colour sequence:
"Bad Beer Rips Our Young Guts But Vodka Gives Well"….referring to Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Gray, and White….
Can you 'create' your own resistors? Sure thing, and not difficult. Here is how to do it: Draw a line on a piece of paper with a soft pencil, HB or 2HB will do fine. Make the line thick and about 2 inches (5cm) long. With your multimeter, measure the ohm's value of this line by putting a probe on each side of the line, make sure the probes are touching the carbon from the pencil. The value would probably be around the 800K to 1.5M depending on your thickness of the line and what type of pencil lead is used. If you double the line the resistance will drop considerably, if you erase some of it (length-wise obviously!) the resistance will increase. You can also use carbon with silicon glue and when it dries measure the resistance, or gypsum with carbon mixed, etc. The reason for mentioning these homebrew resistors is that this method was used in World War II to fix equipment when no spare parts were available.