If water is moving through a hose, we say that it has FLOW. If we restrict the flow, by pinching the hose, we are causing friction at the point of restriction. This friction can be said, is resistance to the flow of the water.
Electricity, according to Benjamin Franklin, acts like a fluid. It flows and has a measurable CURRENT. We can restrict its flow by adding electrical friction. We say that the restriction of electrical flow is called RESISTANCE and that a device which causes such RESISTANCE is called a RESISTOR. All materials, even the very best CONDUCTORS demonstrate a certain amount of RESISTANCE electron flow. In order to compare the resistance of various materials, we need to have some standard unit of measurement. The unit of measurement for resistance is called the Ohm, and is indicated by the Greek letter Omega ( W).
One W is defined as the amount of resistance that a 1000 foot piece of #10 copper wire has. A 3000 foot piece of #10 copper wire would have 3 Ohms of resistance. A 500 foot piece of #10 copper wire would exhibit 1/2 an Ohm, etc.
Although Ohm is the basic unit, KiloOhm and MegOhm are frequently used. 1 KiloOhm (KW) is equal to 1 thousand W. 1 MegOhm (MW) is equal to 1 million W.
There are 4 factors that determine the resistance of a material:
(1) Type of Material
The resistance of various types of materials are different. For instance, gold is a better conductor of electricity than copper, and therefore has less resistance.
The resistance of a material is directly proportional to it's length. The longer the material is, the more resistance it has. This is because the electrons must flow through more material, and therefore meets more friction over the entire distance.
(3) Cross Sectional Area
The resistance of a material is inversely proportional to the cross sectional area of the material. This means that the thicker the substance is across, the lower the resistance. This is because the larger the cross sectional area is, the less friction there is over a given length.
In various types of materials, resistance can vary inversely or directly with the temperature. This is because of the chemical properties of the material. In Carbon, for instance, the resistance decreases as the temperature rises. So we say it varies inversely. In copper, however, the opposite is true, with the rise in temperature, we have a rise in the resistance.
Resistance then, is basically a form of friction which restricts the flow of an electrical current. In basic science class, you learned that by putting your hands together, and rubbing them quickly, your hands get warm. This is because friction generates heat.
Electrical friction - RESISTANCE - also generates heat. So not only can resistance change with heat, but causes heat as well. An important point to remember when working with resistors, especially in high power circuits.