Resistance is opposition to the flow of current . Some materials such as glass and rubber offer great opposition to current flow. Other materials such as silver and copper offer little opposition to current flow.
The size and type of wires in an electrical circuit are chosen to keep the electrical resistance as low as possible. This allows the current to flow easily through the conductor. In an electric circuit, the larger the diameter of the wire, the lower the electrical resistance to current flow.
Temperature also affects the resistance of an electrical conductor. In most conductors (copper, aluminum, and so on), resistance increases with temperature. Carbon is an exception because the resistance decreases as temperature increases. Certain alloys of metals (Manganin and Constantan) have resistance that does not change with temperature.
The term in electricity that is the opposite of resistance is conductance (G). Conductance is the ability of a material to pass electrons. The unit of conductance is a mho, ohm spelled backwards. The symbol used to represent conductance is the inverted Greek letter omega. Conductance is the reciprocal of resistance and is measured in Siemens (S). A reciprocal is obtained by dividing the number into 1.
R = 1/G
G = 1/R
If the resistance of a material is known, dividing its value into 1 gives its conductance. Similarly, if the conductance is known, dividing its value into 1 gives its resistance.
Resistance is a property of all electrical components. Sometimes the effect of resistance is undesirable; other times it is constructive. Resistors are components manufactured to possess a specific value of resistance to the flow of current. A resistor is the most commonly used component in an electronic circuit.
Resistors are available with fixed or variable resistance values. They are available in a variety of shapes and sizes to meet specific circuit, space, and operating requirements .
Resistors are drawn schematically as a series of jagged lines. A resistor’s tolerance is the amount that the resistor may vary and still be acceptable. It is expensive for a manufacturer to hold a resistor to a certain value when an exact value is not needed.
Therefore, the larger the tolerance, the cheaper it is to manufacture. In most electronic circuits, resistors of 10% tolerance are satisfactory.
Resistors fall into three major categories, named for the material they are made of:
- molded carbon composition resistors
- wirewound resistors,
- carbon film resistors.
CONNECTING RESISTORS IN SERIES
CONNECTING RESISTORS IN PARALLEL