Notes: Resistance

Hole Flow Applet: http://mste.illinois.edu/murphy/HoleFlow/HoleFlow.html

Some Definitions

n Resistance (R): The opposition of a material to the flow of electrical current.

       Unit of resistance is the ohm 

        Symbol used to represent resistance is W

 

n Conductance (G): Term that is opposite of resistance. It is the ability of a material to conduct or allow the flow of electrons.

       Unit of conductance is Mho (ohm spelled backward) 

        Symbol used to represent conductance is  (W inverted)

n Resistor: A component manufactured to possess a specific value of resistance to the flow of current. (shown on the left below) 

n Potentiometer: A variable resistor used ot control voltage. (shown in the middle below)

n Rheostat: A variable resistor used to control current. (shown on the right below)

Resistor Configurations

n Resistors in Series Configuration        Finding the total resistance, RT       RT = R1 + R2 + R3 + ... + RN       Notes:        ... means continue with the stated process        The subscript N means it is the last in the         process.

 

n Resistors in Parallel (The circuits on the right are equivalent)        Finding the total resistance RT       1/RT = 1/R1 + 1/R2 + 1/R3 + ... + 1/RN

 

n Resistors in Series and Parallel Combined Configuration        Steps for finding the total resistance.      1.  Find the equivalent resistance for the parallel portion of the circuit           If there are series components within the parallel portion, determine the           equivalent reistance of the series component first.       2.  After the equivalent resistance is determined, redraw the circuit.       3.  Substitute the equivalent resistances for the parallel portion of the circuit.       4.  Perform final calculations

Color Coding for Resistor Values

The electronic color code discussed here is used to indicate the values or ratings of electronic components, very commonly for resistors, but also for capacitors, inductors, and others. A separate code, the 25-pair color code, is used to identify wires in some telecommunications cables.

The electronic color code was developed in the early 1920s by the Radio Manufacturer's Association.

Colorbands were commonly used (especially on resistors) because they were easily printed on tiny components, decreasing construction costs. However, there were drawbacks, especially for color blind people. Overheating of a component, or dirt accumulation, may make it impossible to distinguish brown from red from orange. Advances in printing technology have made printed numbers practical for small components, which are often found in modern electronics.