1. DC Theory - Relationship between Voltage, Current and Resistance

c. Resistance.

The Resistance of a circuit is its ability to resist or prevent the flow of current (electron flow) through it making it necessary to apply a bigger voltage to the circuit to cause the current to flow again. Resistance is measured in Ohms, Greek symbol (Ω, Omega) with prefixes used to denote Kilo-ohms (kΩ = 10³Ω) and Mega-ohms (MΩ = 10⁶Ω). Resistance cannot be negative only positive.

The amount of resistance determines whether the circuit is a "good conductor" - low resistance, or a "bad conductor" - high resistance. Low resistance, for example 1Ω or less implies that the circuit is a good conductor made from materials such as copper, aluminium or carbon while a high resistance, 1MΩ or more implies the circuit is a bad conductor made from insulating materials such as glass, porcelain or plastic. A "semiconductor" on the other hand is a material whose resistance is half way between that of a good conductor and a good insulator such as silicon and germanium and is used to make Diodes and Transistors etc.

Resistance in a circuit prevents short circuits (unless its very low) by limiting and controlling the amount of current flowing in a circuit by the voltage supply connected to it and therefore the transfer of power from source to load. Resistance is not affected by frequency and the AC impedance of a pure resistance is equal to its DC resistance. Resistance also has the ability to change the characteristics of a circuit by the effect of load resistance or by temperature which changes its resistivity.

For very low values of resistance, for example milli-ohms, (mΩ´s) it is sometimes more easier to use the reciprocal of resistance (1/R) rather than resistance (R) itself. The reciprocal of resistance is called Conductance, symbol (G) and it is the ability of a conductor or device to conduct electricity with high values of conductance implying a good conductor and low values of conductance implying a bad conductor. The unit of conductance is the Siemen, symbol (S).

Again, using the water relationship, resistance is the diameter or the length of the pipe the water flows through. The smaller the diameter of the pipe the larger the resistance to the flow of water, and therefore the larger the resistance.

Next Let's move on to Ohms Law