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Electrical Resistance

If an electrical current flows through a conductor, the free charge carriers (free electrons) move between the atoms of the atomic lattice. In the process collisions frequently occur, deflecting the free electrons and retarding their motion. The conductor thus exerts a resistance on the electric current that must be overcome by the voltage. The following illustration demonstrates how the electrons move through the conductor.

electrical-resistance.jpg

The animation below provides an additional illustration of this process.

electrical-resistance-animation.gif

 

A conductor with a low resistance is better at conducting electric current, whereas a conductor with higher resistance is not so good at conducting current. The unit of electrical resistance is named after the German physicist Georg Simon Ohm. The following statements all hold true:

The electrical resistance (unit sign R) has the unit ohm (unit symbol W).

The reciprocal of resistance is the so-called conductance:

Electrical conductance (denoted by G) has the unit Siemens (unit symbol S).

Thus the following is true:

electrical-resistance1.jpg

The unit of conductance originates with the German engineer Werner von Siemens. A material with low resistance has a high conductance and vice versa. Non-conductors or insulators have an extremely high resistance.

Each conductor and every load has electrical resistance. As a rule the resistance in connecting conductors is unwanted. The resistance of a conductor depends material it is made from, its length l and its cross-section (area A). The following equation applies:

electrical-resistance2.jpg

 

The material constant r specifies the so-called specific resistance of the conductor material and has the unit W mm2/m, l is the conductor's length in metres and A the conductor's cross-section in square millimetres. The specific resistance of silver, for example, amounts to 0.0167 W ·mm2/m, that of copper 0.0178 W mm2/m.

The following qualitative conclusions can be drawn from the aforementioned equation:

The resistance of a conductor is greater when the conductor's specific resistance is higher. It also increases in propoportion to the conductor's length and in inverse proportion to the conductor's cross-section.

Furthermore, the resistance of a conductor is also temperature-dependent. We will be considering this factor in more detail later on.

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