Physics - Electricity

 

Electrical resistance

Resistance enters Ohm's Law: U=RI, so that it is a very important topic. Here it is discussed in more detail.

Some materials let the electrons drift more easily than others. The more difficult for the electrons to move, the higher the resistance of the material. If a material has a resistance that is too high, so that electrons cannot move at all, it is called an insulating material. Material that let the electrons move are called conducting materials.

We refer to resistance when we describe electrical components, like light bulbs, resistors, etc. When we describe materials, we refer to another (related) property called resistivity. The lower the resistivity (ρ) of a material, the easier electrons can move, the higher the conductivity (σ), the better is the material for making electrical wires:

σ = 1 / ρ

For some electric applications , materials with high resistivity are also needed. The table below lists the resistivity of some materials:

Material Resistivity (ohm m)
Glass 1012
Mica 9 x 1013
Quartz (molten) 5 x 1016
Copper 1.7 x 10-8

The table shows clearly that glass, mica and molten quartz are insulating materials whereas copper is a conducting material, with a very low resistance. That is why wires are typically made of copper. Silver and gold have even lower electrical resistances but are not used in wiring because of their high cost.

Insulating materials like mica are used in the electronics industry , in the manufacture of capacitors. They are also required in high tech applications including lasers and electronic microscopy.

::The relationship between resistance and resisitivity::

 

The resistance of a piece of wire depends on the material (ρ) .It is also intuitive to think that the longer the wire, the higher the resistance. What about the size of cross section of the wire? The larger it is, the more room for the electrons to move, so that it is inversely proportional to the resistance. the formula below summarizes these ideas:

R = ρ L / A

 

::QUIZ::

1) A current I1 travels through a copper wire submitted to a volatge V1. This wire is substituted by another that has the same length but whose diameter is twice as large. What alternative defines the current I2 that flows through the new wire ?

a) I1=I2

b) 2 * I1= I2

c) I1 = 2 * I2

d) 3 * I1= I2

e) 4 * I1= I2

2) An electric conductor has lenght L, diameter d and resistance R. If we double its length and also its diameter, its resistance will be:

a) R

b) 2R

c) 4R

d)R/2

e) R/4


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