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a) b - École Polytechnique de Montréal

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magnetization, which is a function of a parameter of the system, such as the temperature. A<br />

system’s critical point is the value of the parameter at which the system has a phase transition.<br />

For a given parameter, such as (φ ), with a critical value of ( φ ), the or<strong>de</strong>r parameter (a) is<br />

c<br />

approximated by the following equation:<br />

Equation 2-41. ( ) t<br />

a a φ −φ<br />

= 0<br />

The exponent t is a critical exponent of the system and its value is <strong>de</strong>pen<strong>de</strong>nt on the dimensions<br />

of the lattice(Grimmett, 1999). For a monodispersed distribution of spheres, a value of 0.156 is<br />

predicted for onset of the percolation threshold by percolation theory, which has been verified by<br />

several works.<br />

For example, in the case of an electrically conductive polymer with filler, the effect of filler<br />

loading on the composite resistivity follows a nearly universal pattern regardless of which fillers<br />

are chosen.<br />

2.3 Conductive Materials<br />

In mo<strong>de</strong>rn society, a life without electronics cannot be imagined. All materials un<strong>de</strong>r normal<br />

conditions have some resistance to flowing charges. The electrical resistance of a material is<br />

<strong>de</strong>fined by Ohm’s law as the ratio of the voltage (V) applied to the electric current (I) which<br />

flows through it. A resistance formula is:<br />

<br />

<br />

There are three main groups of materials from an electrical point of view: conductors,<br />

semiconductors, and insulators. Materials containing movable charges of electricity are called<br />

conductors. These mobile charges are forced to move when an electric potential difference is<br />

applied across separate points on a conductor. The measure of the ability of a conductor to carry<br />

electric current is conductivity, which is <strong>de</strong>fined as the ratio of the amount of charges passing<br />

through a unit area of the conductor per second, divi<strong>de</strong>d by the electric field intensity.<br />

c<br />

43

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