Chapter 29 Types of materials Electric current Current density Resistance Resistivity and conductivity Ohm’s law and its applications
Types of materials Natural and artificially made materials show a wide range of electrical properties. These properties are determined partly by the behavior of individual atoms or molecules and partly by the interaction of atoms or molecules in the bulk materials. The ability of a material to conduct electricity may also depend on the conditions of the materials, such as its temperature and pressure. ďƒ˜
Conductors Conductors are materials that permit electrons to flow freely from particle to particle.
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An object made of a conducting material will permit charge to be transferred across the entire surface of the object. Metals such as copper, silver and gold typify conductors.
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In many metals, each atom gives up one or more of its outer or valence electrons to the entire material. These electrons are free to move when an electric field is applied to the material. Under static conditions the electric field in the interior of conductor is zero, even if the conductor carries a net charge.
Types of materials ďƒ˜
Insulators In an Insulator, the electrons are bound rather tightly to the atoms and are not free to move under the electric fields that might be applied under ordinary circumstances.
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Examples of insulators include plastics, paper, rubber, glass and dry air.
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An Insulating material can often be regarded as collection of molecules that are not easily ionized. In this case the electrical properties may depend on the electric dipole moment of molecules.
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Materials in which the molecules have permanent dipole moments are called polar and electric fields can align these dipole moments of molecules.
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In some materials, the alignment of dipoles remain even when we remove the applied field; these materials are called ferroelectric (in analogy with the ferromagnetic materials, in which magnetic dipole moments remained aligned even when the external applied magnetic field is removed)
Types of materials
Ordinary matter is usually electrically neutral. The application of an electric field can remove one or more electrons from atoms of materials. This process is called ionization.
And the resulting positively charged atoms with deficiency of electrons are called ions.
In an insulator, a sufficiently large electric field can ionize the atoms and as a result there are electrons available to move through the material. Under these circumstances an insulator can behave more like a conductor. This situation is called breakdown and requires fields typically in the range of 106 V/m in air to 107 V/m in plastics or ceramics.
Semiconductors
Intermediate between insulators and conductors are semiconductors. In a semiconductor, perhaps one atom in 1010 to 1012 might contribute an electron to the flow of electricity in the material (in contrast to a conductor, in which every atom typically contributes an electron to the flow of electricity).
Commonly used semiconductors include silicon and germanium, as well as many compounds.
Types of materials Superconductors Even the best conductors (copper, silver, and gold) show a small but nonzero resistance to the flow of electricity. Under certain conditions often involving cooling to very low temperatures, electric charge can flow through some materials with no resistance at all. This property of materials is called superconductivity, and materials under these conditions are called superconductors.
Electric current
Question Consider positive and negative charges moving horizontally through the four regions shown in figure below. Rank the current in these four regions from highest to lowest.
Microscopic model of Current & Current density
Resistance, Resistivity & Conductivity
Resistance, Resistivity & Conductivity
The resistivity (or conductivity) of a material is independent of the magnitude and direction of the applied electric field
Resistance, Resistivity & Conductivity
The resistance of an object is independent of the magnitude or sign of the applied potential difference
Ohm’s law Keep in mind that the relationship V = IR is not a statement of Ohm’s law. This equation defines the resistance and is true for both ohmic and non-ohmic objects. Even for non-ohmic devices, we can find a value of the resistance R for a particular value of V.
Figure: (a) a potential difference is applied across the terminals of the device, (b) a current-voltage plot for a material that obeys ohm’s law, (c) a current-voltage plot for a material that does not obey ohm’s law i.e. semiconductor
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