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Electrical Engineering Materials
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Rutherford Atomic Model
Bohrs Atomic Model
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Energy Bands in Crystals
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Selection of Material for Engineering Application
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Thomson Plum Pudding Model (1911)
How a Conductor Conducts Current?The substance of the electrical conductor atom must have no energy gap between its valence band and conduction band. The outer electrons in the valence band are loosely attached to the atom. When an electron gets excited due to electromotive force or thermal effect, it moves from its valence band to conduction band. Conduction band is the band where this electron gets its freedom to move anywhere in the conductor. The conductor is formed of atoms. Thus as a whole, the conduction band is in abundance of electrons. In other word, it can be said that the metallic bonds are present in the conductors. These metallic bonds are based on structure of positive metal ions. These structures are surrounded by electron cloud.
When a potential difference occurs in the conductor across two points, the electrons get sufficient energy to flow from lower potency to higher potency in this conduction band against a small resistance offered by this conductor material. Electricity or current flows in the opposite direction of the flow of the electrons.
How an Electron Flows through a Conductor?Electrons do not move or flow in a straight line. In a conductor, the electrons are in to and fro motion or random velocity i.e. is called Drift Velocity (Vd) or average velocity. Due to this Drift Velocity, the electrons get collisions every moment with atoms or another electron in the conduction band of the conductor. Drift velocity is quite small, as there are so many free electrons. We can estimate the density of free electrons in a conductor, thus we can calculate the drift velocity for a given current. The larger the density, the lower the velocity required for a given current. In the Conductor, flow of the electrons is against the Electric Field (E).
Properties of ConductorThe main properties that should be with a conductor are as follows:
- A conductor always allows free movement of electrons or ions.
- The electric field inside a conductor must be zero to permit the electrons or ions to move through the conductor.
- Charge density inside a conductor is zero i.e. the positive and negative charges cancel inside a conductor.
- As no charge inside the conductor, only free charges can exist only on the surface of a conductor.
- The electric field is perpendicular to the surface of that conductor.
Type of the ConductorsGenerally conductors can be classified based on Ohmic Response. They are
Ohmic ConductorsThis type of conductors always follow Ohm’s Law (V ∝ I) V vs. I graph gives a straight line always. Example Aluminum, Silver, Copper etc.
Non Ohmic ConductorsThis type of conductors never follow the Ohm’s Law (V ∝ I) V vs. I graph does not give a straight line i.e. non linear graph. Example LDR (Light Dependant Resistor), Diode, Filament of Bulb, Thermistor etc. The examples of conductors are given below
- Metallic Conductor: Silver, Copper, Aluminum, Gold etc.
- Non Metallic Conductor: Graphite
- Alloy Conductor: Brass, Bronze etc.
- Metallic Conductor: Mercury
- Non Metallic Conductor: Saline Water, Acid Solution etc.
- Copper Conductor is the most common material used for electrical wiring.
- Gold Conductor is used for high-quality surface-to-surface contacts.
- Silver is the best conductor in the Conductors list.
- Impure Water is listed in Conductor List but it has less conductivity.
What is the Charge of a Conductor During Carrying Electricity?A current carrying conductor at any instance is with zero charge. It is because of at any instance number of electrons (at drift velocity) is equal to the number of protons in this conductor. So the net charge is zero. Suppose a conductor is connected across a battery, i.e. positive end and negative end are connected with a conductor. Now electrons flow through the conductor from negative end to positive end of the battery. This flow of electrons is possible until this battery has EMF producing capability through chemical reaction inside.
At that Situation, is Conductor Positively or Negatively Charged?Just think that here the conductor is the media through which charges can be passed from one electrode to another electrode of the battery. The electrons get rid of negative side of the battery and enter the conduction band of the conductor where already plenty of valence electrons of conductor atoms are available. The free electrons start journey in drift motion (towards positive electrode of the battery) from atom to atom in the conduction band. At any instance each atom holds zero charge because of drift electrons from adjacent atoms fill its valence band electron gaps and it happens continuously i.e. total number of electrons equal to the number of protons in the conductor at any moment. Now the rate of change of charge (q) with respect to time (t) is called current (I), this rate of change of charge with respect to time occurs. Convention wise, Current (I) flows in opposite direction of electron flow. When you remove the conductor from the battery, this conductor does not hold any charge particle, but EMF remains present across the battery electrodes with positive and negative polarity with no flow of electron.
Effect of Temperature on a ConductorThe more effect of temperature, the more vibration in the conductor molecules. This impedes the electrons to flow, i.e. the electrons get obstruction to flow smoothly through the conductor. Thus conductivity decreases gradually with increase the temperature. Again, raise of temperature break some bonds in the conductor molecules and release some electrons. These electrons are less in number. As a whole, it can be said that increase in the temperature opposition against the drifting electron increases in the conductor.
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