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CHEM3440Lec3F06 - Current Convention CONVENTION Electrical...

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CHEM*3440 Chemical Instrumentation Topic 3 Rudimentary Electronics Current Convention CONVENTION: Electrical current flows from a region of positive potential energy to a region of more negative (or less positive) potential energy. Truth is that electron flow is electrical current. They are negative and so flow from negative to positive. Convention is historical. All circuit behaviour can be correctly analyzed based on this convention. Even works when true circuit may consist of mixed carriers (positive ioins and negative electrons flowing in opposite directions. Charge Electrical charge is a fundamental property of nature. Comes in two forms we identify as “positive” and “negative”. Measured in units of coulombs (C) and is commonly represented by Q or q. Charge of an electron is 1.6022 x 10 -19 C. There are 6.2414 x 10 18 electrons in a coulomb of charge. In a mole of electrons there is 96485.31 C of charge. This is called the Faraday constants. It is used to convert electrical measurements (like current) into chemical measurements (like moles). Potential Energy Matter likes to be electrically neutral. Separating a positive charge from a negative charge requires work (energy). Bringing together two like charges requires energy also. The potential difference between two such particles is called the electrical potential difference or the electromotive force (emf). Potential energy difference is measured in joules of energy needed per coulomb of charge separation. It has units of volts (V). 1 V = 1 J/C The space between two separated charges is said to be filled by an electric field. The emf is said to change continuously when moving from one charge towards the other.
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Mobile Charges An ion is formed when an electron is removed (cation) from or added (anion) to a neutral atom or molecule. Matter is held together by the attractive force between oppositely charged particles. This is done by the sharing of the outermost (valence) electrons between atoms. In metals, the valence electrons are shared by so many other atoms, they are essentially free to move through the whole sample. Other materials share their electrons very specifically and they cannot move through the whole material. They are insulators. Other materials donate electrons completely to a neighbour and the material consists of ions. Charges move through this material by the displacement of ions rather than electrons. Current Density Current flow per unit area: J = I/A = Q/tA Positive charge carriers which are mobile move in the direction of the imposed electric field E. Mobile negative carriers move in the opposite direction. Both contribute to the current density J, the rate of charge motion per unit area. Electric Field, E Current I = Q/t Area A + - - + Mobility Current density depends on number density of carriers n, charge on each particle q, and the velocity of the carriers, v.
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