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# ch20_print - Electric Circuits 1 Electric Current and...

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Electric Circuits 1 – Electric Current and Electromotive Force The flow of electric charges: Electric currents power light bulbs, TV sets, computers etc. Definition of electric current : The current is the rate at which charge flows through a surface perpendicular to the flow : I = Δ q Δ t (1) SI unit: ampere (A) : 1 A = 1 C/s It is conventional to give the current the same direction as the flow of positive charge. Dr.D.Wackeroth Spring 2005 PHY102A Electric Circuits Circuits: The transfer of electric energy in form of kinetic energy of the moving charge carriers takes place via electric circuits. A circuit consists of an energy source (e.g., battery, generator) and energy consuming devices (e.g, light bulb, TV,computer) which are connected by conducting wires. “emf” : source that maintains a constant current in a closed circuit. The name originates from electromotive force . The emf is the maximum po- tential difference provided by the source. direct current (dc) circuits: charges move around circuits in the same direction at all times (battery). alternating current (ac) circuits: the direction of charges moving around a circuit changes from momemt to moment (generators at power compa- nies). Dr.D.Wackeroth Spring 2005 PHY102A

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Electric Circuits 2 – Resistance and Ohm’s Law When a voltage, V , is applied across a piece of material, the current, I , is found to be proportional to the applied voltage. The ratio V/I is called the resistance, R , of the material. For many materials, including most metals, the resistance is constant over a wide range of applied voltages. When the resistance is constant Ohm’s law applies: V I = R = constant or V = RI (2) SI unit: ohm ( Ω ) : 1 Ω = 1 V/A Ohm’s law is an empirical relationship. Dr.D.Wackeroth Spring 2005 PHY102A Electric Circuits 3 – Resistivity In a metal conductor, the electric current is carried by moving electrons. Resistance originates from collisions of these electrons with atoms. The resistance of an ohmic conductor is proportional to its length, L , and inversely proportional to its cross-sectional area A : R = ρ L A (3) ρ is called the resistivity of the material.
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ch20_print - Electric Circuits 1 Electric Current and...

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