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Engineering Practice Electric Fitting

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Resistance Electrical resistance is the ratio of voltage drop across a resistor to current flow through the resistor. Polarities are governed by the passive sign convention.

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Current = voltage / resistance I = V / R V = I x R Definitions Voltage = potential energy / unit charge, units = Volts Current = charge flow rate, units = Amps Resistance = friction, units = Ohms Example Voltage drop when current flows through resistor V 1 - V 2 = I R I R V1V1 V2V2 I, V & R

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Power Consumed by Resistors Resistors consume power. v and i are both positive or both negative.

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Creating a Circuit Model A circuit model is usually two or more circuit elements that are connected. A circuit model may have active elements (sources) as well as passive elements (such as resistors). By the assumption that electric signal propagation is instantaneous in a circuit, our circuit model has lumped parameters (A lumped system is one in which the dependent variables of interest are a function of time alone. In general, this will mean solving a set of ordinary differential equations (ODEs)).

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Schematics Symbols represent circuit elements Lines are wires + Battery Resistor Ground + V R I Sample circuit Ground voltage defined = 0

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Example of a Circuit Model

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Kirchhoffs Voltage Law The sum of the voltage drops around a closed path is zero. Example: -120 + V 1 + V 2 + V 3 + V 4 = 0

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Kirchhoffs Current Law A node is a point where two or more circuit elements are connected together. The sum of currents associated with a node is zero.

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Parallel And Series Resistors Series Same current flows through all Parallel Same voltage across all + Note: these points are connected together I V R1R1 R2R2 Series circuit V = R 1 I + R 2 I = R eff I R eff = R 1 + R 2 Parallel circuit I = V/R 1 + V/R 2 = V/R eff 1/R eff = 1/R 1 + 1/R 2 + V R1R1 R2R2 I1I1 I2I2 I

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Resistive voltage divider Series resistor circuit Reduce input voltage to desired level Advantages: simple and accurate complex circuit can use single voltage source Disadvantage: dissipates power easy to overload need R load