Introduction to Circuit Theory

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 Charge is measured in Coulombs (C)  1 C = 6.24 x electrons  Conventional Current flows from positive to negative  Electrons actually flow from negative to positive

 The measure of the rate of electron flow in a circuit  Measured in Amperes (A)  1 mA (milliamp) = A  1 µA (microamp)= mA  Direct Current (DC)  Flow of electricity (current) in an unchanging direction  Alternating Current (AC)  Current flows in different directions

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 Opposition that a component of device offers to the flow of an electric current  Unit of resistance is Ohm Ω  1 kilohm (k Ω) = 1000 Ω  1 megohm (m Ω) = 1,000 k Ω or 1,000,000 Ω  Good conductors have low resistance  Good insulators have high resistance  Assumption in circuit analysis: Resistance of an ideal resistor is constant and does not vary in time

 Standard unit of EMF is the volt (V)  Voltage is the measure of work done to move a charge from one point to another in an electric field  1 mV (millivolt) = V  1 µV (microvolt)= mV  Voltage is referred to as “electric potential” or “electric pressure”  More voltage in a circuit means more potential for current

 V = IR  I = V / R  R = V / I  V – Voltage  I – Current  R - Resistance V IR

 DC is 10 V and potentiometer is 10 Ω. What is the current?  Potentiometer is 100 Ω and current is 10 mA. What is voltage across the resistance?  Potentiometer is uncalibrated. Voltmeter reads 24 V and Ammeter 3A. What is the resistance?

 Measure in Watts (W)  P = IV  P = I 2 R  P = V 2 / R V IR

 Resistance in Series  Values are added to get total resistance  Resistance in Parallel  Overall resistance decreases  Conductance (S) siemens ▪ G= 1 / R  Add conductances to get total resistance

 V1 = V2 = V3  I = I1 + I2 + I3  1 / R eq = 1 /R 1 + 1/R 2 + 1/R 3

 Current Law – Kirchoff’s First Rule  The total current entering a junction in a circuit must equal the sum of the currents leaving that junction  Principle of conservation of electric charge I 1 = I 2 + I 3 I 2 = I 1 – I 3 I 3 = I 1 – I2

 Voltage Law – Kirchoff’s Second Rule  The directed sum of the emfs (potential differences) around any closed circuit it zero  Principle of conservation of energy -V B + V 1 + V 2 = 0 -V 2 - V 3 + V 4 = 0 -V B + V 1 - V 3 + V 4 = 0

 It is possible to simplify a linear circuit, no matter how complex to an equivalent circuit with just a single voltage source and series resistance connected to a load