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EE 221 Review 1. Basic components Electric circuits Voltage and current laws. Basics - SI base units. Basics - SI prefixes. Basics - Charge. Two types of charge positive: (proton) negative: (electron, -1.6 10 -19 C) Continuously transferring charge
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EE 221 Review 1 Basic components Electric circuits Voltage and current laws
Basics - Charge • Two types of charge • positive: (proton) • negative: (electron, -1.6 10-19C) • Continuously transferring charge • total amount of charge never changed • neither created nor destroyed (conservation) • Defined in terms of ampere • Measured in coulomb (C) = As
Representing current • numerical value (+ unit) (e.g., -13.5 A) • direction ( ) • unit is the ampere (A) • represented by I, i, i(t) Symbol for an independent current source Basics - Current • Charge in motion • transfer of energy • related to charge
Basics - Current • Example (a,b) Incomplete, improper, and incorrect definitions of a current. (c) the correct definition of i1(t).
A general two-terminal circuit element Basics - Voltage • General, simple circuit element • two terminals • cannot be decomposed further • completely characterized by itsvoltage-current relationship • Pushing charge • expenditure of energy • electrical voltage (potential difference) • voltage "across" the element
A general two-terminal circuit element Symbols: (a) DC voltage source;(b) battery; (c) ac voltage source. Basics - Voltage • Voltage measures workrequired to move charge • Representing voltage • numerical value (+ unit) (e.g., -2.5 V) • direction (sense) ( + V - )(left terminal is V volts positive withrespect to the right terminal) • unit is volt (V = J / C) • represented by V, v, v(t)
A general two-terminal circuit element Basics - Power • Power is the rate of energyexpenditure: Voltage * Current • Voltage defined in terms of energy • Current is rate at which charge moves • Representing power • numerical value (+ unit) (e.g., -5.6 W) • "direction" by Passive Sign Convention • PSC: Current entering element through positive terminal • unit is watt (W = V *A = J / C *A = J / (As) *A = J / s) • represented by P, p, p(t)
Basics - Passive sign convention (PSC) • Is a choice we make (convention) • The current arrow is directedinto the "+" marked terminal • The power absorbed by the elementis given by the product p = v i • A negative value indicates that poweris actually generated • Or: The power generated by theelement is given by the product p = - v i A general two-terminal circuit element, p = vi represents the power absorbed
Basics - Resistor • Resistance of conducting element • Ohm's law: v = R i • linear, directly proportional • Passive element • Power p = v i = i2 R = v2 / R • Representing resistance • numerical value (+ unit) (e.g., 3 ) • unit is ohm ( = V / A) • represented by R
Circuits • Nodes • Branches • Paths • Loops (a) A circuit containing three nodes and five branches. (b) Node 1 is redrawn to look like two nodes; it is still one node.
Circuits - KCL • Kirchhoff's current law • Conservation of charge The algebraic sum of the currents entering any node is zero. iA + iB - iC - iD = 0
Circuits - KVL • Kirchhoff's voltage law • Conservation of energy The algebraic sum of the voltages around any closed path is zero. v1 = v2 - v3
Circuits - Sources (a) Series connected voltage sources can be replaced by a single source. (b) Parallel current sources can be replaced by a single source.
Circuits - Sources Examples of circuits with multiple sources, some of which are “illegal” as they violate Kirchhoff’s laws.
(a) Series combination of N resistors. (b) Electrically equivalent circuit. Req = R1 + R2 + ... + RN V-I Laws: Resistors
(a) Parallel combination of N resistors. (b) Electrically equivalent circuit. 1/Req = 1/R1 + 1/R2 + ... + 1/RN A special case worth remembering is V-I Laws: Resistors
An illustration of voltage division. Voltage division Using KVL and Ohm's law to find v2. For a string of N series resistors ....
An illustration of current division. Current division Using KCL and Ohm's law to find i2. For a parallel combination of N resistors the current through Rk equals ....
Simplifying circuits (KVL) • What do we count as positive? • Direction of summation determines polarity 1st choice V1 = Va - Vb + Vc 2nd choice V2 = -Va + Vb - Vc