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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

EE 221 Review 1

Basic components

Electric circuits

Voltage and current laws




Basics charge
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


Basics current

  • 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 current1
Basics - Current

  • Example

(a,b) Incomplete, improper, and incorrect definitions of a current. (c) the correct definition of i1(t).


Basics voltage

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


Basics voltage1

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)


Basics power

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
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
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
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
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
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
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 sources1
Circuits - Sources

Examples of circuits with multiple sources, some of which are “illegal” as they violate Kirchhoff’s laws.


V i laws resistors

(a) Series combination of N resistors.

(b) Electrically equivalent circuit.

Req = R1 + R2 + ... + RN

V-I Laws: Resistors


V i laws resistors1

(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


Voltage division

An illustration of voltage division.

Voltage division

Using KVL and Ohm's law to find v2.

For a string of N series resistors ....


Current division

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
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


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