Current Electricity & Circuits W. Sautter 2007

Before we begin our study of circuits - Some basic review

Electrical Meters volts Ammeters measure current in amperes and are always wired in series in the circuit. AMPS Voltmeters measure potential in volts and are always wired in parallel in the circuit. volts

Electrical Symbols A V battery + - junction wiring terminal voltmeter AC generator ammeter A Variable resistance resistance Variable capacitor capacitor

THE MOST BASIC ELECTRICAL CIRCUIT LOAD (RESISTANCE) [ENERGY OUT] ELECTRONS INTO LOAD ELECTRONS OUT OF LOAD CONDUCTOR CONDUCTOR ELECTRONS OUT OF SOURCE ELECTRON PUMP (SOURCE VOLTAGE) [ENERGY IN] ELECTRONS BACK TO SOURCE HIGHER ENERGY ELECTRONS LOWER ENERGY ELECTRONS

Ohm's Law V = I R Potential In volts (joules / coul) Current In amperes (coul / second) Resistance In ohms (volts / amp) Drop across a resistance Current passing Through the resistor

Potential Rise Across a Power Source volts current current Battery Electrons have More Energy Electrons have Less Energy Electrons get An energy boost

Potential Drop Across a Resistor volts current Resistor Electrons have Less Energy Electrons have More Energy Energy is lost In the resistor

There are three generally types of electrical circuits: Series circuits in which the current created by the voltage source passes through each circuit component in succession. R2 A2 R1 R3 A1 Arrows show Current path Through each component

(2) Parallel circuits in which the current created by the voltage source branches with some passing through one component and while the rest of the current passes through other components. A1 R1 R2 R3 A2 A3 A4 R4 Arrows show Current path Through each component Junction or Branching points

(3) Series Parallel circuits or combination circuits which contain series segments and parallel segments. Arrows show Current path Through each component

All electrical circuit analysis requires the use of two fundamental laws called Kirchhoff’s Laws

Kirchhoff's Circuit Laws FIRST LAW All current entering a junction point must equal all current leaving that junction point Current Leaving ( I3 ) Current Leaving ( I2 ) Junction point I1 = I2 + I3 Current Entering ( I1 )

Kirchhoff's Circuit Laws SECOND LAW Around any complete loop, the sum of the voltage rises must equal the sum of voltage drops Resistance 1 (voltage drop 1) Resistance 2 (voltage drop 2) Resistance 3 (voltage drop 3) Current flow Complete loop Battery (voltage rise) + - V(Battery) = V1 + V2 + V3

Current Loops - Loop #2 V2 A2 R2 Loop #3 Complete current V1 Paths in a circuit V1 A1 R1 Loop #1 Kirchhoff’s Laws Around a loop V rises =  V drops A loop is a completed Path for current flow EMF - + At Battery

When using Kirchhoff’s laws we apply the principles of conventional current flow. When current leaves the positive (+) terminal of a voltage source and enters the negative (-) terminal a voltage rise occurs across the source. If the current enters the positive and exits the negative a of a voltage source a voltage drop occurs across the source. When tracing a current loop, if the assumed direction of the current and the loop direction are the same, a voltage drop occurs across a resistance. If the assumed direction of the current and the loop direction are opposite, a voltage rise occurs across the the resistance.

When using Kirchhoff’s laws we apply the principles of conventional current flow. When current leaves the positive (+) terminal of a voltage source and enters the negative (-) terminal a voltage rise occurs across the source. If the current enters the positive and exits the negative a of a voltage source a voltage drop occurs across the source. V = - 6 v Current flow Battery ( 6 volts) + - Current flow V = + 6 v

When tracing a current loop, if the assumed direction of the current and the loop direction are the same, a voltage drop occurs across a resistance. If the assumed direction of the current and the loop direction are opposite, a voltage rise occurs across the the resistance. V = - 6 v A voltage drop Loop direction Assumed Current flow resistor Loop direction Assumed Current flow V = + 6 v A voltage rise

The total resistance of the circuit is the sum of Series Circuit Relationships Derived from Kirchhoff's Laws In a series circuit: The total resistance of the circuit is the sum of the resistance values in the circuit. Series Resistance Rt = R1 + R2 + …. (2) The sum of all voltage drops across the resistors in the circuit equals the voltage rise of the source. EMF = V1 + V2 + V3 + Vi The through each resistance is the same. I TOTAL = I1 = I2 = I3 = Ii

A Series Circuit Voltmeters In parallel Series Resistance Rt = R1 + R2 + …. Ammeters In series V2 EMF = V1 + V2 + V3 + Vi A2 R2 A1 V3 R3 V1 R1 EMF Ammeters read The same everywhere In the circuit A1 = A2 Ri R = Resistance In ohms

A Series Circuit

The reciprocal of the total resistance of the circuit is the sum Parallel Circuit Relationships Derived from Kirchhoff's Laws In a parallel circuit: The reciprocal of the total resistance of the circuit is the sum of the reciprocals of the resistance values in the circuit. Parallel Resistance 1/Rt = 1/R1 + 1/R2 …. (2) The sum of the voltage drops across the resistors in a branch of the circuit equals the voltage rise of the source. V source= V1 = V2 = V3 = Vi (3) All current entering a junction = all current leaving the junction I TOTAL = I1 + I2 + I3 + Ii

A junction = all current A Parallel Circuit Voltmeters In parallel V1 A1 R1 Ammeters In series V2 A2 R2 Parallel Resistance 1/Rt = 1/R1 + 1/R2 …. V3 Junction points A3 R3 Kirchhoff’s Laws All current entering A junction = all current Leaving the junction (2) Around a loop  V rises =  V drops EMF A4 R = Resistance In ohms Battery

A Parallel Circuit

Another Parallel Circuit

A junction = all current V1 Series - Parallel Circuit P A R L E A1 R1 V2 Parallel Resistance 1/Rt = 1/R1 + 1/R2 …. Series Resistance Rt = R1 + R2 + …. A2 R2 V3 A3 R3 Kirchhoff’s Laws All current entering A junction = all current Leaving the junction (2) Around a loop  V rises =  V drops V4 R4 EMF A4 Ri SERIES

A Typical Circuit Board Integrated circuits capacitors resistors

THE END