TEC 284Introduction to Circuit Theory

Structure of Atom

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Electric Charge

Charge is measured in Coulombs (C)1 C = 6.24 x 1018 electronsConventional Current flows from

positive to negativeElectrons actually flow from negative

to positive

Current flow

Current

The measure of the rate of electron flow in a circuit

Measured in Amperes (A) 1 mA (milliamp) = 0.001 A 1 µA (microamp)= 0.001 mA

Direct Current (DC) Flow of electricity (current) in an

unchanging direction Alternating Current (AC)

Current flows in different directions

DC vs AC

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Resistance

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

EMF (Electromotive force) 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) = 0.001 V 1 µV (microvolt)= 0.001 mV

Voltage is referred to as “electric potential” or “electric pressure”

More voltage in a circuit means more potential for current

Ohm’s Law

V = IR I = V / RR = V / I

V – Voltage I – CurrentR - Resistance

V

I R

Resistance

Current, Voltage, Resistance

Exercise: Calculations

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?

Power

Measure in Watts (W)P = IVP = I2RP = V2 / R

V

I R

Resistive Networks

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

Resistors in Parallel

V1 = V2 = V3 I = I1 + I2 + I31 / Req = 1 /R1 + 1/R2 + 1/R3

Voltage Divider

Current Divider

Kirchoff’s Laws

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

I1 = I2 + I3

I2 = I1 – I3

I3 = I1 – I2

Kirchoff’s Laws

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-VB + V1 + V2 = 0

-V2 - V3 + V4 = 0

-VB + V1 - V3 + V4 = 0

Thevenin’s Theorem

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

Units