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TEC 284Introduction to Circuit Theory
Structure of Atom
Image Source: Wikipedia
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
Image Source: Electronics Demystified
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