1.1 Circuit Representation
 Circuit – a mathematical model which approximates the behaviour of a real electrical system
 Circuit elements – represent the electrical behaviour of certain physical situations
1.2 Circuit Elements
 Passive elements absorb power
 Active elements generate power
1.3 Charge Current and Voltage
 Charge
o Coulombs (C)
o Conservative – cannot be created/destroyed
 Current
o Amperes (A)
o Net flow of charge past a point
 Voltage
o Ability of circuit to do work
o Volts (V)
o Measured between two points
o Measure of potential difference
1.4 Voltage/Current Directions
 Arbitrary but must be defined – algebraic quantities
 Current flow denoted by an arrow →
 Voltage direction denoted by + and –
o + high point
o - low point
1.5 Nodes and Branches
 Each element is called a branch
 Junctions of terminals are nodes
 Nodes represent different voltage values – same voltage = same node
1.7 Power Generation/Absorption
 Generation –current flow through a voltage rise. Charge is gaining potential
 Absorption – current flow through a voltage drop. Charge is losing potential
 Passive elements – absorb power
 Active elements – generate power
 Power absorbed in a circuit = power generated
 Resistors must have current flowing from high to low potential
o Resistors are ALWAYS passive elements
 Sign convention – the terminal that the current enters is the sign of the voltage
o Choose an arbitrary current direction
o Label polarities of active elements
o Assign polarities to passive elements such that the current flows into the positive
terminal (voltage drops)
1.8 Kirchhoff’s Current Law and Kirchhoff’s Voltage Law
 Current Law (KCL)
o Sum of all currents entering a node is equal to sum of all currents leaving the node
 Voltage Law (KVL)
o Algebraic sum of voltages around any closed path is zero.
 KVL and KCL =>
o Elements in parallel have same voltage
o Elements in series have same current
1.9 Combining Resistors in Series and Parallel
 Series Resistance - 𝑅𝑇 = 𝑅1 + 𝑅2
 Parallel Resistance- 1/𝑅𝑇 = 1/𝑅1 + 1/𝑅2
1.10 Voltage/Current Division
 Voltage Division
𝑅1

o 𝑉1 = (𝑅1+𝑅2) 𝑉
o V1 is proportional to R1
o Voltage is proportional to its resistance
o ONLY FOR ELEMENTS IN SERIES
Current Division
1
𝑅1
1
1
+
𝑅1 𝑅2
o
𝐼1 = (
o
Current is inversely proportional to its resistance
)𝐼
1.11 Ideal Circuit Elements
 Ideal voltmeter
o Current passing through is zero
o Does not conduct
o Infinite resistance
 Ideal ammeter
o Voltage across is zero
o Must not affect current
o Zero resistance
 In this course all voltmeters/ammeters are ideal
 Voltmeters must be connected in parallel to the element you are measuring
 Ammeters must be connected in series
 Open Circuit – acts like a resistor of infinite resistance
 Short circuit – acts like a resistor of zero resistance
1.12 Dependent Sources
 Strength of source is dependent on a circuit variable elsewhere in the circuit
 Can be voltage controlled (VC) or current controlled (CC)
o Voltage controlled voltage source (VCVS)
o Current controlled voltage source (CCVS)
o Voltage controlled current source (VCCS)
o Current controlled current source (CCCS)
 Treat dependent source in the same ways as an independent source when using KCL and KVL