DC Circuits: Basic Concepts Dr. Hasan Demirel

DC Circuits: Basic Concepts Dr. Hasan Demirel

An electric circuit is an interconnection of electrical elements. A simple electric circuit Electric circuit of a radio receiver

Six basic SI units and one derived unit relevant to this course. Quantity Basic Unit Symbol Length meter m Mass kilogram kg Time second s Electric current ampere A Thermodynamic Temperature kelvin K Luminous intensity candela cd Charge coulomb C

Prefix SI Prefixes Most frequently used micro pico

Charge and Current Charge is an electrical property of the atomic particles of matter, measured in coulombs (C). 1 C of charge requires 6.24 x 10 18 electrons. 1 electron charge e = 1.602 x 10 19 C. Law of Conservation of Charge: Charge can only be transferred. Cannot be created/destroyed. Charge Direction

Charge and Current Electric current is the time rate of change of charge, in other words, the flow of charge, measured in amperes (A). Current is defined by: where i = current in amperes (A), q = charge in coulombs (C), t = time in seconds (s). i dq dt 1 A = 1 C/s Q t Charge transferred between time t 0 and t : t 0 i dt

Charge and Current Direct Current (dc) is the current that remains constant with time. EENG223: CIRCUIT THEORY I Alternating Current (ac) is the current that varries sinusoidally with time. dc current is represented by I and ac current is represented by i. Conventional current flow: Both methods represents the same current. (a) positive current flow, (b) negative current flow.

Charge and Current : Problems Ex. 1.1: EENG223: CIRCUIT THEORY I Ex. 1.2:

Charge and Current : Problems Ex. 1.3: EENG223: CIRCUIT THEORY I

Voltage Voltage (potential difference/electromotive force) is the energy required to move a unit charge through an element, measured in volts (V). The voltage, between two points a and b: υ ab dω dq υ = voltage in volts (V), ω = energy in joules (J), q = charge in coulombs (C). 1 volt = 1 joule/coulomb

Voltage Voltage pushes charge in one direction. We use polarity (+ and ) on batteries to indicate which direction the charge is being pushed. Two equivalent representation of the same voltage: (a) point a is 9 V above point b, (b) point b is 9 V above point a. dc voltage is represented by V and ac voltage is represented by υ.

Power and Energy Power is the rate of expending and absorbing energy, measured in watts(w). p dω dt i p=±υi p = power in watts (W = J/s), ω = energy in joules (J), t = time in seconds (s). υ = voltage in volts (V). i = current in amperes (A). Circuit elements that absorb power has positive value of p. Circuit elements that supply (produce) power has negative value of p. (a) Absorbing power (b) supplying power.

Power and Energy Energy is the capacity to do work, measured in joules(j). t t 0 p dt t t 0 i dt p = power in watts (W), ω = energy in joules (J), t = time in seconds (s). υ = voltage in volts (V). i = current in amperes (A). If current and voltage are constant (dc). The power is: In adddition to joules, Watt-hour can also be used to measure energy. 1 Wh = 3,600 J t t 0 pdt p( t t0) Law of conservation of energy. Total power in a circuit at any instant is must be zero. p 0 +Power absorbed = Power supplied

Power and Energy : Problems Ex. 1.4:

Power and Energy : Problems Ex. 1.5:

Power and Energy : Problems Ex. 1.6:

Passive Sign Convention Passive sign Convention (PSC) is satisfied when current enters through the positive terminal of an element. Most two terminal circuit elements (i.e batteries, light bulbs, resistors, switches) are characterized by a single single equation that relates voltage to current: υ=±f(i) or i=±g(υ). PSC determines the sign relationship If PSC is satisfied: υ=f(i) or i=g(υ). If PSC is not satisfied: υ= f(i) or i= g(υ). This is also true for power If PSC is satisfied: p= υi If PSC is not satisfied: p= υi

Passive Sign Convention Ex. 1.7:

Circuit Elements Ideal Independent Source: provides a specified voltage or current that is completely independent of other circuit variables Ideal Independent Voltage source: (a) Independent voltage source (constant / time varying) (b) Independent voltage source (battery). Ideal Independent Current source:

Circuit Elements Ideal dependent sources: controlled by other voltage or current. (a) dependent voltage source (b) dependent current source