September 6, 2012

ECA-1 - Lecture # 1 & 2: Charge, AC / DC Voltage and Current

This piece of text is the part of our University lecture for Electric Circuits Analysis (ECA) subject. In this post, I will cover my first lecture which is about the basic concepts of Electronic Circuits.

Starting with the definition of Electric Circuit. An electric circuit is interconnection of electrical components.

The analysis of electric circuits is based on electric charge because electric charge is most elementary and the fundamental quantity in analysis of electric circuits. Charge in motion is called current and electric current is the source of transfer the charge from one point to another.

Electric Current is the time rate of change of charge and is measured in amperes, One Ampere = 1 Coulomb per Second (quantity of charge is measured in coulombs and coulomb is the unit of charge).

Mathematically: I = dq/dt ................... (dq over dt)

  • I = Current
  • dq = differential charge
  • dt = differential time
Lecture Key Points:
  • Charge moves from +ve point (higher potential) to -ve point (lower potential) due to emf (applied voltage / potential difference).
  • The quantity of the movement of charges is due to potential difference. Potential difference may be of 1Volts,  2Volts or "n"Volts.
  • This movement of charge (due to applied voltages or P.D) is called charge.
Difference between AC of DC:

The current which remains constant with respect to time is known as DC (direct current) while the current which changes (fluctuates) with respect to time is AC (alternating current).

Voltage is the work done to move the charge. The definition of voltage is work done (energy) which moves the charge. It is also known as electromotive force or simply emf.

Mathematically: V = dw/dq ............................................... (dw over dq)

  • V = Voltage
  • dw = differential work done
  • dq = differential charge
Key Points:
  • In AC neutral (black wire) is always at zero potential. Line, Live (the red wire) changes from higher potential (+220 volts) to lower potential (- 220 volts), that is why it is called alternating current.
  • Current follows due to voltage. Voltage is the energy which exerts a pressure on charge to move and the movement of charge is called current.
  • Generator and Solar Cell are the fundamental source of DC.
  • Alternator (AC Generator) is the fundamental source of AC.
Lecture # 2:

Power and Energy

Power is the time rate of expending and absorbing energy and it is measured in Watts.


P = dw / dt ................................... (dw over dt)

P = (dw / dq) x (dq / dt) ................................... (* multiplying by dq / dq)

P = V x I ............................................. (* V = dw / dq and I = dq / dt)

P = (I^2) x (R)

P = (V^2) / R

  • P = Power
  • dw = differential work done
  • dt = differential time
  • V = Voltage
  • I = Current
Key Points:
  • Voltage doesn't flow it only drops across the load
  • Voltage is a driving force which pushes or the current to flow
  • Power is also known as load
  • Load is applied and energy is consumed
Energy is the ability of a body to do work or measurement of power with respect to time is energy.


Energy = (Power) x (Differential Time)

E = (P) x (dt)

Unit of power is watt-hours. If 1 Ampere of current is flowing through an electrical element for 1 hour, then consumed energy is 1 Kilo Watt-Hours.

  • Reference Book: Basic Engineering Circuits Analysis by J. David Irwin and R. Mark Nelms
  • Student's own knowledge
  • Lecturer: Engr. Shehryar Khan
I hope this will help all the readers and my course fellows for the preparation of ECA notes. This web based content contains no diagrams and images because it takes too time to draw them using computer.

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