Lessons in Electric Circuit


Light bulbLight bulbLight bulb BASIC CONCEPTS OF ELECTRICITY Light bulbLight bulbLight bulb

  • Static Electricity :

    • All materials are made up of tiny ”building blocks” known as atoms.
    • All naturally occurring atoms contain particles called electrons, protons, and neutrons,1with the exception of the protium isotope (1H1) of hydrogen.
    • Electrons have a negative (-) electric charge.
    • Protons have a positive (+) electric charge.
    • Neutrons have no electric charge.
    • Electrons can be dislodged from atoms much easier than protons or neutrons.
    • The number of protons in an atom’s nucleus determines its identity as a unique element.

  • Conductor,Insulator and Electron flow:

    • In conductive materials, the outer electrons in each atom can easily come or go, and are
      called free electrons.
    • In insulating materials, the outer electrons are not so free to move.
    • All metals are electrically conductive.
    • Dynamic electricity, or electric current, is the uniform motion of electrons through a conductor.
    • Static electricity is an unmoving (if on an insulator), accumulated charge formed by either an excess or deficiency of electrons in an object. It is typically formed by charge separation by contact and separation of dissimilar materials.
    • For electrons to flow continuously (indefinitely) through a conductor, there must be a complete, unbroken path for them to move both into and out of that conductor.

  • Electric Circuit:

    • A circuit is an unbroken loop of conductive material that allows electrons to flow through continuously without beginning or end.23
    • If a circuit is ”broken,” that means its conductive elements no longer form a complete path, and continuous electron flow cannot occur in it.
    • The location of a break in a circuit is irrelevant to its inability to sustain continuous electron flow. Any break, anywhere in a circuit prevents electron flow throughout the circuit.

  • Voltage and Current:

    • Electrons can be motivated to flow through a conductor by the same force manifested in static electricity.
    • Voltage is the measure of specific potential energy (potential energy per unit charge) between two locations. In layman’s terms, it is the measure of ”push” available to motivate electrons.4
    • Voltage, as an expression of potential energy, is always relative between two locations, or points. Sometimes it is called a voltage ”drop.”
    • When a voltage source is connected to a circuit, the voltage will cause a uniform flow of electrons through that circuit called a current.
    • In a single (one loop) circuit, the amount of current at any point is the same as the amount of current at any other point.
    • If a circuit containing a voltage source is broken, the full voltage of that source will appear across the points of the break.
    • The +/- orientation of a voltage drop is called the polarity. It is also relative between two points.

  • Resistance:

    • Resistance is the measure of opposition to electric current.

    • A short circuit is an electric circuit offering little or no resistance to the flow of electrons.
    • Short circuits are dangerous with high voltage power sources because the high currents encountered can cause large amounts of heat energy to be released.5
    • An open circuit is one where the continuity has been broken by an interruption in the path for electrons to flow. A closed circuit is one that is complete, with good continuity throughout.
    • A device designed to open or close a circuit under controlled conditions is called a switch.
    • The terms ”open” and ”closed” refer to switches as well as entire circuits. An open switch is one without continuity: electrons cannot flow through it. A closed switch is one that provides a direct (low resistance) path for electrons to flow through.
  • Conventional Vs current flow:
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Light bulbLight bulbLight bulb OMS LAW Light bulbLight bulbLight bulb

  • How voltage, current, and resistance relate?

An electric circuit is formed when a conductive path is created to allow free electrons to continuously
move. This continuous movement of free electrons through the conductors of a circuit is called a current, and it is often referred to in terms of ”flow,” just like the flow of a liquid through a hollow pipe.


The force motivating electrons to ”flow” in a circuit is called voltage. Voltage is a specific measure of potential energy that is always relative between two points. When we speak of a certain amount of voltage being present in a circuit, we are referring to the measurement of how much potential energy exists to move electrons from one particular point in that circuit to another particular point. Without reference to two particular points, the term ”voltage” has no meaning.
Free electrons tend to move through conductors with some degree of friction, or opposition to motion. This opposition to motion is more properly called resistance. The amount of current in a circuit depends on the amount of voltage available to motivate the electrons, and also the amount of resistance in the circuit to oppose electron flow. Just like voltage, resistance is a quantity relative between two points. For this reason, the quantities of voltage and resistance are often stated as being ”between” or ”across” two points in a circuit.
Here are the standard units of measurement for electrical current, voltage, and resistance:
QUANTITY
SYMBOL
UNIT OF MEASUREMENT
UNIT ABBREVIATION
CURRENT
I
Ampere (“AMP”)
A
VOLTAGE
E or V
Volt
V
RESISTANCE
R
Ohm
Ω

Ohm’s principal discovery was that the amount of electric current through a metal conductor in a circuit is directly proportional to the voltage impressed across it, for any given temperature. Ohm expressed his discovery in the form of a simple equation, describing how voltage, current, and resistance interrelate:
                          E = I R
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Ohm’s Law is a very simple and useful tool for analyzing electric circuits. It is used so often in the study of electricity and electronics that it needs to be committed to memory by the serious student. For those who are not yet comfortable with algebra, there’s a trick to remembering how to solve for any one quantity, given the other two. First, arrange the letters E, I, and R in a triangle like this:
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If you know E and I, and wish to determine R, just eliminate R from the picture and see what’s left:
10                       R= clip_image002

If you know E and R, and wish to determine I, eliminate I and see what’s left:
11                      I= clip_image002[12]
Lastly, if you know I and R, and wish to determine E, eliminate E and see what’s left:
12                     E=IR

  • An Analogy for OHM’s law:-

    • Ohm’s Law also makes intuitive sense if you apply it to the water-and-pipe analogy. If we have a water pump that exerts pressure (voltage) to push water around a ”circuit” (current) through a restriction (resistance), we can model how the three variables interrelate. If the resistance to water flow stays the same and the pump pressure increases, the flow rate must also increase.
Pressure = increase                         Voltage = increase
Flow rate = increase                       Current = increase
Resistance = Same                         Resistance = Same 
     If the pressure stays the same and the resistance increases (making it more difficult for the
     water to flow), then the flow rate must decrease:
Pressure = Same                           Voltage = same
  Flow rate = decrease                       Current = decrease
     Resistance = increase                      Resistance = increase 
     If the flow rate were to stay the same while the resistance to flow decreased, the required
     pressure from the pump would necessarily decrease:
Pressure = decrease                         Voltage = decrease
Flow rate = same                              Current = same
Resistance = decrease                     Resistance = decrease 
    • With resistance steady, current follows voltage (an increase in voltage means an increase
      in current, and vice versa).
    • With voltage steady, changes in current and resistance are opposite (an increase in current
      means a decrease in resistance, and vice versa).
    • With current steady, voltage follows resistance (an increase in resistance means an increase
      in voltage).

  • Power :
  • Power is the measure of how much work can be done in a given amount of time.
  • Mechanical power is commonly measured (in America) in ”horsepower.”
  • Electrical power is almost always measured in ”watts,” and it can be calculated by the
    formula P = IE.
  • Electrical power is a product of both voltage and current, not either one separately.
  • Horsepower and watts are merely two different units for describing the same kind of physical measurement, with 1 horsepower equaling 745.7 watts.
 Power measured in watts, symbolized by the letter ”W”.
  • Joule’s Law: P = I2R ; P = IE ; P = E2/R.

    • Resistors :
      • Devices called resistors are built to provide precise amounts of resistance in electric circuits.
        Resistors are rated both in terms of their resistance (ohms) and their ability to dissipate heat energy (watts).
      • Resistor resistance ratings cannot be determined from the physical size of the resistor(s)
        in question, although approximate power ratings can. The larger the resistor is, the more
        power it can safely dissipate without suffering damage.
      • Any device that performs some useful task with electric power is generally known as a load. Sometimes resistor symbols are used in schematic diagrams to designate a nonspecific
        load, rather than an actual resistor.
      • The resistance of most conductive materials is stable over a wide range of conditions, but
        this is not true of all materials.
      • Any function that can be plotted on a graph as a straight line is called a linear function. For circuits with stable resistances, the plot of current over voltage is linear (I=E/R).
      • In circuits where resistance varies with changes in either voltage or current, the plot of current over voltage will be nonlinear (not a straight line).
      • A varistor is a component that changes resistance with the amount of voltage impressed across it. With little voltage across it, its resistance is high. Then, at a certain ”breakdown” or ”firing” voltage, its resistance decreases dramatically.
      • Negative resistance is where the current through a component actually decreases as the applied voltage across it is increased. Some electron tubes and semiconductor diodes (most notably, the tetrode tube and the Esaki, or tunnel diode, respectively) exhibit negative resistance over a certain range of voltages.
    • Circuit Wiring :
      • Connecting wires in a circuit are assumed to have zero resistance unless otherwise stated.
      • Wires in a circuit can be shortened or lengthened without impacting the circuit’s function all that matters is that the components are attached to one another in the same sequence.
      • Points directly connected together in a circuit by zero resistance (wire) are considered to be electrically common.
      • Electrically common points, with zero resistance between them, will have zero voltage dropped between them, regardless of the magnitude of current (ideally).
      • The voltage or resistance readings referenced between sets of electrically common points will be the same.
      • These rules apply to ideal conditions, where connecting wires are assumed to possess absolutely zero resistance. In real life this will probably not be the case, but wire resistances should be low enough so that the general principles stated here still hold.
      • The polarity of the voltage drop across any resistive component is determined by the
        direction of electron flow through it: negative entering, and positive exiting.
    • Series And Parallel Circuit: In a series circuit
      • all components are connected end-to-end, forming a single path for
        electrons to flow.
      • Voltage drops add to equal total voltage.
      • All components share the same (equal) current.
      • Resistances add to equal total resistance.14
    In a parallel circuit
      • All components are connected across each other, forming exactly two
        sets of electrically common points.
      • All components share the same (equal) voltage.
      • Branch currents add to equal total current.
      • Resistances diminish to equal total resistance.14
      • A ”branch” in a parallel circuit is a path for electric current formed by one of the load
        components (such as a resistor).
      • Components in a series circuit share the same current:                               ITotal = I1 = I2 = . . . In
      • Total resistance in a series circuit is equal to the sum of the individual resistances:    RTotal = R1 + R2 + . . . Rn
      • Total voltage in a series circuit is equal to the sum of the individual voltage drops:  ETotal = E1 + E2 + . . . En
      • Components in a parallel circuit share the same voltage:                             ETotal = E1 = E2 = . . . En
      • Total resistance in a parallel circuit is less than any of the individual resistances:     RTotal = 1 / (1/R1 + 1/R2 + . . . 1/Rn)
      • Total current in a parallel circuit is equal to the sum of the individual branch currents: ITotal = I1 + I2 + . . . In.

        • To analyze a series-parallel combination circuit, follow these steps:• Reduce the original circuit to a single equivalent resistor, re-drawing the circuit in each step of reduction as simple series and simple parallel parts are reduced to single, equivalent resistors.
          • Solve for total resistance.
          • Solve for total current (I=E/R).
          • Determine equivalent resistor voltage drops and branch currents one stage at a time,working backwards to the original circuit configuration again.
    • Conductance:
      • Conductance is the opposite of resistance: the measure of how easy it is for electrons to flow through something.
      • Conductance is symbolized with the letter ”G” and is measured in units of mhos or Siemens.
      • Mathematically, conductance equals the reciprocal of resistance: G = 1/R.1414
      • Gtotal = G1 + G2 + G3 + G4
    • Practical Circuits:
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    • What is Soldering?
    School  Soldering is a low-temperature welding process utilizing a lead/tin or tin/silver alloy to bond wires and component leads together, usually with the components secured to a fiberglass board.
    • DIVIDER CIRCUITS AND KIRCHHOFF’S LAWS :
      • Voltage dividers find wide application in electric meter circuits, where specific combinations of series resistors are used to ”divide” a voltage into precise proportions as part of a voltage measurement device.14
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      • Series circuits proportion, or divide, the total supply voltage among individual voltage
        drops, the proportions being strictly dependent upon resistances:                       ERn = ETotal (Rn /RTotal)
      • A potentiometer is a variable-resistance component with three connection points, frequently used as an adjustable voltage divider.
    Kirchhoff’s Voltage Law:
    ”The algebraic sum of all voltages in a loop must equal zero”.
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      • Parallel circuits proportion, or ”divide,” the total circuit current among individual branch
        currents, the proportions being strictly dependent upon resistances:                      In = ITotal (RTotal / Rn)
    Kirchhoff’s current Law:
    ”The algebraic sum of all currents entering and exiting a node must equal zero”.
    Mathematically, we can express this general relationship as such:
    Iexiting = Ientering
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