Electrical Engineerig: Semester No.1

Course Outline
EE 100: Electric Circuit(3+1)
Historical development: Charge, Current, Potential Difference,Current-voltage relationship for resistance, inductance and capacitance, constant voltage and constant current sources, Kirchoff's Law, Loop and node equations ,Thevenin and Norton's Theorems, Concept and use of duality, First order differential equation, determination of current and voltage in R-L and R-C circuits, Sinusoidal Current: Complex Number and Phasor notation,Concept and handling of phasor impedance,Lagging and leading currents and voltages, RMS and average values in general and for sinusoids. Complex power, Power factor and Correction, Maximum power transformer theorem.
Textbook:
The Analysis and Design of Linear Circuits by Ronald E.Thomas, Albert J.Rosa, Gregory J, Toussaint, john Wiley   7th edition                   ||view book||
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Referencebooks:
Basic Engineering Circuit Analysis                                                              

|| Download Book||

Circuit Analysis Theory and Practice 2nd Edition Robbins & Miller          

|| Download Book||

Fundamental of Electric Circuit 4th edition Charles K.Alexander, Matthew N.O Sadiku 

|| Download Book||

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Electric Current

Charges   
passing through any cross-section of a conductor in time deltat is called Electric Current.

Unit: The SI unit of current is Ampere

The current will be 1 ampere if 1 coulomb charge flows through any cross-section of a conductor in 1 second.

Charge Carriers: Electric current is due to flow of charge particles. These charge particles are called charge carriers.

• Metals:  In metals charge carriers are negatively charged  e.g electrons
• Electrolytes: In electrolytes charge carriers are negative and positive ions e.g
• Gases: In gases charge carriers are electrons and ions

Current direction:

Conventional current:

The current which passes from a point at higher potential to a point at lower potential as if it shows the movement of positive charges.

Explanation: 

Early scientists regarded an electric current as flow of positive charges from positive to negative terminal of a battery through an external circuit as shown in fig.3.1 below

Fig.3.1 (conventional current direction)

Later on it was found that a current in metallic conductors is actually due to the flow of negative charge carriers called electrons moving in the opposite direction i.e from negative to positive terminal of the battery. But it is convention to take the direction in which positive charges flow.This current is called Conventional Current.

Reason:

The reason is that it has been found experimentally that positive charges moving in one direction is equivalent in all external effects to the negative charges moving in opposite direction. As the current is measured by the external effects, so the current due to the motion of negative charges, after reversing its direction of flow, can be considered as the current flowing due to the positive charges.
Source of current:
A source which produces a constant potential difference across the ends of a conductors is called Source of Current.
Explanation:
 when two conductors at different potential are connected by a wire , current starts flowing through the wire from high potential to the low potential. This current flows until both the conductors are at the same level of  potential. Then the current stops flowing. So current through the wire decreases from maximum value to zero. In order to get a constant current the potential difference across the ends of the wire should remain constant. This is done by connecting the ends of the wire to the terminals of a device called Source of Current.

fig.3.2

Types of Current Source: 

Every source of current converts some non-electrical energy into electrical energy. A few example are mentioned.

• Cell : It converts chemical energy into electrical energy.
• Electric generator: it converts heat energy into electrical energy.
• Thermo couples: It converts heat energy into electrical energy
• Solar cell: It converts light energy into electrical energy.

Fig.3.3(cell source)

Effects of Current:

The presence of current can be detected by the various effects which it produces, e.g

1. Heating effect
2. Magnetic effect
3. Chemical effect

Heating effect:

   Current flows through a metallic wire due to motion of free electrons. These electrons collide with the atoms of the metal and give some of their kinetic energy to these atoms. As a result, it increases the kinetic energy of the vibrations of the atoms of the metal, i.e it generates heat in  the wire. The heat 'H' produced by a current 'I' in the wire of the resistance 'R' during time 't'

Equation

Uses: 

Heating effect of current is used in:

• Electric heater
• Kettles
• Toaster
• Electric iron

Magnetic effect:
When current flows in a straight wire, coil or solenoid, a magnetic field is produced around them as shown in fig.3.4. The strength of field depends on the value of current and distance from the wire.

Uses:
Magnetic effect is used in galvanometers to detect the current. Motors, fans,drill machines and grinders etc use this effect.

Fig.3.5(magnetic effect)

Circuit Diagrams

Electric circuits are constructed using components such as batteries, switches, resistors, capacitors, transistors, interconnecting wires, etc. To represent these circuits on paper, diagrams are used.

There are three types of circuit diagrams:

1. Block diagrams
2.  Schematic diagrams
3.  Pictorials

Block Diagrams:

Block diagrams describe a circuit or system in simplified form. The overall problem is broken into blocks, each representing a portion of the system or circuit. Blocks are labelled to indicate what they do or what they contain, then interconnected to show their relationship to each other. General signal flow is usually from left to right and top to bottom. In fig 2.1 a block diagram is shown represents an audio amplifier

Working of audio amplifier:

Sound is picked up by the microphone, converted to an electrical signal, amplified by a pair of amplifiers, then output to the speaker, where it is converted back to sound. A power supply energizes the system. The advantage of a block diagram is that it gives you the overall picture and helps you understand the general nature of a problem. However, it does not provide detail.

Fig.2.1(An example block diagram. Pictured is a simplified representation of an
audio amplification system.) 

 Pictorial Diagrams:

Pictorial diagrams are one of the types of diagrams that provide detail. They help you visualize circuits and their operation by showing components as they actually appear. For example, the circuit of Fig 2.2 consists of a battery, a switch, and an electric lamp, all interconnected by wire. Operation is easy visualize—when the switch is closed, the battery causes current in the circuit, which lights the lamp. The battery is referred to as the source and the lamp as the load.

  

Fig.2.2(A pictorial diagram. The battery is referred to as a source while the lamp
is referred to as a load.)

 Schematic diagrams:

Schematic diagrams get around this by using simplified, standard symbols to represent components.

Fig.2.3 shows an example of Schematic diagrams. In this, a battery, switch and resistor symbols are used.

Fig.2.3(Schematic

Systems of units

   Electric Circuit: Electrical circuit is a system of interconnected components such as resistors, capacitors, inductors, voltage sources and so on.

   Circuit Theory:The concepts, mathematical relationships, methods of analysis which have evolved from these circuits are known as circuit theory.

   Electric Charge:Charge is an electrical property of the atomic particles of which matter consists, measured in coulombs (C).

The following points should be noted about electric charge:

1. The coulomb is a large unit for charges. In 1 C of charge, there 1/1.602X10 ^-19 =    6.24X10 ¹⁸ are electrons. Thus realistic or laboratory values of charges are on the order of pC, nC, or uC .
2. According to experimental observations, the only charges that occur in nature are integral multiples of the electronic charge e= -1.602X10 ^-19
3. The law of conservation of charge states that charge can neither be created nor destroyed, only transferred. Thus the algebraic sum of the electric charges in a system does not change.

System of Units:

System International:

The system of units is Known as System international.

Length:   The distance travelled by light in a vacuum in 1/299 792 458 of a second

Second:   The second is defined in terms of the period of a cesium-based atomic clock.

Kilogram:  The definition of the kilogram is the mass of a specific platinum-iridium cylinder (the international prototype), preserved at the International Bureau of Weights and Measures in France.

Electric Current: Electric current is the time rate of change of charge, measured in amperes (A).

Temperature: The degree of hotness of a body is called Temperature

Basic units are shown in Table 1.1

Table 1.1

Force: Force is a physical quantity, which changes or tend to changes the position of a body. Force is measured in Newton. The force,which produces an acceleration of 1-meter-second square in a body of mass 1 kg is 1 Newton. It is denoted by N.
Energy: The capacity of a body to do work is called Energy.The unit of the energy is joule, if a force of 1 N acts on a body and displaces it through a distance of 1 m in its direction, then the work done or energy will be 1 joule. It is denoted by j.
Power: Work done per unit time is called power. or The rate of doing work by an agency is called power.  The unit of the power is Watt,  if an agency performs 1 joule of work in 1 sec, then the power will be 1 watt.It is denoted by W.
Voltage(Potential difference): The potential energy per unit charge is called potential of that point.The difference of the potential energy per unit charge between two points is called Potential Difference or voltage.                                                                                OR
Work done in moving a unit positive charge from one point to other point against the electric field keeping the charge in electrostatics equilibrium is called Potential Difference or voltage.The unit of the voltage is volt, The voltage between two points will be one Volt if one joule of work is done to move a positive charge of one coulomb from one point to other point keeping the charge in equilibrium.
Charge: Charge is an electrical property of the atomic particles of which matter consists, measured in coulombs (C).
Resistance:The opposition offered by the atoms of the conductor to the flow of electric current is called Resistance.It is measured in ohms, the resistance of a conductor is said to be one ohm if a potential difference of 1 volt produces a current of 1 ampere in it.
Capacitance:The ability or capability of capacitor to store charge is called Capacitance. The unit of capacitance is Farad, The capacitance of the capacitor is one Farad if a charge of one coulomb given to one of the plates of a parallel plate capacitor, produces a voltage of one volt between them.
Inductance:  The ratio of the average emf to the change of current with time in the coil. The unit of  inductance is Henry. One henry is the inductance of the coil in which a rate of change of current of one ampere per second causes an induced emf of one volt in the coil itself. It is denoted by H. 
Frequency: The number of waves passing through a point is called Frequency.It is measured in Hertz denoted by Hz.
Magnetic Flux: The total number of magnetic lines of forces passing through any surface placed perpendicular to magnetic field is called magnetic flux. The unit of Magnetic flux is weber. It is denoted by Wb.
Magnetic Flux density: The flux per unit area held perpendicular to magnetic field is called Magnetic flux density. The unit of the flux density is Tesla denoted by T.
 Derived units are shown in Table 1.2 

Table 1.2

 Conversions of Units is shown in the Table 1.3

Table 1.3

 The System International prefixes are shown in Table 1.4

Table 1.4