System of Units:

The unit system we used in engineering is the international system of units, they also referred as SI units. The basic units are meter (m), kilogram (kg), second (s), ampere (A).

The standard prefixes which we employ in SI units are:

Basic Quantities:

The most commonly used word in Electrical Engineering is circuit. We can define it as interconnection of electrical components. The most elementary quantity in an analysis of electric circuit is the electric charge we are dealing with it because when charge in motion cause energy transfer. An electric circuit is a structure in which charge flows from one point to another. The rate of change of charge or the flow of charge is called Current. Mathematically it can be represented as

 

while q is charge and i is current. The SI unit of current is ampere  (A). We can define it as 1 Coulomb per second.

There are two types of currents 1. Alternating Current 2. Direct Current. AC current is most commonly used current in household appliances such as refrigerator,stove,washing machine and so on but small electronic components/devices such as mobile batteries adopters are run on DC current. The figure given below graphically AC

                                                                    

and DC current. We can also called DC current as static current because it remains constant with time but not same in case of AC current.

Story

“We know that in metals inner small molecules (electrons) are continuously in random motion but when we apply potential on them to move in a certain path then applied potential is called Voltage and rate of change of charge is called Current as mentioned above”

Voltage also called potential difference(difference of electric potential between two points),electromotive force(emf)

Power is product of voltage and current and it’s SI unit is watt (W). The figure given below is a electric circuit in

 

which current and voltage are given and arrow is representing the direction of current.Passive sign convention is a convention in which when current (I) enters from +ve side and leaves from -ve side then it’s power is +ve it means power is absorbed by the element and if current (I) enters from -ve side and leaves from +ve side then it’s power is    -ve which means power is supplied by the element.

Example: Determine the power and also determine that circuit is supplying and absorbing the power.

 

In this figure current I=2A and voltage V=4V we can see that current is entering from +ve terminal so

P=(2)(4)

P=8W

as power is +ve so it is a absorbing circuit.

Example: Determine the unknown variables

for figure 1 we have current I=-2A and power P=40W we know that P=VI by using this formula we can find V1

V=P/I

                                                                                                     V=40W/-2A                        =-20V

for figure 2 we have power P=-50W and voltage V=10V apply above formula to find I

I=P/V

                                I=-50W/10v           =-5A

Circuit Elements:

We can classify electric circuits by two things either they can absorb or supply energy.An active element is capable of generating energy (batteries,generators) and a passive element cannot generate energy but they can store energy(resistors,capacitors,inductors). We can classify active element as

1. Independent Sources:

Independent Voltage Source is a circuit element in which specific voltage is present between the two terminals and it is represented by a circle while Independent Current Source is a circuit element in which specific current is present between the two terminals regardless of voltage across it’s terminals. It is represented by a circle and a specific arrow inside it.

These elements provide power to the other components of the circuit we can also use them in a circuit as a passive element.

Example: Determine the power absorb or supplied by the elements

for the main voltage source we see that current is entering from -ve terminal so it’s a supplying element it’s power is

P=(24V) (2A)=48W for element 1 current I=2A and voltage V=6V so power P=(2A) (6V)=12W absorbing power for element 3 current I=2A and voltage V=18V so power is P=(18V) (2A)=36W absorbing power.

2.Dependent Sources:

Dependent voltage and current sources generate current or voltage that is determined by voltage or current at a specific location in the circuit. For example metal-oxide semiconductor field-effect transistors (MOSFET’S). They are represented by diamond shape dependent source. Following figures showing them

 

Example: Determine the output

 

from this figure we can see that we have to calculate V0 we know the value of Vs=3V just insert that value in V0 so

V0=20Vs

                      V0=(20)(3)             =60V

 

Next Page:

Resistive Circuits

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