VOLTAGE

As explained in the previous section, to move the electron in a conductor in a conductor in a particular direction requires some work or energy transfer. This work is done by an external electromotive force (emf), typically represented by battery. This emf is also known as potential difference or voltage. Actually, whenever positive and negative charges are separated, energy is expanded.
Voltage is the energy per unit charge created by the separation. Thus, the voltage V₁₂ between two point 1 and 2 in an electric circuit is the energy or work needed to move a unit charge from 1 to 2. We express this ratio in differential form as :-
v=V₁₂=dw/dq................ (1.3)

where, 

w=the energy in joule

q=the charge in coulombs

v=V₁₂=the voltage in volts

From equation (1.3) it is evident that
 1 volt=1 joule/coulomb=1 newton-meter/coulomb

Thus, voltage or potential difference is the energy required to move a unit charge through an element.

Figure below shows the voltage across a lamp connected between point 1 and 2.

The plus (+) and minus (-) sign s are used to represent reference direction or voltage polarity. The voltage V₁₂ can be interpreted in two ways. 

1. Point 1 is at a potential of V₁₂ volts higher than point 2.
2. The potential at point 1 with respect to potential 2 is V₁₂.

Therefore, logically it follows that,

In figure (a), point 1 is +10V above point 2; in figure (b), point 2 is -10V above point 1. We can say that in figure (a) there is a 10V voltage drop from point 1 to 2 or equivalently a 10V voltage rise from point 2 to 1. In general, a voltage drop from 1 to 2 is equivalent to a voltage rise from 2 to 1.

CHARGE AND CURRENT

The concept of electric charge is the underlying principle for explaining all electrical phenomena. The most basic quantity in an electric circuit is the electric charge. Important characteristics of electric charge are :-

1. The charge is bipolar, meaning the electrical effects are described in terms of positive and negative charges.
2. According to experimental observation, the only charges that occurs in nature are integral multiple of electronic charge e=-1.602*10-19 coulomb.
3. Electrical effects are attributed to both the separation of charge and charges in motion.
4. The law of conservation of charges states that charge can neither be created nor destroyed, only transferred. Thus, the algebraic sum of electric charges in a system does not change.

Effect of electric charge can be experienced when we try to remove our woolen sweater and have it stick to our body or walk across a carpet and receive shock.

Charge is an electric property of the atomic particles and is measured in coulombs(C).

Now, consider the flow of electric charges. A unique feature of electric charge is that it can be transferred from one place to another, that means it is mobile, where it can be converted to another form of energy. The motion of charge creates an electric fluid(current).

We know that a conducting wire consists of several atoms and a battery is a source of electromotive force. When a conducting wire is connected to a battery, the charges are compelled to move. Positive charges move in one direction while negative charges move in opposite direction.

This motion of charges creates electric current. Convention is to take the current flow as the movement of positive charges, that is, opposite to the flow of negative charges as shown in figure below: -

The electric effects caused by charges in motion depends on the rate of charge flow. The rate of charge flow is known as the electric current.

Mathematically, the relationship between current, charge and time is,

                                        
$\; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \;$

where, 
i=the current in Ampere

q=the charge in Coulomb

t=the time Second

The charge transferred between time t₀ and t is obtained by integrating both sides of equation (1.1) we get;

Equation (1.1) suggests that current need not be a constant valued function.

When a current is constant with time, we say that we have direct current (dc). Thus, a direct current (dc) is a current that remains constant with time.

On the other hand, a current that varies with time, reversing direction periodically is called alternating current (ac). Thus, an alternating current is a current that varies with time periodically. Alternating current is used in our household, to run the refrigerator, toaster, air conditioner and other electrical appliances.

 As mentioned earlier, direction of current flow is conveniently taken as the direction of positive charge movement. Based on this convention, a current of 4Amp may be represented positively or negativity. This is shown in figure below where a lamp is connected in series with a battery.

In above figure i₁₂ =4Amp, this means the current through the lamp with its reference direction pointing from 1 to 2. Similarly, i₂₁ is the current with its reference directed from 2 to 1. Of course, i₁₂ and i₂₁, are the same in magnitude and opposite in sign, because they denote the same current but with opposite direction. Thus, we have,

SYSTEM OF UNITS

System of Units

We all electrical engineers deals with several measurable quantities. However, our measurement must be communicated in standard language such that an engineering professional can understand, irrespective of the country where the measurement is conducted.

Such an international measurement language is the International System of Units (SI), adopted by the General Conference on weights and measures in 1960. In this international system there are six principle units from which the units of all the other physical quantities can be obtained. One major advantage of the SI unit is that it uses prefixes based on the power of 10 to relate smaller and larger units to the basic unit.

Quantity	Basic Unit	Symbol
Luminous Intensity	Candela	Cd
Thermodynamics Temperature	Kelvin	K
Length	Meter	m
Mass	Kilogram	Kg
Time	Second	s
Electric Current	Ampere	A

The SI unit Prefix are: -

Multiplier	Prefix	Symbol
1018	exa	E
1015	penta	P
1012	tera	T
109	giga	G
106	mega	M
103	kilo	K
102	hecto	H
10	deka	da
10-1	deci	d
10-2	centi	c
10-3	milli	m
10-6	micro	µ
10-9	nano	n
10-12	pico	p
10-15	femto	f
10-18	atto	a

INTRODUCTION TO CIRCUIT

All branches of electrical engineering are based on electric circuit theory and electromagnetic theory. Several branches of electrical engineering such as Power System, Control System, Electrical Machine, Instrumentation, Electronics, Communication are based on electric circuit theory. Therefore, basic electric theory course is very important course for an electrical engineering and of course an excellent starting point for a first semester students in Electrical Engineering. Circuit Theory course is also very important to students specializing in other branches of the physical sciences because of applied mathematics, physics, and topology involved. Also circuits are a good model for study of energy systems.

In Electrical Engineering, we are often interested in transferring energy from one point to another. To do this, we require an interconnection of electrical devices and such interconnection is known as an electric circuit. Each component of the electric circuit is known as an element. Therefore, an electric circuit is an interconnection of electrical element.

Electrical charge(electrons) will flow through the circuit due to chemical forces in the battery. The charge gain energy from the chemicals in the battery and delivers energy to the lamp. The battery voltage is a measure of the energy gained by unit of charge as it moves through the battery.

The wires are made of copper conductor and are insulated from one another by electrical insulation coating. Electrons readily move through the copper conductor but not through the plastic insulation.

The switch is used to allow or disallow the flow of current through the circuit. Current will flow through the circuit when the conducting metallic parts of switch make contact and we say that the switch is closed. On the other hand, when the conducting parts of the switch don't make contact, current doesn't flow through the circuit and we say that switch is open.

The lamp contains Tungsten wire that can withstand high temperature. The Tungsten is not as good as copper, and the electrons experience collisions with the atoms of the Tungsten wires, resulting in heating of the Tungsten. We can say that Tungsten wire has electrical resistance. Thus energy is transferred by the chemical action in the battery to the electrons and then to the Tungsten where it appears as heat. The Tungsten becomes hot enough so that light is emitted.