CHARGE

The most basic quantity in an electric circuit is the electric charge. We all experience the effect of electric charge when we try to remove our wool sweater and have it stick to our body or walk across a carpet and receive a shock.Charge is an electrical property of the atomic particles of which matter consists, measured in coulombs (C). Charge, positive or negative, is denoted by the letter q or Q.
We know from elementary physics that all matter is made of fundamental building blocks known as atoms and that each atom consists of electrons, protons, and neutrons. We also know that the charge ‘e’ on an electron is negative and equal in magnitude to 1.602x10-19 C, while a proton carries a
positive charge of the same magnitude as the electron and the neutron has no charge. The presence of equal numbers of protons and electrons leaves an atom neutrally charged.


 CURRENT

Electricity may be defined as a form of energy. It involves making and using energy. It may also be defined as a way in which materials behave.
Sometimes people use the term ‘electricity’ as the name for a material that flows through a
solid wire, motion of this strange material is called electric current.
Current can be defined as the motion of charge through a conducting material, measured in Ampere (A). Electric current, is denoted by the letter i or I.
The unit of current is the ampere abbreviated as (A) and corresponds to the quantity of total charge that passes through an arbitrary cross section of a conducting material per unit second.
Mathematically,
 
 Where is the symbol of charge measured in Coulombs (C), I is the current in amperes (A) and t is the time in second (s).
The current can also be defined as the rate of charge passing through a point in an electric circuit.
Mathematically,

The charge transferred between time t1 and t2 is obtained as
 
A constant current (also known as a direct current or DC) is denoted by symbol I whereas a time￾varying current (also known as alternating current or AC) is represented by the symbol i or i(t).
Figure shows direct current and alternating current.
Two types of currents:
1) A direct current (DC) is a current that remains constant with time.
2) An alternating current (AC) is a current that varies with time.

VOLTAGE (or) POTENTIAL DIFFERENCE

To move the electron in a conductor in a particular direction requires some work or energy transfer. This work is performed by an external electromotive force (emf), typically represented by the battery in Fig. 1.3. This emf is also known as voltage or potential difference. The voltage Vab between two points and b in an electric circuit is the energy (or work) needed to move a unit charge from a to b.

Voltage (or potential difference) is the energy required to move charge from one point to the other, measured in volts (V). Voltage is denoted by the letter v or V.
Mathematically,
                        Vab=dw/dt
where w is energy in joules (J) and q is charge in coulombs (C). The voltage Vab or simply V is measured in volts (V).
1 volt = 1 joule/coulomb = 1 newton-meter/coulomb

Fig. 1.3 shows the voltage across an element (represented by a rectangular block) connected to points a and b. The plus (+) and minus (-) signs are used to define reference direction or voltage polarity. The Vab can be interpreted in two ways: (1) point a is at a potential of Vab volts higher than point b, or (2) the potential at point a with respect to point b is Vab . It follows logically that in general
                                             Vab= -Vba

POWER

Power is the time rate of expending or absorbing energy, measured in watts (W). Power, is denoted by the letter p or P.
Mathematically,
                         p=dw/dt
Where p is power in watts (W), w is energy in joules (J), and t is time in seconds (s).
From voltage and current equations, it follows that;
                                             p=dw/dt=dw/dq*dq/dt=V*I
Thus, ifthe magnitude of current I and voltage are given, then power can be evaluated as the product of the two quantities and is measured in watts (W).
Sign of power:
Plus sign: Power is absorbed by the element. (Resistor, Inductor)
Minus sign: Power is supplied by the element. (Battery, Generator)
Passive sign convention:
If the current enters through the positive polarity of the voltage, p = +vi
If the current enters through the negative polarity of the voltage, p = – vi

ENERGY

Energy is the capacity to do work, and is measured in joules (J).
The energy absorbed or supplied by an element from time 0 to t is given by,
 
The electric power utility companies measure energy in watt-hours (WH) or Kilo watt-hours (KWH)
                 1 WH = 3600 J

Electron Theory

An element is defined as a substance which cannot be decomposed into other substances.
The smallest particle of an element which takes part in chemical reaction is known as an
atom.
- All matter is composed of atoms which are infinitesimally small.
- The atom, itself, is composed of electrons, protons and neutrons. The number and
arrangement of these particles determines the type of atom : oxygen, carbon, copper, lead
or any other element.
- Weight, colour, density, and all other characteristics of an element are determined by the
structure of the atom. Electrons from lead would be the same as electrons from any other
element.
- The ‘electron’ is a very light particle that spins around the centre of the atom. Electrons
move in an orbit. The number of electrons orbiting around the centre or nucleus of the
atom varies from element to element. The electron has a negative (–) electric charge.
- The ‘proton’ is a very large and heavy particle in relationship to the electron. One or more
protons will form the centre or nucleus of the atom. The proton has a positive (+) electrical
charge.
-The ‘neutron’ consists of an electron and proton bound tightly together. Neutrons are
located near the centre of the atom. The neutron is electrically neutral ; it has no electrical
charge
-Normally, the atoms are electrically neutral, that, the number of electrons and protons
are the same, cancelling out each other’s electrical force. Atoms ‘‘stay together’’ because
unlike electrical charges attract each other. The electrical force of the protons holds the
electrons in their orbits. Like electrical charges repel each other so negatively charged
electrons will not collide with each other.

 Electric Circuit

An ‘‘electric circuit’’ is a conducting path through which either an electric current flows, or is
intended to flow. It can be divided into four categories :
(i) Closed circuit (ii) Open circuit
(iii) Short circuit (iv) Earth or leakage circuit.

(i) Closed circuit. It is the complete path for flow of electric current through the load.
Example. The glowing of a bulb.
(ii) Open circuit. In case any one of the supply wires is disconnected or the fuse burns out,
then the current will not flow through the bulb, which is an example of open circuit.
(iii) Short circuit. If the supply mains are connected directly by a piece of wire without any
load, then the value of current will be much greater than that in closed circuit. Hence, the fuse gets
blown off and this circuit is known as short circuit.
(iv) Earth or leakage circuit. If any wire of supply mains touches the ‘body of an appliance’,
then it is known as earth or leakage circuit.
Depending upon the type of current flowing in it, an electric circuit may be further classified
as :
(i) D.C. circuit (ii) A.C. circuit.