Charge

The most basic quantity in an electric circuit is the electric 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 1.602×10^-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

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  or  ( ).   Picture below shows direct current and alternating current.

Current is always measured through a circuit element as shown below:

Current through Resistor (R)

Two types of currents – A direct current (DC) is a current that remains constant with & An alternating current (AC) is a current that varies with:

Two common types of current: (a) direct current (DC), (b) alternative current (AC)