Electric charge, current, voltage, power, energy and circuit elements

INTRODUCTION

Electric circuit is a very fundamental knowledge required for anyone who interested to be an electrical engineer. In order to understand electrical circuit, all the components, quantities and units must be understood beforehand. The quantities can be divided into two categories that are electrical and magnetic.

ELECTRICAL AND MAGNETIC UNIT

Table 1 and Table 2 are electrical quantities and magnetic quantities respectively. There is no other ways to understand this table except memorizing it.

Table 1: Electrical Quantities

Table 2: Magnetic Quantities

ELECTRIC CHARGE

Charge is an electrical property of the atomic particles of which matter consists, which is measured in coulombs (C). Electronic charge for one electron is 1.602 x 10^-19 C. The charges that occur are integral multiples of the electronic charge. Therefore, in one coulombs it contains 6.24 x 10^18 electrons.

CURRENT

Electric current is the time rate of charge, which is measured in amperes (A). 1A=1C/s. That means in one amperes the charge is one coulomb in one seconds but one coulomb is equal to 6.24 x 10^18 electrons. Mathematically, the relationship between current (i), charge (q) and time t is i = dq/dt. The relationship says that current is basically equal to the variation of charge with time. There are two types of currents that are direct current (DC) and alternating current (AC). Direct current is a current that remains constant with time and alternating current is a current that is varies with time. In details, direct current is always 5V (for example) at one second, two seconds or even twenty seconds. Alternating current however is the current that is 2V (for example) at one second, 5V (for example) at two seconds and perhaps 0V (for example) at fifteen seconds. Where is the direction of current flow? Current flow is followed the direction of positive ions or against the negative ions.

VOLTAGE

Voltage (or potential difference) is the energy required to move a unit charge through an element, measured in volts (V). Electric voltage is always across the circuit element or between two points in a circuit. Mathematically, V = dw/dq where w is energy in joules and q is charge in coulombs.

POWER AND ENERGY

Power is the time rate of expending or absorbing energy, measured in watts (W). Mathematical expression of power is p=dw/dt=(dw/dq)*(dq/dt)=vi. If the current enters the positive terminal of an element, this is absorbing power whereas if the current enters through the negative terminals, it is supplying power. Energy is the capacity to do work, measured in joules (J).

CIRCUIT ELEMENTS

There are two types of elements that we will cover in this chapter. The elements are active and passive elements. An active element is an element that is capable of generating energy while a passive element is an element that is not capable of generating energy. Example of passive elements are resistor, inductor and capacitor whereas for active elements are sources. Sources in the other hand can be divided into two categories that are independent and dependant sources. In details, an independent source is an active element that provides a specified voltage or current that is completely independent of other circuit elements. A dependent source however is an active element in which the source quantity is controlled by another voltage or current. In order to differentiate the symbol of independent and dependent sources, we have to look at the shape that is square shape for dependence source but circle shape for an independent source.