Electricity – An Introduction

We explore some basic questions about electricity like What constitutes electricity? How does it flow in an electric circuit? What are the factors that control or regulate the current through an electric circuit?

ELECTRIC CURRENT

(i)  Electrons are key to understanding electric current.  Electron possesses a fundamental property called electric charge. 

(ii)  The charge of an electron is negative1.6 × 10–19 C.  The SI unit of electric charge is coulomb (C).

(iii)  1 coulomb is equivalent to the charge contained in nearly 6 × 1018 electrons.

(iv)  The flow of electrons in a conductor constitutes an electric current.

(v)  Electric current is expressed by the amount of charge flowing through a particular area in unit time.

(vi)  If a net charge Q, flows across any cross-section of a conductor in time t, then the current I, through the cross-section is

Current Equation

(vii)  The electric current is expressed by a unit called ampere (A). One ampere is the flow of one coulomb of charge per second, that is,

1 A = 1 C/1 s

(viii)  Conventionally, in an electric circuit the direction of electric current is taken as opposite to the direction of the flow of electrons, which are negative charges.

(ix)  Ammeter is an instrument used measures electric current in a circuit. It is connected in series in a circuit.  We cannot see current as it is due to flow of electrons, which being very small are invisible to the naked eye.

(x)  For electrons to flow, in other words, for electric current to flow two things are required:

(a) A source of potential difference; and

(b) A circuit which is continuous/unbroken.

ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE

(i)  Electric Potential at a point is defined as the amount of work done in bringing a unit charge to that point.

(ii)  The difference in electric potential at two points say A and B are termed as potential difference.

(iii)  Potential difference is the reason, due to which electrons flow in a conductor or a circuit from one point to another.

(iv)  Potential difference may be produced by a battery, consisting of one or more electric cells. The chemical action within a cell generates the potential difference across the terminals of the cell, even when no current is drawn from it.

(v)  When the cell is connected to a conducting circuit element, the potential difference sets the charges in motion in the conductor and produces an electric current.

(vi)  We define the electric potential difference between two points as the work done to move a unit charge from one point to the other.

Potential difference (V) = Work done (W)/ Charge (Q) between two points

    V = W/Q

(vii)  The SI unit of electric potential difference is volt (V). One volt is the potential difference between two points in a current carrying conductor when 1 joule of work is done to move a charge of 1 coulomb from one point to the other.

1 volt =1 joule/1 coulomb

1 V = 1 JC–1

(viii)  The potential difference is measured by means of an instrument called the voltmeter. Voltmeter is always connected in parallel across the points between which the potential difference is to be measured.

CIRCUIT DIAGRAM

(i)  A continuous and closed path of an electric current is called an electric circuit.

(ii)  Here is a diagram of an electric circuit:

(iii)  Electric circuit comprises a cell (or a battery), a plug key, electrical component(s), and connecting wires.

(iv)  One interesting thing.  Electrons flow from negative terminal of a battery to its positive terminal.  But, flow of current is taken from positive terminal of the battery to the negative terminal.  Blame this confusion on Benjamin Franklin.

(v)  Different components of the circuit are represented by the symbols shown below:

Electric Symbols
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