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Electric Current – dc basic concepts

DC BASIC CONCEPTS, The controlled movement of electrons through a

substance is called the electric current.

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Electric current may be also defined as the time rate of net motion of

an electric charge across a cross-sectional boundary (Fig.).

A random motion of electrons in a metal does not constitute a current

unless there is a net transfer of charge with time.

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I.e., Electric current, 

I = Rate of transfer of electric charge 

= dQ / dt …….. [1]

Coulomb is the practical as well as SI unit for measurement of electric charge.

One coulomb is approximately equal to 624 x 10¹⁶ Electrons. 

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Since current is the rate of flow of electric charge through a conductor and

coulomb is the unit of electric charge, the current may be specified in coulombs

per second. In practice the term coulomb per second is seldom used, a shorter

term, ampere is used instead.

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Electromotive Force and Potential Difference [ dc basic concepts ]

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Electromotive force ( emf ) is the force that causes an electric current to

flow in an electric circuit while the potential difference ( pd ) between

two points in an electric circuit is that difference in their electrical state

which tends to cause flow of electric current between them. 

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Volt is a unit of electromotive force as well as potential difference in practical

as well as in SI system of units. 

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The volt is defined as that potential difference between two points of a conductor

carrying a current of one ampere when the power dissipated between these points

is equal to one watt. 

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Resistance

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Resistance may be defined as that property of a substance which opposes

(or restricts) the flow of an electric current (or electrons) through it. 

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The practical as well as MKS (or SI) unit of resistance is ohm (Ω), which

is defined as that resistance between two points of a conductor when a

potential difference of one volt, applied Between two points, produces

in this conductor a current of one ampere, the conductor not being

source of any emf.

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for insulation having high resistance, much bigger units kilo ohm or

kΩ (10³ ohm) and mega ohm or MΩ (10⁶ ohm) are used. In case of very

small resistances smaller units like milli ohm (10^-3 ohm) or micro ohm

(10^-6 ohm) are employed. 

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Ohm’s Law

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[ DC BASIC CONCEPTS ……….. —- ]

The current flowing through a conductor is directly proportional to the

potential difference across the ends of the conductor and inversely

proportional to the conductor resistance. 

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This relation was discovered by Georg Simon Ohm and so it is known as Ohm’s law. 

If I is the current flowing through a conductor of resistance R across which a

potential difference V is applied then according to Ohm’s law. 

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V / I = Constant = R  —— [2] 

for no change in physical state. 

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Ohm’s law cannot be applied to 

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OHM'S LAW CANNOT APPLIED

(I) Magnetically coupled circuits such as transformers 

(II) Vacuum tubes 

(III) Gas filled tubes 

(IV) semiconductor devices such as diodes, transistors, thyristors 

(V) Temperature dependent devices such as thermistors 

(VI) Photo-sensitive devices such as photo Electric devices, LDRS 

etc.; and 

(VII) Non-linear elements such as powdered carbon, thyrite, electric 

are etc.

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Types of Supply

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There are two types of supply viz. DC and AC.

The voltage or current available from batteries or solar cells is direct

in the sense that the polarity remains the same. Such sources are called

direct current sources.

A plot of output of such sources with respect to time is a straight line parallel

to time axis as illustrated in Fig. 

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This shows that the voltage/current output of a de source is constant with

respect to time, unless the chemicals in the battery are exhausted or the light

incident on solar cell varies.

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Power obtainable from batteries or solar cells is very small as compared to

the total power we need.

The de power in bulk can be generated by using dc

generators. Standard dc voltages are 230 and 460 V.

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A current (or voltage) is called alternating if it periodically changes its direction

and magnitude where as direct currents are steady and in one direction.

In more restricted sense, alternating current is a periodically varying current,

the average value of which, over a period, is zero. 

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