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rcu t na ys sChapter # 1

Basic Concepts

Mazhar Javed

Faculty of Electronics EngineeringGhulam Ishaq Khan Institute of Engineering Sciencesand Technologies, Topi, Pakistan

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Todays Lecture

Electric Current

Electric Voltage

Electric Energy

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Basic Quantities

1. Electric Charge (elementary quantity) and current

.Electric circuit facilitates the motion of charge from one pointto another in closed path.

Time rate of change of charge is electric current (equation 1.1)

Basic unit of Current is 1 Ampere = 1 Coulomb per second

or I = Q/s

ELECTRONONEOFCHARGETHEIS(e)

(e)106.28COULOMB1 18=

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Strictly speaking current is a basic quantity and charge is derived. However,physically the electric current is created by a movement of charged particles.

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More concepts on current

More concepts on current:

c arge par c e may e a ree e ec ron s su ec eto a steady state force (F=qE) under a steady electricfield E

In a conductor, this charge undergoes collision withmassive stationary ions of the material, causing theirran om c ange n t e mot on o t e ree e ectrons.

The net effect of the electric field E is that , there is aslow net motion or DRIFT of the moving charges as a

group in the direction of the electric force F. This motion is described in terms of the drift velocityv

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of the particle. As a result there is a net current in theconductor

Heat is also generated due to the random collision ofcharges with Ions causing the rise in temperature

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More on Current For different current carrying materials, the charged particlecan be negative or positive: Metal has negative (electron)moving charge. Ionized gas (Plasma) or ionic solution has both

e ectrons an pos t ve y c arge ons. em con uctors anGa) has electrons and vacancies (holes) as moving parts)

In circuit analysis, the conventional current is considered as themotion/flow of the positive charge mainly for simplicity. Sign ofmoving charge is not of any significance in analyzing thee ectr c c rcu ts. ere we assume t e e ectr c c arge to move nthe same direction as that of the current.

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More on Current

Finally, Current through the cross sectional area A is the netcharge flowing through the area per unit time. Thus the timerate of change of charge constitutes an electric current.

at emat ca y:.

==t

dxxitqordt

tdqti )()(

)()(

Note that current is not a vectorquantity. Current is always along

the length of wire.

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EXAMPLE

s nq =

=)(ti )120cos(120104 3 t ][A

][)120cos(480.0)( mAtti =

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EXAMPLE