electric current and ohm's law.pdf

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4/17/2016 Electric Current and Ohm's Law : Theory Page 1

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Electric Circuits and Ohm's Law : Concepts

BASIC DATA ABOUT ATOM :Mass of electron = 9.11 x 10 kg

Mass of proton = 1.67 x 10 kg

Mass of neutron = mass of proton

Mass of electron = 1/1840 mass of proton

Diameter of nucleus is of the order of 10 m

Diameter of orbits = 10 times dia of molecule

Diameter of electron = 10 m

Charge on electron = - 1.602 x 10 coulomb

Charge on proton = + 1.602 x 10 coulomb.

UNIT OF CURRENT :

The charge on an electron is measured in terms of coulomb. The unit of current is coulomb per second and is called

ampere.Thus

I (Ampere) = coulomb/second = ∆ q / ∆ t

One coulomb is equivalent to the charge of 6.28 x 10 electrons.

1 emu of current = 3 x 10 esu of current.

ELECTROMOTIVE FORCE :Electromotive force or potential of a body is the work done in joules to bring a unit electric charge from inတnity to the

body. It is expressed in terms of volts (V).

The potential dierence is deတned as that which causes current to ow in the closed circuit.

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RESISTANCE :Resistance is the property of a substance due to which it opposes the ow of electrons (i.e., electric current) through it.

The unit of resistance is ohm ( ).

Metals, acids and salt solutions are good conductors of electricity. Silver, copper and aluminium oer least resistance

to ow of current and are called very good conductor of electricity. The electrons while owing through the molecules

or the atoms of the conductor, collide with other atoms and electrons, thereby producing heat.

Some substances oer relatively greater di†culty or hindrance to the passage of these electrons. Such substances are

called poor conductors or insulators of electricity. Some of the insulators are glass, bakelite, mica, rubber, polyvinyl

chloride (P.V.C.), dry wood, etc.

The resistance of a conductor depends on:

1. Length of conductor- it varies directly with the length

2. Cross-sectional area of the conductor - it varies inversely with the cross-sectional area

3. Its resistivity i.e. the nature of composition, etc., of the material of which the conductor is made up

4. Temperature of the conductor - it almost varies directly with the temperature. Thus R, the resistance of a

conductor is given by

R = l / A

where

= speciတc resistance or resistivity of the material,

l = length of the conductors,

A

= cross-sectional area of conductor.

Ohm s Law :

If the temperature and other conditions remain constant, the current through a conductor is proportional to the

applied potential dierence and it remains constant. Thus


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Current = Applied Voltage / Resistance of the circuit

Resistance = Applied voltage / Current in the circuit

Potential across resistance = Current x Resistance.

Conditions for Ohm s Law :

1. Ohm's law can be applied either to the entire circuit or a part of a circuit.

2. When ohm's law is applied to a part circuit, part resistance and the potential across the part resistance should be

used.

3. The Ohm's law can be applied to DC as well as AC circuits. However, in case of AC circuits impedance Z, is used in

place of resistance. Thus

I = E / Z = Applied voltage / Impedance in the circuit

Conductance (G ) :Conductance is the reciprocal of ( R ) and is measure of the ease with which the current will ow through a substance.

Thus

G= 1 / R

The unit of conductance is mho.

ELECTRICAL POWER :Electrical power is expressed in terms of watts (W) and is given by

W= E x I = I x R = E / R

2


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Electric Circuit > Theory Page 2 → (/electric-circuits/electric-circuits-2.htm)

Power is also expressed in terms of

kW ( kilowatt ) ( =1000 W ) or

MW ( megawatt ) which is 1000 kW or 1000,000 W.

Electrical Energy :Electrical energy is expressed in terms of kilowatt hours (kWh). Thus

1 kWh = 1 kW x 1 hour = 1000 watt-hours = 1000 x 60 x 60

watt-sec.

RESISTANCE COMBINATIONS :

Resistances in series :

When resistances are connected in series, same current ows through all resistances, and overall resistance R, is givenby

R = R + R + R

Also,

V = V + V + V = IR + IR + IR .

Discuss Electric Circuit Concept : Page 1 !!!

1 2 3

1 2 3 1 2 3

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