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Electrical Engineering andElectronics (EEE)

Course on

(Esh114t)LECTURE 2

School of Petroleum Technology

Pandit Deendayal Petroleum University, Gandhinagar

Brijesh Tripathi

2nd Year, III Semester

1

Common prefix multipliers

2

Resistance Vs current for constant voltage

E

Current versus resistance through an electricdevice when the voltage is constant at 1 V.

R=

3

Ohms law with different resistance

E

Relative current versus relative voltage fordifferent resistances

R

4

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Voltage measurement

Whenever a resistance carries acurrent, there is a voltage across it

5

DC (Direct Current)

A representation of pure DC

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Kirchhoff Current Law (KCL)

The physicist Gustav Robert Kirchhoff (1824-1887)was a researcher and experimentalist in electricity

back in the time before radio, before electric lighting,and before much was understood about how currents

.

Kirchhoff reasoned that current must work somethinglike water in a network of pipes, and that the currentgoing into any point has to be the same as thecurrent going out. This is true for any point in a

circuit, no matter how many branches lead into or outof the point.

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Kirchhoff Current Law

In a network of water pipes that does not

leak, and into which no water is addedalongthe way, the total volume of water going in

The current into Zequals currentout of Z(I1+I2=I3+I4+I5 )

going out. Water cant form from nothing,nor can it disappear, inside a closedsystem ofpipes. Electric current, thought Kirchhoff,must act the same way in an electriccircuit.

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Kirchhoff Current Law

The Current into X or Y is the same as

the current out of X or Y (I= I1+ I2)

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Kirchhoff Current Law

The algebraic sum of the branch currents at a

node is zero at every instant of time

(Alternative form) The sum of the branch currentsentering a node at a given instant of time is equal tothe sum of the currents leaving the node at thatnstant o t me

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Kirchhoff Voltage Law (KVL)

The algebraic sum of the product of current &

resistance of various branches of a closed mesh

of a circuit plus the algebraic sum of the emfs in

that closed mesh is equal to zero.

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The sum of the voltages across theresistors is equal to, but has opposite

polarity from, the supply voltage E.Thus E1 + E2 + E3 + E4 = E

KVL

321VVVV

s++=

As the current I passes in the circuit, the sum of thevoltage drops around the loop is equal to the totalvoltage in that loop.

The direction of current : It leaves thepositive terminal of the voltage sourceand enters into the negative terminal

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KVLConsider the problem offinding out the currentdelivered by the source

Vs.

11IRV =

13

22=

33IRV =

321VVVV

s++=

321IRIRIRV

s++=

321 RRR

VI

++

=

Problem on KVL

What is the current in the

circuit shown in figure? Alsodetermine the voltage acrosseach resistor.

Problem on KCL

Determine the current in all resistors in the circuit shown infigure.

Series Circuit

Light bulbs in series. An

ammeter, A is placed in thecircuit to measure current.

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Parallel Circuit

Light bulbs in parallel.

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The sum of all the Ins in the circuit is equalto the total current, I, drawn from thesource.

Parallel resistance

The total resistance of the circuit decreases as thenumber of resistors connected in parallel increases

For resistance R1, R2 and R3 connected in paralleltheir combined resistance RT is given by

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321

1111

RRRRT

++=

Certain axioms for series and parallel

circuits

The current in a series circuit is the same at every pointalong the way.

The voltage across any component in a parallel circuit is

the same as the voltage across any other, or across thewhole set

The voltages across elements in a series circuit alwaysadd up to the supply voltage.

The currents through elements in a parallel circuit alwaysadd up to the total current drawn from the supply.

The total power consumed in a series or parallel circuit is

always equal to the sum of the wattages dissipated ineach of the elements.

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Voltage division

20

s

T

VR

RV

1

1=

321RRRR

T++=

The total voltage drop across any resistor in a series circuit is equalto the ratio of that resistance value to the total resistance, multipliedby the source voltage.

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Current division

In parallel circuit, the current divides in all branches

The current in any branch is equal to the ratio of

opposite branch resistance to the total resistancevalue, multiplied by the total current in the circuit

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