Illustration Of Ohm's Law And Kirchhoff's Laws


It is the student’s mentality to think that electrical engineering is difficult. Electrical laws are basic building blocks for all the subjects of electrical engineering. Staring from Ohm’s law, every law is important in electrical engineering. This article introduces the important laws of electrical engineering and also gives the idea of apply them correctly.

Ohm's law



Ohm's law is defined as the current flowing through the circuit is directly proportional to the voltage applied across the circuit at constant room temperature.

i.e. I a V or I=V/R ………(i)

1/R is the constant of proportionality and called as conductance and its unit is mho or Siemens.
Ohm's law can also be given as V a I or V=IR….. (ii)
'R' is the constant known as resistance of the circuit. It is defined as the opposition offered by the circuit for the flow of current. Its unit is ohm (?).

Fig.(i)
Ohm

In the fig(i)
'R' is resistance of the circuit.
'V' is voltage applied across the circuit.
'I' is current flowing through the resistor.

Ohm's law is applicable to both AC and DC circuits, but can not be applied under the conditions of variable temperature and to any semiconductor devices.

Kirchhoff's laws:



Kirchhoff's Current law (KCL):

This fundamental law results from the conservation of charge. It states that at any junction or node algebraic sum of currents entering the node is equal to algebraic sum of currents leaving the node. In general the summation of currents meeting at the junction is always zero.

i.e. ? I = 0

Consider the network shown below.
Fig(ii)

Kirchhoff
From the circuit shown KCL is written as

I1+I2=I3+I4 or I1+I2-I3-I4=0

Kirchhoff's voltage law (KVL) :

It states that in any network, the algebraic sum of the voltage drops across the circuit elements of any closed path or loop is equal to the algebraic sum of the e.m.f s in the path. In other words "the algebraic sum of all the branch voltages, around any closed path or closed loop is always zero."

i.e. ? V = 0

This law can be applied to both AC and DC circuits.

Fig(iii)

Concept of potential drop

Before proceeding with the illustration of the law, let us know about the concept of potential rise and potential drop. Consider a resistor in which current I is flowing from end 'a' to end 'b'. Polarities are as shown. '+' sign shown at end 'a' indicates that it is at higher potential and end 'b' is at lower potential. From 'a' to 'b' there is drop in the voltage and is indicated by '-'sign. Hence the drop from 'a' to 'b' is –IR. Without changing the direction of the current, voltage measured from 'b' to 'a' is voltage rise and indicated by '+' sign and hence drop from 'b' to 'a' is +IR. This concept is very important to understand the application of KVL.

Consider the circuit shown below.
Fig(iv)

Illustration of KVL

In this cirucit there are two loops, 3resistors and one voltage source. Assume the directions of currents arbitrarily.

Applying KVL to the loops
i)abefa

V1-I1R1-I3R3=0 or V1=I1R1+I3R3
I3=I1-I2

ii)bcdeb

-I2R2+I3R3=0

iii)abcdefa (Though there are three loops, abcdefa also forms the outer loop which can also be considered)

V1-I1R1-I2R2=0 or V1=I1R1+I2R2

Consider one more example where two voltage sources are included.
Fig(v)

Illustration of KVL

Applying KVL to the loops
i)abefa

V1-I1R1-I2R2-V2=0 or V1-V2=I1R1+I2R2
I3=I1-I2

ii)bcdeb

-I2R2 -V2+I3R3=0 or V2= I3R3-I2R2

iii)abcdefa

V1-I1R1-I2R2-V2=0

For the same circuit with two voltage sources, directions of currents can also be assumed as follows.

Fig(vi)

Illustration of KVL

Hence, the corresponding KVL equations are written as

i)abefa

V1-I1R1-I3R3=0 or V1=I1R1+I3R3
I3=I1+I2

ii)bcdeb

I2R2-V2+I3R3=0

iii)abcdefa

V1-I1R1+I2R2-V2=0

Points to remember:

Naming and following the order of the loop is very important aspect to avoid doing mistakes. For ex: if name of the loop is 'abcda' then starting from noe 'a' you have to proceed as 'a' to 'b', then cda. Remember, 'abcd' is not the loop. Loop should end with the starting node. i.e. 'a' in this case. Once the equtaions are written it can be simplified according to convenience. Number of unkown currents are equal to number of loop equations. For the circuits shown above, out of the three euations written, two are suffiecient to solve for the unknown circuit currents.


Comments

Author: Ravi Shankar Kumar G19 Jul 2015 Member Level: Silver   Points : 2

This is the basic information every electrical student should be aware of this particular topic in-order to workout or to know more about the Network Analysis or Electrical Circuit Analysis.
Not only KCL , KVL and Ohm's law even few more basic topics like Voltage Divider Rule , Current Divider Rule this are also play very crucial role in solving circuits in less time. But few of them were not aware of this rules who takes little bit of more time in solving step by step process.

However, thank you for revising the basic topics in networks. Of course am always in touch with this particular subject because as a part time teacher i get this one subject frequently for teaching.

Author: Kailash Kumar19 Jul 2015 Member Level: Platinum   Points : 1

This is a wonderful idea of the author to write an article on such a basic concepts. The points are well explained with sketches/illustrations. I recommend to the author to add her 'Article Footer' so that we can know about her academic profile. The author can do it by visiting dash board, then clicking on 'Edit Forum Footer' under 'Site Settings', though I myself have not yet tried it. A brief summary of her academic profile can be added there.

Author: DEEPANKAR DAS10 Apr 2016 Member Level: Silver   Points : 2

Ohm's Law can be applied only in DC Circuits because DC is constant but AC is sinusoidal or current changes periodically.



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