Post on 17-Aug-2015
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SILVER OAK COLLEGE OF ENGINEERING AND
TECHNOLOGY
TOPIC :- KIRCHHOFF’S CURRENT LAWSTUDENT NAME :- SIDDHI SHRIVAS (130770107163) DIVISON :- COMPUTER – C GUIDED BY :-MR. BIKAS MUDULI, SOCET
SUBJECT :- BASIC ELECTRONIC(2110003)
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KIRCHHOFF’S CURRENT LAW (KCL)
This law is also called Kirchhoff's first law, Kirchhoff's point rule, or Kirchhoff's
junction rule (or nodal rule).
This fundamental law results from the conservation of charge
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KIRCHHOFF’S CURRENT LAW
IT STATES THAT : “At any node (junction) in an electrical circuit, the sum
of currents flowing into that node is equal to the sum of currents
flowing out of that node, or The algebraic sum of currents in a
network of conductors meeting at a point is zero.”
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WHAT IS A JUNCTION OR NODE
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A junction is an intersection of three or more conductors.
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KIRCHHOFF'S RULES: JUNCTION RULEThe algebraic sum of the currents into any junction is zero.
• I = 0 ( at any junction )• I is + when it enters a junction.• I is - when it enters a junction.
• I = 0• I1 + I2 - I3 = 0• I1 + I2 = I3
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KIRCHHOFF'S CURRENT LAW
It also states that the sum of the currents entering a node must equal the sum of the currents leaving a node. This law is a consequence of the conservation of charge (electrons) in electrical networks.
I1 + I2 = I3 + I4 + I5
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EXPLANATION OF KIRCHHOFF'S LAW
WITH EXAMPLE
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SOME FUNDAMENTAL ASSUMPTIONS – NODES
A node is defined as a place where two or more components are connected.
The key thing to remember is that we connect components with wires. It doesn’t matter how many wires are being used; it only matters how many components are connected together.
+
-vA
RC
RD
iBRF
RE
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HOW MANY NODES?
To test our understanding of nodes, let’s look at the example circuit schematic given here.
How many nodes are there in this circuit?
+
-vA
RC
RD
iBRF
RE
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HOW MANY NODES – CORRECT ANSWER
In this schematic, there are three nodes. These nodes are shown in dark blue here.
Some students count more than three nodes in a circuit like this. When they do, it is usually because they have considered two points connected by a wire to be two nodes.
+
-
vA
RC
RD
iB
RF
RE
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HOW MANY NODES – WRONG ANSWER
In the example circuit schematic given here, the two red nodes are really the same node. There are not four nodes.
Remember, two nodes connected by a wire were really only one node in the first place.
+
-
vA
RC
RD
iB
RF
RE
Wire connecting two nodes means that these are really a single node.
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SOME FUNDAMENTAL ASSUMPTIONS – CLOSED LOOPS
A closed loop can be defined in this way: Start at any node and go in any direction and end up where you start. This is a closed loop.
Note that this loop does not have to follow components. It can jump across open space. Most of the time we will follow components, but we will also have situations where we need to jump between nodes that have no connections.
+
-
vA
RC
RD
iB
RF
RE
vX
+
-
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HOW MANY CLOSED LOOPS
To test our understanding of closed loops, let’s look at the example circuit schematic given here.
How many closed loops are there in this circuit?
+
-
vA
RC
RD
iB
RF
RE
vX
+
-
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HOW MANY CLOSED LOOPS – AN ANSWER
There are several closed loops that are possible here. We will show a few of them, and allow you to find the others.
The total number of simple closed loops in this circuit is 13.
Finding the number will not turn out to be important. What is important is to recognize closed loops when you see them.
+
-
vA
RC
RD
iB
RF
RE
vX
+
-
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+
-
vA
RC
RD
iB
RF
RE
vX
+
-
CLOSED LOOPS – LOOP #1 Here is a loop we
will call Loop #1. The path is shown in red.
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+
-
vA
RC
RD
iB
RF
RE
vX
+
-
Here is Loop #2. The path is shown in red.
CLOSED LOOPS – LOOP #2
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+
-
vA
RC
RD
iB
RF
RE
vX
+
-
CLOSED LOOPS – LOOP #3 Here is Loop #3.
The path is shown in red.
Note that this path is a closed loop that jumps across the voltage labeled vX. This is still a closed loop.
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+
-
vA
RC
RD
iB
RF
RE
vX
+
-
CLOSED LOOPS – LOOP #4 Here is Loop #4.
The path is shown in red.
Note that this path is a closed loop that jumps across the voltage labeled vX. This is still a closed loop. The loop also crossed the current source. Remember that a current source can have a voltage across it.
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+
-
vA
RC
RD
iB
RF
RE
vX
+
-
A NOT-CLOSED LOOP The path is shown
in red here is not closed.
Note that this path does not end where it started.
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CURRENT POLARITIES
Here, the issue of the sign, or polarity, or direction, of the current arises. When we write a Kirchhoff Current Law equation, we attach a sign to each reference current polarity, depending on whether the reference current is entering or leaving the closed surface. This can be done in different ways.
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KIRCHHOFF’S CURRENT LAW (KCL) – AN EXAMPLE
For this set of material, we will always assign a positive sign to a term that refers to a current that leaves a closed surface, and a negative sign to a term that refers to a current that enters a closed surface.
In this example, we have already assigned reference polarities for all of the currents for the nodes indicated in darker blue.
For this circuit, and using my rule, we have the following equation:
+
-
vA
RC
RD
iB
RF
RE
iA
iB
iC
iE
iD
0A C D E Bi i i i i
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KIRCHHOFF’S CURRENT LAW (KCL) – EXAMPLE DONE ANOTHER WAY
Some prefer to write this same equation in a different way; they say that the current entering the closed surface must equal the current leaving the closed surface. Thus, they write :
+
-
vA
RC
RD
iB
RF
RE
iA
iB
iC
iE
iD
0A C D E Bi i i i i
A D B C Ei i i i i • Compare this to the equation that we wrote in the last slide:
• These are the same equation. Use either method.
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Example Find the current I x.
4 A2 A
-1 A 6 A
IX9 A
Ans: IX = 22 A 22
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-8 A
-3 A
-5 A
-2 A
TRY BY YOURSELF ExampleFind the currents IW, I X, IY, IZ.
1 2 A
I X I Y
I Z
6 A
9 A
2 A
I W
IW = IX =IY =
IZ =
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ExampleFind the currents IA, IB, and IC in the circuit below.
s u rfa ce1
s u rfa ce2
-2 A
4 A
I B I C
9 A
2 A I A
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Solution for the above Example
s u rfa ce1
s u rfa ce2
-2 A
4 A
I B I C
9 A
2 A I A
n o de 1 n o de 2
At surface 1: IB = 2A At node 1: Ic = 0A At node 2: IA = 9A